From 5299d8a191246cf55c8ead7b8be68c8aeca78d35 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 15:28:14 +0200
Subject: [PATCH 01/34] parser: extract `ban_unexpected_or_or`.

---
 src/libsyntax/parse/parser/pat.rs | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 8cfa6abbe6270..4cda14907e46c 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -31,14 +31,7 @@ impl<'a> Parser<'a> {
             pats.push(self.parse_top_level_pat()?);
 
             if self.token == token::OrOr {
-                self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
-                    .span_suggestion(
-                        self.token.span,
-                        "use a single `|` to specify multiple patterns",
-                        "|".to_owned(),
-                        Applicability::MachineApplicable
-                    )
-                    .emit();
+                self.ban_unexpected_or_or();
                 self.bump();
             } else if self.eat(&token::BinOp(token::Or)) {
                 // This is a No-op. Continue the loop to parse the next
@@ -49,6 +42,17 @@ impl<'a> Parser<'a> {
         };
     }
 
+    fn ban_unexpected_or_or(&mut self) {
+        self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
+            .span_suggestion(
+                self.token.span,
+                "use a single `|` to specify multiple patterns",
+                "|".to_owned(),
+                Applicability::MachineApplicable
+            )
+            .emit();
+    }
+
     /// A wrapper around `parse_pat` with some special error handling for the
     /// "top-level" patterns in a match arm, `for` loop, `let`, &c. (in contrast
     /// to subpatterns within such).
@@ -116,9 +120,7 @@ impl<'a> Parser<'a> {
         let mut pats = vec![first_pat];
 
         while self.eat(&token::BinOp(token::Or)) {
-            pats.push(self.parse_pat_with_range_pat(
-                true, expected
-            )?);
+            pats.push(self.parse_pat_with_range_pat(true, expected)?);
         }
 
         let or_pattern_span = lo.to(self.prev_span);

From 1ba7550a8996cffc07c6af89dcd6e1cdc63b24cf Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 15:31:34 +0200
Subject: [PATCH 02/34] parser: type alias `type Expected = Option<&'static
 str>;`.

---
 src/libsyntax/parse/parser/pat.rs | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 4cda14907e46c..36d5ed5c4aa64 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -12,12 +12,11 @@ use crate::ThinVec;
 
 use errors::{Applicability, DiagnosticBuilder};
 
+type Expected = Option<&'static str>;
+
 impl<'a> Parser<'a> {
     /// Parses a pattern.
-    pub fn parse_pat(
-        &mut self,
-        expected: Option<&'static str>
-    ) -> PResult<'a, P<Pat>> {
+    pub fn parse_pat(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
         self.parse_pat_with_range_pat(true, expected)
     }
 
@@ -105,7 +104,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a pattern, that may be a or-pattern (e.g. `Some(Foo | Bar)`).
-    fn parse_pat_with_or(&mut self, expected: Option<&'static str>) -> PResult<'a, P<Pat>> {
+    fn parse_pat_with_or(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
         let first_pat = self.parse_pat(expected)?;
 
@@ -135,7 +134,7 @@ impl<'a> Parser<'a> {
     fn parse_pat_with_range_pat(
         &mut self,
         allow_range_pat: bool,
-        expected: Option<&'static str>,
+        expected: Expected,
     ) -> PResult<'a, P<Pat>> {
         maybe_recover_from_interpolated_ty_qpath!(self, true);
         maybe_whole!(self, NtPat, |x| x);
@@ -257,7 +256,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `&pat` / `&mut pat`.
-    fn parse_pat_deref(&mut self, expected: Option<&'static str>) -> PResult<'a, PatKind> {
+    fn parse_pat_deref(&mut self, expected: Expected) -> PResult<'a, PatKind> {
         self.expect_and()?;
         let mutbl = self.parse_mutability();
 
@@ -363,7 +362,7 @@ impl<'a> Parser<'a> {
     fn fatal_unexpected_non_pat(
         &mut self,
         mut err: DiagnosticBuilder<'a>,
-        expected: Option<&'static str>,
+        expected: Expected,
     ) -> PResult<'a, P<Pat>> {
         self.cancel(&mut err);
 

From 0bbea47794d28f78cf313fde475a35a83d0e9842 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 15:45:44 +0200
Subject: [PATCH 03/34] parser: refactor `parse_pat_with_or` + use it in [p0,
 p1, ..] pats.

---
 src/libsyntax/parse/parser/pat.rs | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 36d5ed5c4aa64..ca5a9f2a5a882 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -104,12 +104,12 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a pattern, that may be a or-pattern (e.g. `Some(Foo | Bar)`).
-    fn parse_pat_with_or(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
+    fn parse_pat_with_or(&mut self, expected: Expected, gate_or: bool) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
         let first_pat = self.parse_pat(expected)?;
 
-        // If the next token is not a `|`, this is not an or-pattern and
-        // we should exit here.
+        // If the next token is not a `|`,
+        // this is not an or-pattern and we should exit here.
         if !self.check(&token::BinOp(token::Or)) {
             return Ok(first_pat)
         }
@@ -124,7 +124,10 @@ impl<'a> Parser<'a> {
 
         let or_pattern_span = lo.to(self.prev_span);
 
-        self.sess.gated_spans.or_patterns.borrow_mut().push(or_pattern_span);
+        // Feature gate the or-pattern if instructed:
+        if gate_or {
+            self.sess.gated_spans.or_patterns.borrow_mut().push(or_pattern_span);
+        }
 
         Ok(self.mk_pat(or_pattern_span, PatKind::Or(pats)))
     }
@@ -145,7 +148,11 @@ impl<'a> Parser<'a> {
             token::OpenDelim(token::Paren) => self.parse_pat_tuple_or_parens()?,
             token::OpenDelim(token::Bracket) => {
                 // Parse `[pat, pat,...]` as a slice pattern.
-                PatKind::Slice(self.parse_delim_comma_seq(token::Bracket, |p| p.parse_pat(None))?.0)
+                let (pats, _) = self.parse_delim_comma_seq(
+                    token::Bracket,
+                    |p| p.parse_pat_with_or(None, true),
+                )?;
+                PatKind::Slice(pats)
             }
             token::DotDot => {
                 self.bump();
@@ -273,7 +280,7 @@ impl<'a> Parser<'a> {
     /// Parse a tuple or parenthesis pattern.
     fn parse_pat_tuple_or_parens(&mut self) -> PResult<'a, PatKind> {
         let (fields, trailing_comma) = self.parse_paren_comma_seq(|p| {
-            p.parse_pat_with_or(None)
+            p.parse_pat_with_or(None, true)
         })?;
 
         // Here, `(pat,)` is a tuple pattern.
@@ -517,7 +524,7 @@ impl<'a> Parser<'a> {
             err.span_label(self.token.span, msg);
             return Err(err);
         }
-        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or(None))?;
+        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or(None, true))?;
         Ok(PatKind::TupleStruct(path, fields))
     }
 
@@ -661,7 +668,7 @@ impl<'a> Parser<'a> {
             // Parsing a pattern of the form "fieldname: pat"
             let fieldname = self.parse_field_name()?;
             self.bump();
-            let pat = self.parse_pat_with_or(None)?;
+            let pat = self.parse_pat_with_or(None, true)?;
             hi = pat.span;
             (pat, fieldname, false)
         } else {

From 30b841dce0a1c0f26588f4b5791a9eda1c1f42f4 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 16:35:19 +0200
Subject: [PATCH 04/34] parser: improve or-patterns recovery.

---
 src/libsyntax/parse/parser/pat.rs | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index ca5a9f2a5a882..e52d0bc9d483b 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -110,18 +110,25 @@ impl<'a> Parser<'a> {
 
         // If the next token is not a `|`,
         // this is not an or-pattern and we should exit here.
-        if !self.check(&token::BinOp(token::Or)) {
+        if !self.check(&token::BinOp(token::Or)) && self.token != token::OrOr {
             return Ok(first_pat)
         }
 
         let lo = first_pat.span;
-
         let mut pats = vec![first_pat];
+        loop {
+            if self.token == token::OrOr {
+                // Found `||`; Recover and pretend we parsed `|`.
+                self.ban_unexpected_or_or();
+                self.bump();
+            } else if self.eat(&token::BinOp(token::Or)) {
+                // Found `|`. Working towards a proper or-pattern.
+            } else {
+                break;
+            }
 
-        while self.eat(&token::BinOp(token::Or)) {
             pats.push(self.parse_pat_with_range_pat(true, expected)?);
         }
-
         let or_pattern_span = lo.to(self.prev_span);
 
         // Feature gate the or-pattern if instructed:

From d34ee769b061975bf637776052179058c1f00bd7 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 16:45:08 +0200
Subject: [PATCH 05/34] parser: move `multiple-pattern-typo` -> `or-patterns`
 directory.

---
 .../ui/{did_you_mean => or-patterns}/multiple-pattern-typo.rs     | 0
 .../ui/{did_you_mean => or-patterns}/multiple-pattern-typo.stderr | 0
 2 files changed, 0 insertions(+), 0 deletions(-)
 rename src/test/ui/{did_you_mean => or-patterns}/multiple-pattern-typo.rs (100%)
 rename src/test/ui/{did_you_mean => or-patterns}/multiple-pattern-typo.stderr (100%)

diff --git a/src/test/ui/did_you_mean/multiple-pattern-typo.rs b/src/test/ui/or-patterns/multiple-pattern-typo.rs
similarity index 100%
rename from src/test/ui/did_you_mean/multiple-pattern-typo.rs
rename to src/test/ui/or-patterns/multiple-pattern-typo.rs
diff --git a/src/test/ui/did_you_mean/multiple-pattern-typo.stderr b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
similarity index 100%
rename from src/test/ui/did_you_mean/multiple-pattern-typo.stderr
rename to src/test/ui/or-patterns/multiple-pattern-typo.stderr

From 5f57feec0a3038ffc085ba897717b4ffd75445ee Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 16:52:49 +0200
Subject: [PATCH 06/34] parser: `multiple-pattern-typo`: cover more or-pattern
 places.

---
 .../ui/or-patterns/multiple-pattern-typo.rs   | 33 +++++++++++++++
 .../or-patterns/multiple-pattern-typo.stderr  | 42 ++++++++++++++++++-
 2 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/src/test/ui/or-patterns/multiple-pattern-typo.rs b/src/test/ui/or-patterns/multiple-pattern-typo.rs
index 14ad33d53b08c..5d1da56674ba4 100644
--- a/src/test/ui/or-patterns/multiple-pattern-typo.rs
+++ b/src/test/ui/or-patterns/multiple-pattern-typo.rs
@@ -1,7 +1,40 @@
+#![feature(or_patterns)]
+//~^ WARN the feature `or_patterns` is incomplete and may cause the compiler to crash
+
 fn main() {
     let x = 3;
+
     match x {
         1 | 2 || 3 => (), //~ ERROR unexpected token `||` after pattern
         _ => (),
     }
+
+    match x {
+        (1 | 2 || 3) => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
+
+    match (x,) {
+        (1 | 2 || 3,) => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
+
+    struct TS(u8);
+
+    match TS(x) {
+        TS(1 | 2 || 3) => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
+
+    struct NS { f: u8 }
+
+    match (NS { f: x }) {
+        NS { f: 1 | 2 || 3 } => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
+
+    match [x] {
+        [1 | 2 || 3] => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
 }
diff --git a/src/test/ui/or-patterns/multiple-pattern-typo.stderr b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
index a29fa584b2924..97f3470a54aa2 100644
--- a/src/test/ui/or-patterns/multiple-pattern-typo.stderr
+++ b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
@@ -1,8 +1,46 @@
 error: unexpected token `||` after pattern
-  --> $DIR/multiple-pattern-typo.rs:4:15
+  --> $DIR/multiple-pattern-typo.rs:8:15
    |
 LL |         1 | 2 || 3 => (),
    |               ^^ help: use a single `|` to specify multiple patterns: `|`
 
-error: aborting due to previous error
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:13:16
+   |
+LL |         (1 | 2 || 3) => (),
+   |                ^^ help: use a single `|` to specify multiple patterns: `|`
+
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:18:16
+   |
+LL |         (1 | 2 || 3,) => (),
+   |                ^^ help: use a single `|` to specify multiple patterns: `|`
+
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:25:18
+   |
+LL |         TS(1 | 2 || 3) => (),
+   |                  ^^ help: use a single `|` to specify multiple patterns: `|`
+
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:32:23
+   |
+LL |         NS { f: 1 | 2 || 3 } => (),
+   |                       ^^ help: use a single `|` to specify multiple patterns: `|`
+
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:37:16
+   |
+LL |         [1 | 2 || 3] => (),
+   |                ^^ help: use a single `|` to specify multiple patterns: `|`
+
+warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
+  --> $DIR/multiple-pattern-typo.rs:1:12
+   |
+LL | #![feature(or_patterns)]
+   |            ^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: aborting due to 6 previous errors
 

From f852c7ce1c6f55bc816d90c6e7f8e9205bb6c6f2 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 16:55:52 +0200
Subject: [PATCH 07/34] parser: simplify `parse_pat_with_or`.

---
 src/libsyntax/parse/parser/pat.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index e52d0bc9d483b..89688a287a7f3 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -127,7 +127,7 @@ impl<'a> Parser<'a> {
                 break;
             }
 
-            pats.push(self.parse_pat_with_range_pat(true, expected)?);
+            pats.push(self.parse_pat(expected)?);
         }
         let or_pattern_span = lo.to(self.prev_span);
 

From 21d9b85c0da1b639f8d8b3585e08759f96d1c286 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 17:11:12 +0200
Subject: [PATCH 08/34] parser: extract `maybe_recover_unexpected_comma`.

---
 src/libsyntax/parse/parser/pat.rs | 67 +++++++++++++++++--------------
 1 file changed, 36 insertions(+), 31 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 89688a287a7f3..588e5aef8a2f4 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -57,40 +57,45 @@ impl<'a> Parser<'a> {
     /// to subpatterns within such).
     pub(super) fn parse_top_level_pat(&mut self) -> PResult<'a, P<Pat>> {
         let pat = self.parse_pat(None)?;
-        if self.token == token::Comma {
-            // An unexpected comma after a top-level pattern is a clue that the
-            // user (perhaps more accustomed to some other language) forgot the
-            // parentheses in what should have been a tuple pattern; return a
-            // suggestion-enhanced error here rather than choking on the comma
-            // later.
-            let comma_span = self.token.span;
-            self.bump();
-            if let Err(mut err) = self.skip_pat_list() {
-                // We didn't expect this to work anyway; we just wanted
-                // to advance to the end of the comma-sequence so we know
-                // the span to suggest parenthesizing
-                err.cancel();
-            }
-            let seq_span = pat.span.to(self.prev_span);
-            let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
-            if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
-                err.span_suggestion(
-                    seq_span,
-                    "try adding parentheses to match on a tuple..",
-                    format!("({})", seq_snippet),
-                    Applicability::MachineApplicable
-                ).span_suggestion(
-                    seq_span,
-                    "..or a vertical bar to match on multiple alternatives",
-                    format!("{}", seq_snippet.replace(",", " |")),
-                    Applicability::MachineApplicable
-                );
-            }
-            return Err(err);
-        }
+        self.maybe_recover_unexpected_comma(pat.span)?;
         Ok(pat)
     }
 
+    fn maybe_recover_unexpected_comma(&mut self, lo: Span) -> PResult<'a, ()> {
+        if self.token != token::Comma {
+            return Ok(());
+        }
+
+        // An unexpected comma after a top-level pattern is a clue that the
+        // user (perhaps more accustomed to some other language) forgot the
+        // parentheses in what should have been a tuple pattern; return a
+        // suggestion-enhanced error here rather than choking on the comma later.
+        let comma_span = self.token.span;
+        self.bump();
+        if let Err(mut err) = self.skip_pat_list() {
+            // We didn't expect this to work anyway; we just wanted to advance to the
+            // end of the comma-sequence so we know the span to suggest parenthesizing.
+            err.cancel();
+        }
+        let seq_span = lo.to(self.prev_span);
+        let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
+        if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
+            err.span_suggestion(
+                seq_span,
+                "try adding parentheses to match on a tuple..",
+                format!("({})", seq_snippet),
+                Applicability::MachineApplicable
+            )
+            .span_suggestion(
+                seq_span,
+                "..or a vertical bar to match on multiple alternatives",
+                format!("{}", seq_snippet.replace(",", " |")),
+                Applicability::MachineApplicable
+            );
+        }
+        Err(err)
+    }
+
     /// Parse and throw away a parentesized comma separated
     /// sequence of patterns until `)` is reached.
     fn skip_pat_list(&mut self) -> PResult<'a, ()> {

From a4a34ab62df777e885cac71ab171225b2cd1a812 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 17:44:27 +0200
Subject: [PATCH 09/34] parser: integrate `maybe_recover_unexpected_comma` in
 `parse_pat_with_or`.

---
 src/libsyntax/parse/parser/pat.rs | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 588e5aef8a2f4..b2a026d0071e4 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -57,12 +57,12 @@ impl<'a> Parser<'a> {
     /// to subpatterns within such).
     pub(super) fn parse_top_level_pat(&mut self) -> PResult<'a, P<Pat>> {
         let pat = self.parse_pat(None)?;
-        self.maybe_recover_unexpected_comma(pat.span)?;
+        self.maybe_recover_unexpected_comma(pat.span, true)?;
         Ok(pat)
     }
 
-    fn maybe_recover_unexpected_comma(&mut self, lo: Span) -> PResult<'a, ()> {
-        if self.token != token::Comma {
+    fn maybe_recover_unexpected_comma(&mut self, lo: Span, top_level: bool) -> PResult<'a, ()> {
+        if !top_level || self.token != token::Comma {
             return Ok(());
         }
 
@@ -109,9 +109,15 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a pattern, that may be a or-pattern (e.g. `Some(Foo | Bar)`).
-    fn parse_pat_with_or(&mut self, expected: Expected, gate_or: bool) -> PResult<'a, P<Pat>> {
+    fn parse_pat_with_or(
+        &mut self,
+        expected: Expected,
+        gate_or: bool,
+        top_level: bool
+    ) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
         let first_pat = self.parse_pat(expected)?;
+        self.maybe_recover_unexpected_comma(first_pat.span, top_level)?;
 
         // If the next token is not a `|`,
         // this is not an or-pattern and we should exit here.
@@ -132,7 +138,9 @@ impl<'a> Parser<'a> {
                 break;
             }
 
-            pats.push(self.parse_pat(expected)?);
+            let pat = self.parse_pat(expected)?;
+            self.maybe_recover_unexpected_comma(pat.span, top_level)?;
+            pats.push(pat);
         }
         let or_pattern_span = lo.to(self.prev_span);
 
@@ -162,7 +170,7 @@ impl<'a> Parser<'a> {
                 // Parse `[pat, pat,...]` as a slice pattern.
                 let (pats, _) = self.parse_delim_comma_seq(
                     token::Bracket,
-                    |p| p.parse_pat_with_or(None, true),
+                    |p| p.parse_pat_with_or(None, true, false),
                 )?;
                 PatKind::Slice(pats)
             }
@@ -292,7 +300,7 @@ impl<'a> Parser<'a> {
     /// Parse a tuple or parenthesis pattern.
     fn parse_pat_tuple_or_parens(&mut self) -> PResult<'a, PatKind> {
         let (fields, trailing_comma) = self.parse_paren_comma_seq(|p| {
-            p.parse_pat_with_or(None, true)
+            p.parse_pat_with_or(None, true, false)
         })?;
 
         // Here, `(pat,)` is a tuple pattern.
@@ -536,7 +544,7 @@ impl<'a> Parser<'a> {
             err.span_label(self.token.span, msg);
             return Err(err);
         }
-        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or(None, true))?;
+        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or(None, true, false))?;
         Ok(PatKind::TupleStruct(path, fields))
     }
 
@@ -680,7 +688,7 @@ impl<'a> Parser<'a> {
             // Parsing a pattern of the form "fieldname: pat"
             let fieldname = self.parse_field_name()?;
             self.bump();
-            let pat = self.parse_pat_with_or(None, true)?;
+            let pat = self.parse_pat_with_or(None, true, false)?;
             hi = pat.span;
             (pat, fieldname, false)
         } else {

From 7b59b4f14dae8c859718d60794021230e1e3ac29 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 18:13:19 +0200
Subject: [PATCH 10/34] parser: extract `eat_or_separator`.

---
 src/libsyntax/parse/parser/pat.rs | 58 +++++++++++++++----------------
 1 file changed, 28 insertions(+), 30 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index b2a026d0071e4..3af64cef74f26 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -29,27 +29,10 @@ impl<'a> Parser<'a> {
         loop {
             pats.push(self.parse_top_level_pat()?);
 
-            if self.token == token::OrOr {
-                self.ban_unexpected_or_or();
-                self.bump();
-            } else if self.eat(&token::BinOp(token::Or)) {
-                // This is a No-op. Continue the loop to parse the next
-                // pattern.
-            } else {
+            if !self.eat_or_separator() {
                 return Ok(pats);
             }
-        };
-    }
-
-    fn ban_unexpected_or_or(&mut self) {
-        self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
-            .span_suggestion(
-                self.token.span,
-                "use a single `|` to specify multiple patterns",
-                "|".to_owned(),
-                Applicability::MachineApplicable
-            )
-            .emit();
+        }
     }
 
     /// A wrapper around `parse_pat` with some special error handling for the
@@ -127,17 +110,7 @@ impl<'a> Parser<'a> {
 
         let lo = first_pat.span;
         let mut pats = vec![first_pat];
-        loop {
-            if self.token == token::OrOr {
-                // Found `||`; Recover and pretend we parsed `|`.
-                self.ban_unexpected_or_or();
-                self.bump();
-            } else if self.eat(&token::BinOp(token::Or)) {
-                // Found `|`. Working towards a proper or-pattern.
-            } else {
-                break;
-            }
-
+        while self.eat_or_separator() {
             let pat = self.parse_pat(expected)?;
             self.maybe_recover_unexpected_comma(pat.span, top_level)?;
             pats.push(pat);
@@ -152,6 +125,31 @@ impl<'a> Parser<'a> {
         Ok(self.mk_pat(or_pattern_span, PatKind::Or(pats)))
     }
 
+    /// Eat the or-pattern `|` separator.
+    /// If instead a `||` token is encountered, recover and pretend we parsed `|`.
+    fn eat_or_separator(&mut self) -> bool {
+        match self.token.kind {
+            token::OrOr => {
+                // Found `||`; Recover and pretend we parsed `|`.
+                self.ban_unexpected_or_or();
+                self.bump();
+                true
+            }
+            _ => self.eat(&token::BinOp(token::Or)),
+        }
+    }
+
+    fn ban_unexpected_or_or(&mut self) {
+        self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
+            .span_suggestion(
+                self.token.span,
+                "use a single `|` to specify multiple patterns",
+                "|".to_owned(),
+                Applicability::MachineApplicable
+            )
+            .emit();
+    }
+
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
     /// allowed).
     fn parse_pat_with_range_pat(

From dc5bbaf7b2df8dc2be6c0f1a9973867e5519300b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 18:15:17 +0200
Subject: [PATCH 11/34] parser: improve `parse_pat_with_or` docs.

---
 src/libsyntax/parse/parser/pat.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 3af64cef74f26..14ac509d6f706 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -91,7 +91,8 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
-    /// Parses a pattern, that may be a or-pattern (e.g. `Some(Foo | Bar)`).
+    /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
+    /// Corresponds to `pat<allow_top_alt>` in RFC 2535.
     fn parse_pat_with_or(
         &mut self,
         expected: Expected,

From 6498959377421876040515af39b6491a2ec2a0c5 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 18:34:35 +0200
Subject: [PATCH 12/34] parser: use `eat_or_separator` for leading vert.

---
 src/libsyntax/parse/parser/pat.rs                    | 4 ++--
 src/test/ui/or-patterns/multiple-pattern-typo.rs     | 5 +++++
 src/test/ui/or-patterns/multiple-pattern-typo.stderr | 8 +++++++-
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 14ac509d6f706..1063e34753095 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -22,8 +22,8 @@ impl<'a> Parser<'a> {
 
     /// Parses patterns, separated by '|' s.
     pub(super) fn parse_pats(&mut self) -> PResult<'a, Vec<P<Pat>>> {
-        // Allow a '|' before the pats (RFC 1925 + RFC 2530)
-        self.eat(&token::BinOp(token::Or));
+        // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
+        self.eat_or_separator();
 
         let mut pats = Vec::new();
         loop {
diff --git a/src/test/ui/or-patterns/multiple-pattern-typo.rs b/src/test/ui/or-patterns/multiple-pattern-typo.rs
index 5d1da56674ba4..e308c0adb4eb8 100644
--- a/src/test/ui/or-patterns/multiple-pattern-typo.rs
+++ b/src/test/ui/or-patterns/multiple-pattern-typo.rs
@@ -37,4 +37,9 @@ fn main() {
         [1 | 2 || 3] => (), //~ ERROR unexpected token `||` after pattern
         _ => (),
     }
+
+    match x {
+        || 1 | 2 | 3 => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
 }
diff --git a/src/test/ui/or-patterns/multiple-pattern-typo.stderr b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
index 97f3470a54aa2..765c7879b12ca 100644
--- a/src/test/ui/or-patterns/multiple-pattern-typo.stderr
+++ b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
@@ -34,6 +34,12 @@ error: unexpected token `||` after pattern
 LL |         [1 | 2 || 3] => (),
    |                ^^ help: use a single `|` to specify multiple patterns: `|`
 
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:42:9
+   |
+LL |         || 1 | 2 | 3 => (),
+   |         ^^ help: use a single `|` to specify multiple patterns: `|`
+
 warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
   --> $DIR/multiple-pattern-typo.rs:1:12
    |
@@ -42,5 +48,5 @@ LL | #![feature(or_patterns)]
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 

From 39f5e5bec42a4c05588db45d12ab9aafc01776aa Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 18:37:41 +0200
Subject: [PATCH 13/34] parser: move `maybe_recover_unexpected_comma` to a more
 appropriate place.

---
 src/libsyntax/parse/parser/pat.rs | 99 +++++++++++++++----------------
 1 file changed, 49 insertions(+), 50 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 1063e34753095..680a587205686 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -35,62 +35,12 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// A wrapper around `parse_pat` with some special error handling for the
-    /// "top-level" patterns in a match arm, `for` loop, `let`, &c. (in contrast
-    /// to subpatterns within such).
     pub(super) fn parse_top_level_pat(&mut self) -> PResult<'a, P<Pat>> {
         let pat = self.parse_pat(None)?;
         self.maybe_recover_unexpected_comma(pat.span, true)?;
         Ok(pat)
     }
 
-    fn maybe_recover_unexpected_comma(&mut self, lo: Span, top_level: bool) -> PResult<'a, ()> {
-        if !top_level || self.token != token::Comma {
-            return Ok(());
-        }
-
-        // An unexpected comma after a top-level pattern is a clue that the
-        // user (perhaps more accustomed to some other language) forgot the
-        // parentheses in what should have been a tuple pattern; return a
-        // suggestion-enhanced error here rather than choking on the comma later.
-        let comma_span = self.token.span;
-        self.bump();
-        if let Err(mut err) = self.skip_pat_list() {
-            // We didn't expect this to work anyway; we just wanted to advance to the
-            // end of the comma-sequence so we know the span to suggest parenthesizing.
-            err.cancel();
-        }
-        let seq_span = lo.to(self.prev_span);
-        let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
-        if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
-            err.span_suggestion(
-                seq_span,
-                "try adding parentheses to match on a tuple..",
-                format!("({})", seq_snippet),
-                Applicability::MachineApplicable
-            )
-            .span_suggestion(
-                seq_span,
-                "..or a vertical bar to match on multiple alternatives",
-                format!("{}", seq_snippet.replace(",", " |")),
-                Applicability::MachineApplicable
-            );
-        }
-        Err(err)
-    }
-
-    /// Parse and throw away a parentesized comma separated
-    /// sequence of patterns until `)` is reached.
-    fn skip_pat_list(&mut self) -> PResult<'a, ()> {
-        while !self.check(&token::CloseDelim(token::Paren)) {
-            self.parse_pat(None)?;
-            if !self.eat(&token::Comma) {
-                return Ok(())
-            }
-        }
-        Ok(())
-    }
-
     /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
     /// Corresponds to `pat<allow_top_alt>` in RFC 2535.
     fn parse_pat_with_or(
@@ -151,6 +101,55 @@ impl<'a> Parser<'a> {
             .emit();
     }
 
+    /// Some special error handling for the "top-level" patterns in a match arm,
+    /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
+    fn maybe_recover_unexpected_comma(&mut self, lo: Span, top_level: bool) -> PResult<'a, ()> {
+        if !top_level || self.token != token::Comma {
+            return Ok(());
+        }
+
+        // An unexpected comma after a top-level pattern is a clue that the
+        // user (perhaps more accustomed to some other language) forgot the
+        // parentheses in what should have been a tuple pattern; return a
+        // suggestion-enhanced error here rather than choking on the comma later.
+        let comma_span = self.token.span;
+        self.bump();
+        if let Err(mut err) = self.skip_pat_list() {
+            // We didn't expect this to work anyway; we just wanted to advance to the
+            // end of the comma-sequence so we know the span to suggest parenthesizing.
+            err.cancel();
+        }
+        let seq_span = lo.to(self.prev_span);
+        let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
+        if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
+            err.span_suggestion(
+                seq_span,
+                "try adding parentheses to match on a tuple..",
+                format!("({})", seq_snippet),
+                Applicability::MachineApplicable
+            )
+            .span_suggestion(
+                seq_span,
+                "..or a vertical bar to match on multiple alternatives",
+                format!("{}", seq_snippet.replace(",", " |")),
+                Applicability::MachineApplicable
+            );
+        }
+        Err(err)
+    }
+
+    /// Parse and throw away a parentesized comma separated
+    /// sequence of patterns until `)` is reached.
+    fn skip_pat_list(&mut self) -> PResult<'a, ()> {
+        while !self.check(&token::CloseDelim(token::Paren)) {
+            self.parse_pat(None)?;
+            if !self.eat(&token::Comma) {
+                return Ok(())
+            }
+        }
+        Ok(())
+    }
+
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
     /// allowed).
     fn parse_pat_with_range_pat(

From 8f6a0cdb0fd453580bed74586c6930b1498aa26f Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 18:38:36 +0200
Subject: [PATCH 14/34] parser: document `ban_unexpected_or_or`.

---
 src/libsyntax/parse/parser/pat.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 680a587205686..8fab8884ca018 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -90,6 +90,7 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// We have parsed `||` instead of `|`. Error and suggest `|` instead.
     fn ban_unexpected_or_or(&mut self) {
         self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
             .span_suggestion(

From b7178ef9836fe8e98ffb3f8d4d870c94e6fe816d Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 22:04:28 +0200
Subject: [PATCH 15/34] parser: `parse_pats` -> `parse_top_pat{_unpack}`.

---
 src/libsyntax/parse/parser/expr.rs | 12 +++++++-----
 src/libsyntax/parse/parser/pat.rs  | 28 +++++++++++++++++-----------
 2 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/src/libsyntax/parse/parser/expr.rs b/src/libsyntax/parse/parser/expr.rs
index 5da9b75d53b04..b9dd851817160 100644
--- a/src/libsyntax/parse/parser/expr.rs
+++ b/src/libsyntax/parse/parser/expr.rs
@@ -1241,11 +1241,12 @@ impl<'a> Parser<'a> {
         Ok(cond)
     }
 
-    /// Parses a `let $pats = $expr` pseudo-expression.
+    /// Parses a `let $pat = $expr` pseudo-expression.
     /// The `let` token has already been eaten.
     fn parse_let_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
-        let pats = self.parse_pats()?;
+        // FIXME(or_patterns, Centril | dlrobertson): use `parse_top_pat` instead.
+        let pat = self.parse_top_pat_unpack(false)?;
         self.expect(&token::Eq)?;
         let expr = self.with_res(
             Restrictions::NO_STRUCT_LITERAL,
@@ -1253,7 +1254,7 @@ impl<'a> Parser<'a> {
         )?;
         let span = lo.to(expr.span);
         self.sess.gated_spans.let_chains.borrow_mut().push(span);
-        Ok(self.mk_expr(span, ExprKind::Let(pats, expr), attrs))
+        Ok(self.mk_expr(span, ExprKind::Let(pat, expr), attrs))
     }
 
     /// `else` token already eaten
@@ -1387,7 +1388,8 @@ impl<'a> Parser<'a> {
     crate fn parse_arm(&mut self) -> PResult<'a, Arm> {
         let attrs = self.parse_outer_attributes()?;
         let lo = self.token.span;
-        let pats = self.parse_pats()?;
+        // FIXME(or_patterns, Centril | dlrobertson): use `parse_top_pat` instead.
+        let pat = self.parse_top_pat_unpack(false)?;
         let guard = if self.eat_keyword(kw::If) {
             Some(self.parse_expr()?)
         } else {
@@ -1448,7 +1450,7 @@ impl<'a> Parser<'a> {
 
         Ok(ast::Arm {
             attrs,
-            pats,
+            pats: pat, // FIXME(or_patterns, Centril | dlrobertson): this should just be `pat,`.
             guard,
             body: expr,
             span: lo.to(hi),
diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 8fab8884ca018..e4a9dc0097752 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -20,19 +20,25 @@ impl<'a> Parser<'a> {
         self.parse_pat_with_range_pat(true, expected)
     }
 
-    /// Parses patterns, separated by '|' s.
-    pub(super) fn parse_pats(&mut self) -> PResult<'a, Vec<P<Pat>>> {
-        // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
-        self.eat_or_separator();
-
-        let mut pats = Vec::new();
-        loop {
-            pats.push(self.parse_top_level_pat()?);
+    // FIXME(or_patterns, Centril | dlrobertson):
+    // remove this and use `parse_top_pat` everywhere it is used instead.
+    pub(super) fn parse_top_pat_unpack(&mut self, gate_or: bool) -> PResult<'a, Vec<P<Pat>>> {
+        self.parse_top_pat(gate_or)
+            .map(|pat| pat.and_then(|pat| match pat.node {
+                PatKind::Or(pats) => pats,
+                node => vec![self.mk_pat(pat.span, node)],
+            }))
+    }
 
-            if !self.eat_or_separator() {
-                return Ok(pats);
-            }
+    /// Entry point to the main pattern parser.
+    /// Corresponds to `top_pat` in RFC 2535 and allows or-pattern at the top level.
+    pub(super) fn parse_top_pat(&mut self, gate_or: bool) -> PResult<'a, P<Pat>> {
+        // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
+        if self.eat_or_separator() && gate_or {
+            self.sess.gated_spans.or_patterns.borrow_mut().push(self.prev_span);
         }
+
+        self.parse_pat_with_or(None, gate_or, true)
     }
 
     pub(super) fn parse_top_level_pat(&mut self) -> PResult<'a, P<Pat>> {

From 92d66a131711bc7817e599c81d081847f689654c Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 22:35:53 +0200
Subject: [PATCH 16/34] parser: document `parse_pat`.

---
 src/libsyntax/parse/parser/pat.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index e4a9dc0097752..021e52798af26 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -16,6 +16,10 @@ type Expected = Option<&'static str>;
 
 impl<'a> Parser<'a> {
     /// Parses a pattern.
+    ///
+    /// Corresponds to `pat<no_top_alt>` in RFC 2535 and does not admit or-patterns
+    /// at the top level. Used when parsing the parameters of lambda expressions,
+    /// functions, function pointers, and `pat` macro fragments.
     pub fn parse_pat(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
         self.parse_pat_with_range_pat(true, expected)
     }

From 95792b4d5a12276068e32f54c5d1561b8528a952 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 22:54:07 +0200
Subject: [PATCH 17/34] parser: `let` stmts & `for` exprs: allow or-patterns.

---
 src/libsyntax/parse/parser/expr.rs         | 2 +-
 src/libsyntax/parse/parser/pat.rs          | 6 ------
 src/libsyntax/parse/parser/stmt.rs         | 2 +-
 src/test/ui/parser/bad-match.rs            | 2 +-
 src/test/ui/parser/bad-match.stderr        | 4 ++--
 src/test/ui/parser/bad-name.stderr         | 4 ++--
 src/test/ui/parser/issue-22647.rs          | 2 +-
 src/test/ui/parser/issue-22647.stderr      | 4 ++--
 src/test/ui/parser/issue-22712.rs          | 2 +-
 src/test/ui/parser/issue-22712.stderr      | 4 ++--
 src/test/ui/parser/issue-24197.rs          | 2 +-
 src/test/ui/parser/issue-24197.stderr      | 4 ++--
 src/test/ui/parser/mut-patterns.rs         | 3 ++-
 src/test/ui/parser/mut-patterns.stderr     | 4 ++--
 src/test/ui/parser/pat-lt-bracket-5.rs     | 2 +-
 src/test/ui/parser/pat-lt-bracket-5.stderr | 4 ++--
 src/test/ui/parser/pat-ranges-1.rs         | 2 +-
 src/test/ui/parser/pat-ranges-1.stderr     | 4 ++--
 src/test/ui/parser/pat-ranges-2.rs         | 2 +-
 src/test/ui/parser/pat-ranges-2.stderr     | 4 ++--
 src/test/ui/parser/pat-ranges-3.rs         | 2 +-
 src/test/ui/parser/pat-ranges-3.stderr     | 4 ++--
 src/test/ui/parser/pat-ranges-4.rs         | 2 +-
 src/test/ui/parser/pat-ranges-4.stderr     | 4 ++--
 24 files changed, 35 insertions(+), 40 deletions(-)

diff --git a/src/libsyntax/parse/parser/expr.rs b/src/libsyntax/parse/parser/expr.rs
index b9dd851817160..25a858b47353f 100644
--- a/src/libsyntax/parse/parser/expr.rs
+++ b/src/libsyntax/parse/parser/expr.rs
@@ -1284,7 +1284,7 @@ impl<'a> Parser<'a> {
             _ => None,
         };
 
-        let pat = self.parse_top_level_pat()?;
+        let pat = self.parse_top_pat(true)?;
         if !self.eat_keyword(kw::In) {
             let in_span = self.prev_span.between(self.token.span);
             self.struct_span_err(in_span, "missing `in` in `for` loop")
diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 021e52798af26..e77d9120bce78 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -45,12 +45,6 @@ impl<'a> Parser<'a> {
         self.parse_pat_with_or(None, gate_or, true)
     }
 
-    pub(super) fn parse_top_level_pat(&mut self) -> PResult<'a, P<Pat>> {
-        let pat = self.parse_pat(None)?;
-        self.maybe_recover_unexpected_comma(pat.span, true)?;
-        Ok(pat)
-    }
-
     /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
     /// Corresponds to `pat<allow_top_alt>` in RFC 2535.
     fn parse_pat_with_or(
diff --git a/src/libsyntax/parse/parser/stmt.rs b/src/libsyntax/parse/parser/stmt.rs
index c911caba4cd41..3c8cb4ea5a582 100644
--- a/src/libsyntax/parse/parser/stmt.rs
+++ b/src/libsyntax/parse/parser/stmt.rs
@@ -207,7 +207,7 @@ impl<'a> Parser<'a> {
     /// Parses a local variable declaration.
     fn parse_local(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Local>> {
         let lo = self.prev_span;
-        let pat = self.parse_top_level_pat()?;
+        let pat = self.parse_top_pat(true)?;
 
         let (err, ty) = if self.eat(&token::Colon) {
             // Save the state of the parser before parsing type normally, in case there is a `:`
diff --git a/src/test/ui/parser/bad-match.rs b/src/test/ui/parser/bad-match.rs
index 79bc7eec3113e..04100d1701ddb 100644
--- a/src/test/ui/parser/bad-match.rs
+++ b/src/test/ui/parser/bad-match.rs
@@ -1,4 +1,4 @@
 fn main() {
-  let isize x = 5; //~ ERROR expected one of `:`, `;`, `=`, or `@`, found `x`
+  let isize x = 5; //~ ERROR expected one of `:`, `;`, `=`, `@`, or `|`, found `x`
   match x;
 }
diff --git a/src/test/ui/parser/bad-match.stderr b/src/test/ui/parser/bad-match.stderr
index 2f29b978e9c91..d5baaf5e93b35 100644
--- a/src/test/ui/parser/bad-match.stderr
+++ b/src/test/ui/parser/bad-match.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `x`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `x`
   --> $DIR/bad-match.rs:2:13
    |
 LL |   let isize x = 5;
-   |             ^ expected one of `:`, `;`, `=`, or `@` here
+   |             ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/bad-name.stderr b/src/test/ui/parser/bad-name.stderr
index 15e61cf06cae9..dce4dabedf5c8 100644
--- a/src/test/ui/parser/bad-name.stderr
+++ b/src/test/ui/parser/bad-name.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `.`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `.`
   --> $DIR/bad-name.rs:4:8
    |
 LL |   let x.y::<isize>.z foo;
-   |        ^ expected one of `:`, `;`, `=`, or `@` here
+   |        ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/issue-22647.rs b/src/test/ui/parser/issue-22647.rs
index 25cd7ffce5a97..a6861410682cb 100644
--- a/src/test/ui/parser/issue-22647.rs
+++ b/src/test/ui/parser/issue-22647.rs
@@ -1,5 +1,5 @@
 fn main() {
-    let caller<F> = |f: F|  //~ ERROR expected one of `:`, `;`, `=`, or `@`, found `<`
+    let caller<F> = |f: F|  //~ ERROR expected one of `:`, `;`, `=`, `@`, or `|`, found `<`
     where F: Fn() -> i32
     {
         let x = f();
diff --git a/src/test/ui/parser/issue-22647.stderr b/src/test/ui/parser/issue-22647.stderr
index 2dc56a5eca3a6..4b1ef4f3dfc78 100644
--- a/src/test/ui/parser/issue-22647.stderr
+++ b/src/test/ui/parser/issue-22647.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `<`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `<`
   --> $DIR/issue-22647.rs:2:15
    |
 LL |     let caller<F> = |f: F|
-   |               ^ expected one of `:`, `;`, `=`, or `@` here
+   |               ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/issue-22712.rs b/src/test/ui/parser/issue-22712.rs
index b03d578e3d638..774de9c7e6448 100644
--- a/src/test/ui/parser/issue-22712.rs
+++ b/src/test/ui/parser/issue-22712.rs
@@ -3,7 +3,7 @@ struct Foo<B> {
 }
 
 fn bar() {
-    let Foo<Vec<u8>>  //~ ERROR expected one of `:`, `;`, `=`, or `@`, found `<`
+    let Foo<Vec<u8>>  //~ ERROR expected one of `:`, `;`, `=`, `@`, or `|`, found `<`
 }
 
 fn main() {}
diff --git a/src/test/ui/parser/issue-22712.stderr b/src/test/ui/parser/issue-22712.stderr
index 167eaf962e0f4..d9e83144b367a 100644
--- a/src/test/ui/parser/issue-22712.stderr
+++ b/src/test/ui/parser/issue-22712.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `<`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `<`
   --> $DIR/issue-22712.rs:6:12
    |
 LL |     let Foo<Vec<u8>>
-   |            ^ expected one of `:`, `;`, `=`, or `@` here
+   |            ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/issue-24197.rs b/src/test/ui/parser/issue-24197.rs
index 005ff9fa2e0ec..aaf5137461fa6 100644
--- a/src/test/ui/parser/issue-24197.rs
+++ b/src/test/ui/parser/issue-24197.rs
@@ -1,3 +1,3 @@
 fn main() {
-    let buf[0] = 0; //~ ERROR expected one of `:`, `;`, `=`, or `@`, found `[`
+    let buf[0] = 0; //~ ERROR expected one of `:`, `;`, `=`, `@`, or `|`, found `[`
 }
diff --git a/src/test/ui/parser/issue-24197.stderr b/src/test/ui/parser/issue-24197.stderr
index 2dfb31432bc99..24818db622ad4 100644
--- a/src/test/ui/parser/issue-24197.stderr
+++ b/src/test/ui/parser/issue-24197.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `[`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `[`
   --> $DIR/issue-24197.rs:2:12
    |
 LL |     let buf[0] = 0;
-   |            ^ expected one of `:`, `;`, `=`, or `@` here
+   |            ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/mut-patterns.rs b/src/test/ui/parser/mut-patterns.rs
index a5eb48252394f..bffeb1e2e7c40 100644
--- a/src/test/ui/parser/mut-patterns.rs
+++ b/src/test/ui/parser/mut-patterns.rs
@@ -2,5 +2,6 @@
 
 pub fn main() {
     struct Foo { x: isize }
-    let mut Foo { x: x } = Foo { x: 3 }; //~ ERROR: expected one of `:`, `;`, `=`, or `@`, found `{`
+    let mut Foo { x: x } = Foo { x: 3 };
+    //~^ ERROR: expected one of `:`, `;`, `=`, `@`, or `|`, found `{`
 }
diff --git a/src/test/ui/parser/mut-patterns.stderr b/src/test/ui/parser/mut-patterns.stderr
index 286956440ec34..b39209afd4295 100644
--- a/src/test/ui/parser/mut-patterns.stderr
+++ b/src/test/ui/parser/mut-patterns.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `{`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `{`
   --> $DIR/mut-patterns.rs:5:17
    |
 LL |     let mut Foo { x: x } = Foo { x: 3 };
-   |                 ^ expected one of `:`, `;`, `=`, or `@` here
+   |                 ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/pat-lt-bracket-5.rs b/src/test/ui/parser/pat-lt-bracket-5.rs
index c4b9dd469f54c..aaece1f6bd9cb 100644
--- a/src/test/ui/parser/pat-lt-bracket-5.rs
+++ b/src/test/ui/parser/pat-lt-bracket-5.rs
@@ -1,3 +1,3 @@
 fn main() {
-    let v[0] = v[1]; //~ ERROR expected one of `:`, `;`, `=`, or `@`, found `[`
+    let v[0] = v[1]; //~ ERROR expected one of `:`, `;`, `=`, `@`, or `|`, found `[`
 }
diff --git a/src/test/ui/parser/pat-lt-bracket-5.stderr b/src/test/ui/parser/pat-lt-bracket-5.stderr
index ce4cc05db19b2..167314dde0650 100644
--- a/src/test/ui/parser/pat-lt-bracket-5.stderr
+++ b/src/test/ui/parser/pat-lt-bracket-5.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, `=`, or `@`, found `[`
+error: expected one of `:`, `;`, `=`, `@`, or `|`, found `[`
   --> $DIR/pat-lt-bracket-5.rs:2:10
    |
 LL |     let v[0] = v[1];
-   |          ^ expected one of `:`, `;`, `=`, or `@` here
+   |          ^ expected one of `:`, `;`, `=`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/pat-ranges-1.rs b/src/test/ui/parser/pat-ranges-1.rs
index ce953b2eb2299..1dafb5a07bb5a 100644
--- a/src/test/ui/parser/pat-ranges-1.rs
+++ b/src/test/ui/parser/pat-ranges-1.rs
@@ -1,5 +1,5 @@
 // Parsing of range patterns
 
 fn main() {
-    let macropus!() ..= 11 = 12; //~ error: expected one of `:`, `;`, or `=`, found `..=`
+    let macropus!() ..= 11 = 12; //~ error: expected one of `:`, `;`, `=`, or `|`, found `..=`
 }
diff --git a/src/test/ui/parser/pat-ranges-1.stderr b/src/test/ui/parser/pat-ranges-1.stderr
index 6e0deccab8ca2..4e2c5d28381d8 100644
--- a/src/test/ui/parser/pat-ranges-1.stderr
+++ b/src/test/ui/parser/pat-ranges-1.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, or `=`, found `..=`
+error: expected one of `:`, `;`, `=`, or `|`, found `..=`
   --> $DIR/pat-ranges-1.rs:4:21
    |
 LL |     let macropus!() ..= 11 = 12;
-   |                     ^^^ expected one of `:`, `;`, or `=` here
+   |                     ^^^ expected one of `:`, `;`, `=`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/pat-ranges-2.rs b/src/test/ui/parser/pat-ranges-2.rs
index 9f736ed328c5c..1593222acca1b 100644
--- a/src/test/ui/parser/pat-ranges-2.rs
+++ b/src/test/ui/parser/pat-ranges-2.rs
@@ -1,5 +1,5 @@
 // Parsing of range patterns
 
 fn main() {
-    let 10 ..= makropulos!() = 12; //~ error: expected one of `::`, `:`, `;`, or `=`, found `!`
+    let 10 ..= makropulos!() = 12; //~ error: expected one of `::`, `:`, `;`, `=`, or `|`, found `!`
 }
diff --git a/src/test/ui/parser/pat-ranges-2.stderr b/src/test/ui/parser/pat-ranges-2.stderr
index d180bb429110a..64df56f5a61b1 100644
--- a/src/test/ui/parser/pat-ranges-2.stderr
+++ b/src/test/ui/parser/pat-ranges-2.stderr
@@ -1,8 +1,8 @@
-error: expected one of `::`, `:`, `;`, or `=`, found `!`
+error: expected one of `::`, `:`, `;`, `=`, or `|`, found `!`
   --> $DIR/pat-ranges-2.rs:4:26
    |
 LL |     let 10 ..= makropulos!() = 12;
-   |                          ^ expected one of `::`, `:`, `;`, or `=` here
+   |                          ^ expected one of `::`, `:`, `;`, `=`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/pat-ranges-3.rs b/src/test/ui/parser/pat-ranges-3.rs
index 65da55e3bda8c..8976dcf0d90f9 100644
--- a/src/test/ui/parser/pat-ranges-3.rs
+++ b/src/test/ui/parser/pat-ranges-3.rs
@@ -1,5 +1,5 @@
 // Parsing of range patterns
 
 fn main() {
-    let 10 ..= 10 + 3 = 12; //~ expected one of `:`, `;`, or `=`, found `+`
+    let 10 ..= 10 + 3 = 12; //~ expected one of `:`, `;`, `=`, or `|`, found `+`
 }
diff --git a/src/test/ui/parser/pat-ranges-3.stderr b/src/test/ui/parser/pat-ranges-3.stderr
index aaa85e3c2ddd8..c32c18d98dce7 100644
--- a/src/test/ui/parser/pat-ranges-3.stderr
+++ b/src/test/ui/parser/pat-ranges-3.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:`, `;`, or `=`, found `+`
+error: expected one of `:`, `;`, `=`, or `|`, found `+`
   --> $DIR/pat-ranges-3.rs:4:19
    |
 LL |     let 10 ..= 10 + 3 = 12;
-   |                   ^ expected one of `:`, `;`, or `=` here
+   |                   ^ expected one of `:`, `;`, `=`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/pat-ranges-4.rs b/src/test/ui/parser/pat-ranges-4.rs
index 7f4a5f3239e76..61188976b028c 100644
--- a/src/test/ui/parser/pat-ranges-4.rs
+++ b/src/test/ui/parser/pat-ranges-4.rs
@@ -2,5 +2,5 @@
 
 fn main() {
     let 10 - 3 ..= 10 = 8;
-    //~^ error: expected one of `...`, `..=`, `..`, `:`, `;`, or `=`, found `-`
+    //~^ error: expected one of `...`, `..=`, `..`, `:`, `;`, `=`, or `|`, found `-`
 }
diff --git a/src/test/ui/parser/pat-ranges-4.stderr b/src/test/ui/parser/pat-ranges-4.stderr
index 0a1d7a1f6b8ab..53e38bc670beb 100644
--- a/src/test/ui/parser/pat-ranges-4.stderr
+++ b/src/test/ui/parser/pat-ranges-4.stderr
@@ -1,8 +1,8 @@
-error: expected one of `...`, `..=`, `..`, `:`, `;`, or `=`, found `-`
+error: expected one of `...`, `..=`, `..`, `:`, `;`, `=`, or `|`, found `-`
   --> $DIR/pat-ranges-4.rs:4:12
    |
 LL |     let 10 - 3 ..= 10 = 8;
-   |            ^ expected one of `...`, `..=`, `..`, `:`, `;`, or `=` here
+   |            ^ expected one of 7 possible tokens here
 
 error: aborting due to previous error
 

From 5f6bec8ecf5a9018bb368becc0aaea703334795a Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 22:57:34 +0200
Subject: [PATCH 18/34] parser: drive-by: simplify `parse_arg_general`.

---
 src/libsyntax/parse/parser.rs | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 89725d8b3395c..002e9bccec72a 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -971,15 +971,12 @@ impl<'a> Parser<'a> {
     /// Skips unexpected attributes and doc comments in this position and emits an appropriate
     /// error.
     /// This version of parse arg doesn't necessarily require identifier names.
-    fn parse_arg_general<F>(
+    fn parse_arg_general(
         &mut self,
         is_trait_item: bool,
         allow_c_variadic: bool,
-        is_name_required: F,
-    ) -> PResult<'a, Arg>
-    where
-        F: Fn(&token::Token) -> bool
-    {
+        is_name_required: impl Fn(&token::Token) -> bool,
+    ) -> PResult<'a, Arg> {
         let lo = self.token.span;
         let attrs = self.parse_arg_attributes()?;
         if let Some(mut arg) = self.parse_self_arg()? {

From a9af18bed544ede37b6b817cca1d4613aad4696f Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 23:07:21 +0200
Subject: [PATCH 19/34] move `ui/or-pattern-mismatch` -> `ui/or-patterns/`.

---
 src/test/ui/{ => or-patterns}/or-pattern-mismatch.rs     | 0
 src/test/ui/{ => or-patterns}/or-pattern-mismatch.stderr | 0
 2 files changed, 0 insertions(+), 0 deletions(-)
 rename src/test/ui/{ => or-patterns}/or-pattern-mismatch.rs (100%)
 rename src/test/ui/{ => or-patterns}/or-pattern-mismatch.stderr (100%)

diff --git a/src/test/ui/or-pattern-mismatch.rs b/src/test/ui/or-patterns/or-pattern-mismatch.rs
similarity index 100%
rename from src/test/ui/or-pattern-mismatch.rs
rename to src/test/ui/or-patterns/or-pattern-mismatch.rs
diff --git a/src/test/ui/or-pattern-mismatch.stderr b/src/test/ui/or-patterns/or-pattern-mismatch.stderr
similarity index 100%
rename from src/test/ui/or-pattern-mismatch.stderr
rename to src/test/ui/or-patterns/or-pattern-mismatch.stderr

From f35432e1889a4361388adf514634b122aefa746b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 23:53:08 +0200
Subject: [PATCH 20/34] or-patterns: add syntactic tests.

---
 .../or-patterns/or-patterns-syntactic-fail.rs | 56 +++++++++++++
 .../or-patterns-syntactic-fail.stderr         | 75 ++++++++++++++++++
 .../or-patterns/or-patterns-syntactic-pass.rs | 78 +++++++++++++++++++
 .../or-patterns-syntactic-pass.stderr         |  8 ++
 4 files changed, 217 insertions(+)
 create mode 100644 src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
 create mode 100644 src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
 create mode 100644 src/test/ui/or-patterns/or-patterns-syntactic-pass.rs
 create mode 100644 src/test/ui/or-patterns/or-patterns-syntactic-pass.stderr

diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs b/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
new file mode 100644
index 0000000000000..43c9214bd98f7
--- /dev/null
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
@@ -0,0 +1,56 @@
+// Test some cases where or-patterns may ostensibly be allowed but are in fact not.
+// This is not a semantic test. We only test parsing.
+
+#![feature(or_patterns)]
+//~^ WARN the feature `or_patterns` is incomplete and may cause the compiler to crash
+
+fn main() {}
+
+// Test the `pat` macro fragment parser:
+macro_rules! accept_pat {
+    ($p:pat) => {}
+}
+
+accept_pat!(p | q); //~ ERROR no rules expected the token `|`
+accept_pat!(| p | q); //~ ERROR no rules expected the token `|`
+
+// Non-macro tests:
+
+enum E { A, B }
+use E::*;
+
+fn no_top_level_or_patterns() {
+    // We do *not* allow or-patterns at the top level of lambdas...
+    let _ = |A | B: E| (); //~ ERROR binary operation `|` cannot be applied to type `E`
+    //           -------- This looks like an or-pattern but is in fact `|A| (B: E | ())`.
+
+    // ...and for now neither do we allow or-patterns at the top level of functions.
+    fn fun(A | B: E) {} //~ ERROR expected one of `:` or `@`, found `|`
+}
+
+// We also do not allow a leading `|` when not in a top level position:
+
+#[cfg(FALSE)]
+fn no_leading_parens() {
+    let ( | A | B); //~ ERROR expected pattern, found `|`
+}
+
+#[cfg(FALSE)]
+fn no_leading_tuple() {
+    let ( | A | B,); //~ ERROR expected pattern, found `|`
+}
+
+#[cfg(FALSE)]
+fn no_leading_slice() {
+    let [ | A | B ]; //~ ERROR expected pattern, found `|`
+}
+
+#[cfg(FALSE)]
+fn no_leading_tuple_struct() {
+    let TS( | A | B ); //~ ERROR expected pattern, found `|`
+}
+
+#[cfg(FALSE)]
+fn no_leading_struct() {
+    let NS { f: | A | B }; //~ ERROR expected pattern, found `|`
+}
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr b/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
new file mode 100644
index 0000000000000..809ff272f6204
--- /dev/null
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
@@ -0,0 +1,75 @@
+error: expected one of `:` or `@`, found `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:28:14
+   |
+LL |     fn fun(A | B: E) {}
+   |              ^ expected one of `:` or `@` here
+
+error: expected pattern, found `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:35:11
+   |
+LL |     let ( | A | B);
+   |           ^ expected pattern
+
+error: expected pattern, found `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:40:11
+   |
+LL |     let ( | A | B,);
+   |           ^ expected pattern
+
+error: expected pattern, found `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:45:11
+   |
+LL |     let [ | A | B ];
+   |           ^ expected pattern
+
+error: expected pattern, found `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:50:13
+   |
+LL |     let TS( | A | B );
+   |             ^ expected pattern
+
+error: expected pattern, found `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:55:17
+   |
+LL |     let NS { f: | A | B };
+   |                 ^ expected pattern
+
+error: no rules expected the token `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:14:15
+   |
+LL | macro_rules! accept_pat {
+   | ----------------------- when calling this macro
+...
+LL | accept_pat!(p | q);
+   |               ^ no rules expected this token in macro call
+
+error: no rules expected the token `|`
+  --> $DIR/or-patterns-syntactic-fail.rs:15:13
+   |
+LL | macro_rules! accept_pat {
+   | ----------------------- when calling this macro
+...
+LL | accept_pat!(| p | q);
+   |             ^ no rules expected this token in macro call
+
+warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
+  --> $DIR/or-patterns-syntactic-fail.rs:4:12
+   |
+LL | #![feature(or_patterns)]
+   |            ^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0369]: binary operation `|` cannot be applied to type `E`
+  --> $DIR/or-patterns-syntactic-fail.rs:24:22
+   |
+LL |     let _ = |A | B: E| ();
+   |                  ----^ -- ()
+   |                  |
+   |                  E
+   |
+   = note: an implementation of `std::ops::BitOr` might be missing for `E`
+
+error: aborting due to 9 previous errors
+
+For more information about this error, try `rustc --explain E0369`.
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-pass.rs b/src/test/ui/or-patterns/or-patterns-syntactic-pass.rs
new file mode 100644
index 0000000000000..5fe72caf9c1ff
--- /dev/null
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-pass.rs
@@ -0,0 +1,78 @@
+// Here we test all the places `|` is *syntactically* allowed.
+// This is not a semantic test. We only test parsing.
+
+// check-pass
+
+#![feature(or_patterns)]
+
+fn main() {}
+
+// Test the `pat` macro fragment parser:
+macro_rules! accept_pat {
+    ($p:pat) => {}
+}
+
+accept_pat!((p | q));
+accept_pat!((p | q,));
+accept_pat!(TS(p | q));
+accept_pat!(NS { f: p | q });
+accept_pat!([p | q]);
+
+// Non-macro tests:
+
+#[cfg(FALSE)]
+fn or_patterns() {
+    // Top level of `let`:
+    let | A | B;
+    let A | B;
+    let A | B: u8;
+    let A | B = 0;
+    let A | B: u8 = 0;
+
+    // Top level of `for`:
+    for | A | B in 0 {}
+    for A | B in 0 {}
+
+    // Top level of `while`:
+    while let | A | B = 0 {}
+    while let A | B = 0 {}
+
+    // Top level of `if`:
+    if let | A | B = 0 {}
+    if let A | B = 0 {}
+
+    // Top level of `match` arms:
+    match 0 {
+        | A | B => {},
+        A | B => {},
+    }
+
+    // Functions:
+    fn fun((A | B): _) {}
+
+    // Lambdas:
+    let _ = |(A | B): u8| ();
+
+    // Parenthesis and tuple patterns:
+    let (A | B);
+    let (A | B,);
+
+    // Tuple struct patterns:
+    let A(B | C);
+    let E::V(B | C);
+
+    // Struct patterns:
+    let S { f1: B | C, f2 };
+    let E::V { f1: B | C, f2 };
+
+    // Slice patterns:
+    let [A | B, .. | ..];
+
+    // These bind as `(prefix p) | q` as opposed to `prefix (p | q)`:
+    let box 0 | 1; // Unstable; we *can* the precedence if we want.
+    let &0 | 1;
+    let &mut 0 | 1;
+    let x @ 0 | 1;
+    let ref x @ 0 | 1;
+    let ref mut x @ 0 | 1;
+}
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-pass.stderr b/src/test/ui/or-patterns/or-patterns-syntactic-pass.stderr
new file mode 100644
index 0000000000000..3145a2e9f2a6e
--- /dev/null
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-pass.stderr
@@ -0,0 +1,8 @@
+warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
+  --> $DIR/or-patterns-syntactic-pass.rs:6:12
+   |
+LL | #![feature(or_patterns)]
+   |            ^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+

From d3b3bceffb1f0fa7727c4d63aa37ecb9444e88c5 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 18 Aug 2019 23:57:43 +0200
Subject: [PATCH 21/34] move `feature-gate-or_patterns.*` -> `ui/or-patterns/`

---
 .../ui/{feature-gate => or-patterns}/feature-gate-or_patterns.rs  | 0
 .../{feature-gate => or-patterns}/feature-gate-or_patterns.stderr | 0
 2 files changed, 0 insertions(+), 0 deletions(-)
 rename src/test/ui/{feature-gate => or-patterns}/feature-gate-or_patterns.rs (100%)
 rename src/test/ui/{feature-gate => or-patterns}/feature-gate-or_patterns.stderr (100%)

diff --git a/src/test/ui/feature-gate/feature-gate-or_patterns.rs b/src/test/ui/or-patterns/feature-gate-or_patterns.rs
similarity index 100%
rename from src/test/ui/feature-gate/feature-gate-or_patterns.rs
rename to src/test/ui/or-patterns/feature-gate-or_patterns.rs
diff --git a/src/test/ui/feature-gate/feature-gate-or_patterns.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
similarity index 100%
rename from src/test/ui/feature-gate/feature-gate-or_patterns.stderr
rename to src/test/ui/or-patterns/feature-gate-or_patterns.stderr

From 1ffea18ddbe9ebaba4ff301a3c42e44a55741355 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 19 Aug 2019 00:19:40 +0200
Subject: [PATCH 22/34] or-patterns: harden feature gating tests.

---
 .../or-patterns/feature-gate-or_patterns.rs   |  47 ++++-
 .../feature-gate-or_patterns.stderr           | 182 +++++++++++++++++-
 2 files changed, 227 insertions(+), 2 deletions(-)

diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns.rs b/src/test/ui/or-patterns/feature-gate-or_patterns.rs
index 036a6095965bd..560db20e66862 100644
--- a/src/test/ui/or-patterns/feature-gate-or_patterns.rs
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns.rs
@@ -1,4 +1,4 @@
-#![crate_type="lib"]
+fn main() {}
 
 pub fn example(x: Option<usize>) {
     match x {
@@ -7,3 +7,48 @@ pub fn example(x: Option<usize>) {
         _ => {}
     }
 }
+
+// Test the `pat` macro fragment parser:
+macro_rules! accept_pat {
+    ($p:pat) => {}
+}
+
+accept_pat!((p | q)); //~ ERROR or-patterns syntax is experimental
+accept_pat!((p | q,)); //~ ERROR or-patterns syntax is experimental
+accept_pat!(TS(p | q)); //~ ERROR or-patterns syntax is experimental
+accept_pat!(NS { f: p | q }); //~ ERROR or-patterns syntax is experimental
+accept_pat!([p | q]); //~ ERROR or-patterns syntax is experimental
+
+// Non-macro tests:
+
+#[cfg(FALSE)]
+fn or_patterns() {
+    // Gated:
+
+    let | A | B; //~ ERROR or-patterns syntax is experimental
+    //~^ ERROR or-patterns syntax is experimental
+    let A | B; //~ ERROR or-patterns syntax is experimental
+    for | A | B in 0 {} //~ ERROR or-patterns syntax is experimental
+    //~^ ERROR or-patterns syntax is experimental
+    for A | B in 0 {} //~ ERROR or-patterns syntax is experimental
+    fn fun((A | B): _) {} //~ ERROR or-patterns syntax is experimental
+    let _ = |(A | B): u8| (); //~ ERROR or-patterns syntax is experimental
+    let (A | B); //~ ERROR or-patterns syntax is experimental
+    let (A | B,); //~ ERROR or-patterns syntax is experimental
+    let A(B | C); //~ ERROR or-patterns syntax is experimental
+    let E::V(B | C); //~ ERROR or-patterns syntax is experimental
+    let S { f1: B | C, f2 }; //~ ERROR or-patterns syntax is experimental
+    let E::V { f1: B | C, f2 }; //~ ERROR or-patterns syntax is experimental
+    let [A | B]; //~ ERROR or-patterns syntax is experimental
+
+    // Top level of `while`, `if`, and `match` arms are allowed:
+
+    while let | A = 0 {}
+    while let A | B = 0 {}
+    if let | A = 0 {}
+    if let A | B = 0 {}
+    match 0 {
+        | A => {},
+        A | B => {},
+    }
+}
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
index aaabb54c1f017..e2abfbfd5254d 100644
--- a/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
@@ -7,6 +7,186 @@ LL |         Some(0 | 1 | 2) => {}
    = note: for more information, see https://github.com/rust-lang/rust/issues/54883
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
-error: aborting due to previous error
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:28:9
+   |
+LL |     let | A | B;
+   |         ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:28:11
+   |
+LL |     let | A | B;
+   |           ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:30:9
+   |
+LL |     let A | B;
+   |         ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:31:9
+   |
+LL |     for | A | B in 0 {}
+   |         ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:31:11
+   |
+LL |     for | A | B in 0 {}
+   |           ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:33:9
+   |
+LL |     for A | B in 0 {}
+   |         ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:34:13
+   |
+LL |     fn fun((A | B): _) {}
+   |             ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:35:15
+   |
+LL |     let _ = |(A | B): u8| ();
+   |               ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:36:10
+   |
+LL |     let (A | B);
+   |          ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:37:10
+   |
+LL |     let (A | B,);
+   |          ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:38:11
+   |
+LL |     let A(B | C);
+   |           ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:39:14
+   |
+LL |     let E::V(B | C);
+   |              ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:40:17
+   |
+LL |     let S { f1: B | C, f2 };
+   |                 ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:41:20
+   |
+LL |     let E::V { f1: B | C, f2 };
+   |                    ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:42:10
+   |
+LL |     let [A | B];
+   |          ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:16:14
+   |
+LL | accept_pat!((p | q));
+   |              ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:17:14
+   |
+LL | accept_pat!((p | q,));
+   |              ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:18:16
+   |
+LL | accept_pat!(TS(p | q));
+   |                ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:19:21
+   |
+LL | accept_pat!(NS { f: p | q });
+   |                     ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns.rs:20:14
+   |
+LL | accept_pat!([p | q]);
+   |              ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error: aborting due to 21 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

From b205055c7bc92c0f873755996e6fac3e694c7e72 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 19 Aug 2019 02:40:24 +0200
Subject: [PATCH 23/34] parser: better recovery for || in inner pats.

---
 src/libsyntax/parse/parser/pat.rs             | 27 ++++++-
 .../or-patterns/or-patterns-syntactic-fail.rs | 39 ++++------
 .../or-patterns-syntactic-fail.stderr         | 77 ++++++++++++++-----
 3 files changed, 96 insertions(+), 47 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index e77d9120bce78..b9871be229ce5 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -155,6 +155,25 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
+    /// Recursive possibly-or-pattern parser with recovery for an erroneous leading `|`.
+    /// See `parse_pat_with_or` for details on parsing or-patterns.
+    fn parse_pat_with_or_inner(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
+        // Recover if `|` or `||` is here.
+        // The user is thinking that a leading `|` is allowed in this position.
+        if let token::BinOp(token::Or) | token::OrOr = self.token.kind {
+            let span = self.token.span;
+            let rm_msg = format!("remove the `{}`", pprust::token_to_string(&self.token));
+
+            self.struct_span_err(span, "a leading `|` is only allowed in a top-level pattern")
+                .span_suggestion(span, &rm_msg, String::new(), Applicability::MachineApplicable)
+                .emit();
+
+            self.bump();
+        }
+
+        self.parse_pat_with_or(expected, true, false)
+    }
+
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
     /// allowed).
     fn parse_pat_with_range_pat(
@@ -173,7 +192,7 @@ impl<'a> Parser<'a> {
                 // Parse `[pat, pat,...]` as a slice pattern.
                 let (pats, _) = self.parse_delim_comma_seq(
                     token::Bracket,
-                    |p| p.parse_pat_with_or(None, true, false),
+                    |p| p.parse_pat_with_or_inner(None),
                 )?;
                 PatKind::Slice(pats)
             }
@@ -303,7 +322,7 @@ impl<'a> Parser<'a> {
     /// Parse a tuple or parenthesis pattern.
     fn parse_pat_tuple_or_parens(&mut self) -> PResult<'a, PatKind> {
         let (fields, trailing_comma) = self.parse_paren_comma_seq(|p| {
-            p.parse_pat_with_or(None, true, false)
+            p.parse_pat_with_or_inner(None)
         })?;
 
         // Here, `(pat,)` is a tuple pattern.
@@ -547,7 +566,7 @@ impl<'a> Parser<'a> {
             err.span_label(self.token.span, msg);
             return Err(err);
         }
-        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or(None, true, false))?;
+        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or_inner(None))?;
         Ok(PatKind::TupleStruct(path, fields))
     }
 
@@ -691,7 +710,7 @@ impl<'a> Parser<'a> {
             // Parsing a pattern of the form "fieldname: pat"
             let fieldname = self.parse_field_name()?;
             self.bump();
-            let pat = self.parse_pat_with_or(None, true, false)?;
+            let pat = self.parse_pat_with_or_inner(None)?;
             hi = pat.span;
             (pat, fieldname, false)
         } else {
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs b/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
index 43c9214bd98f7..7959812f5b39c 100644
--- a/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
@@ -30,27 +30,20 @@ fn no_top_level_or_patterns() {
 
 // We also do not allow a leading `|` when not in a top level position:
 
-#[cfg(FALSE)]
-fn no_leading_parens() {
-    let ( | A | B); //~ ERROR expected pattern, found `|`
-}
-
-#[cfg(FALSE)]
-fn no_leading_tuple() {
-    let ( | A | B,); //~ ERROR expected pattern, found `|`
-}
-
-#[cfg(FALSE)]
-fn no_leading_slice() {
-    let [ | A | B ]; //~ ERROR expected pattern, found `|`
-}
-
-#[cfg(FALSE)]
-fn no_leading_tuple_struct() {
-    let TS( | A | B ); //~ ERROR expected pattern, found `|`
-}
-
-#[cfg(FALSE)]
-fn no_leading_struct() {
-    let NS { f: | A | B }; //~ ERROR expected pattern, found `|`
+fn no_leading_inner() {
+    struct TS(E);
+    struct NS { f: E }
+
+    let ( | A | B) = E::A; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let ( | A | B,) = (E::B,); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let [ | A | B ] = [E::A]; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let TS( | A | B ); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let NS { f: | A | B }; //~ ERROR a leading `|` is only allowed in a top-level pattern
+
+    let ( || A | B) = E::A; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let [ || A | B ] = [E::A]; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let TS( || A | B ); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let NS { f: || A | B }; //~ ERROR a leading `|` is only allowed in a top-level pattern
+
+    let recovery_witness: String = 0; //~ ERROR mismatched types
 }
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr b/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
index 809ff272f6204..dd4c309ce85a3 100644
--- a/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
@@ -4,35 +4,59 @@ error: expected one of `:` or `@`, found `|`
 LL |     fn fun(A | B: E) {}
    |              ^ expected one of `:` or `@` here
 
-error: expected pattern, found `|`
-  --> $DIR/or-patterns-syntactic-fail.rs:35:11
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:37:11
    |
-LL |     let ( | A | B);
-   |           ^ expected pattern
+LL |     let ( | A | B) = E::A;
+   |           ^ help: remove the `|`
 
-error: expected pattern, found `|`
-  --> $DIR/or-patterns-syntactic-fail.rs:40:11
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:38:11
    |
-LL |     let ( | A | B,);
-   |           ^ expected pattern
+LL |     let ( | A | B,) = (E::B,);
+   |           ^ help: remove the `|`
 
-error: expected pattern, found `|`
-  --> $DIR/or-patterns-syntactic-fail.rs:45:11
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:39:11
    |
-LL |     let [ | A | B ];
-   |           ^ expected pattern
+LL |     let [ | A | B ] = [E::A];
+   |           ^ help: remove the `|`
 
-error: expected pattern, found `|`
-  --> $DIR/or-patterns-syntactic-fail.rs:50:13
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:40:13
    |
 LL |     let TS( | A | B );
-   |             ^ expected pattern
+   |             ^ help: remove the `|`
 
-error: expected pattern, found `|`
-  --> $DIR/or-patterns-syntactic-fail.rs:55:17
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:41:17
    |
 LL |     let NS { f: | A | B };
-   |                 ^ expected pattern
+   |                 ^ help: remove the `|`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:43:11
+   |
+LL |     let ( || A | B) = E::A;
+   |           ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:44:11
+   |
+LL |     let [ || A | B ] = [E::A];
+   |           ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:45:13
+   |
+LL |     let TS( || A | B );
+   |             ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:46:17
+   |
+LL |     let NS { f: || A | B };
+   |                 ^^ help: remove the `||`
 
 error: no rules expected the token `|`
   --> $DIR/or-patterns-syntactic-fail.rs:14:15
@@ -70,6 +94,19 @@ LL |     let _ = |A | B: E| ();
    |
    = note: an implementation of `std::ops::BitOr` might be missing for `E`
 
-error: aborting due to 9 previous errors
+error[E0308]: mismatched types
+  --> $DIR/or-patterns-syntactic-fail.rs:48:36
+   |
+LL |     let recovery_witness: String = 0;
+   |                                    ^
+   |                                    |
+   |                                    expected struct `std::string::String`, found integer
+   |                                    help: try using a conversion method: `0.to_string()`
+   |
+   = note: expected type `std::string::String`
+              found type `{integer}`
+
+error: aborting due to 14 previous errors
 
-For more information about this error, try `rustc --explain E0369`.
+Some errors have detailed explanations: E0308, E0369.
+For more information about an error, try `rustc --explain E0308`.

From b2966e651de3bf83ab9c712a1afaeba84162cab1 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 24 Aug 2019 21:43:28 +0200
Subject: [PATCH 24/34] parser: bool -> GateOr.

---
 src/libsyntax/parse/parser/expr.rs |  7 ++++---
 src/libsyntax/parse/parser/pat.rs  | 16 ++++++++++------
 src/libsyntax/parse/parser/stmt.rs |  3 ++-
 3 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/src/libsyntax/parse/parser/expr.rs b/src/libsyntax/parse/parser/expr.rs
index 25a858b47353f..83e5a84a8c6ae 100644
--- a/src/libsyntax/parse/parser/expr.rs
+++ b/src/libsyntax/parse/parser/expr.rs
@@ -1,6 +1,7 @@
 use super::{Parser, PResult, Restrictions, PrevTokenKind, TokenType, PathStyle};
 use super::{BlockMode, SemiColonMode};
 use super::{SeqSep, TokenExpectType};
+use super::pat::GateOr;
 
 use crate::maybe_recover_from_interpolated_ty_qpath;
 use crate::ptr::P;
@@ -1246,7 +1247,7 @@ impl<'a> Parser<'a> {
     fn parse_let_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
         // FIXME(or_patterns, Centril | dlrobertson): use `parse_top_pat` instead.
-        let pat = self.parse_top_pat_unpack(false)?;
+        let pat = self.parse_top_pat_unpack(GateOr::No)?;
         self.expect(&token::Eq)?;
         let expr = self.with_res(
             Restrictions::NO_STRUCT_LITERAL,
@@ -1284,7 +1285,7 @@ impl<'a> Parser<'a> {
             _ => None,
         };
 
-        let pat = self.parse_top_pat(true)?;
+        let pat = self.parse_top_pat(GateOr::Yes)?;
         if !self.eat_keyword(kw::In) {
             let in_span = self.prev_span.between(self.token.span);
             self.struct_span_err(in_span, "missing `in` in `for` loop")
@@ -1389,7 +1390,7 @@ impl<'a> Parser<'a> {
         let attrs = self.parse_outer_attributes()?;
         let lo = self.token.span;
         // FIXME(or_patterns, Centril | dlrobertson): use `parse_top_pat` instead.
-        let pat = self.parse_top_pat_unpack(false)?;
+        let pat = self.parse_top_pat_unpack(GateOr::No)?;
         let guard = if self.eat_keyword(kw::If) {
             Some(self.parse_expr()?)
         } else {
diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index b9871be229ce5..3d89ec56ffafa 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -14,6 +14,10 @@ use errors::{Applicability, DiagnosticBuilder};
 
 type Expected = Option<&'static str>;
 
+/// Whether or not an or-pattern should be gated when occurring in the current context.
+#[derive(PartialEq)]
+pub enum GateOr { Yes, No }
+
 impl<'a> Parser<'a> {
     /// Parses a pattern.
     ///
@@ -26,7 +30,7 @@ impl<'a> Parser<'a> {
 
     // FIXME(or_patterns, Centril | dlrobertson):
     // remove this and use `parse_top_pat` everywhere it is used instead.
-    pub(super) fn parse_top_pat_unpack(&mut self, gate_or: bool) -> PResult<'a, Vec<P<Pat>>> {
+    pub(super) fn parse_top_pat_unpack(&mut self, gate_or: GateOr) -> PResult<'a, Vec<P<Pat>>> {
         self.parse_top_pat(gate_or)
             .map(|pat| pat.and_then(|pat| match pat.node {
                 PatKind::Or(pats) => pats,
@@ -36,9 +40,9 @@ impl<'a> Parser<'a> {
 
     /// Entry point to the main pattern parser.
     /// Corresponds to `top_pat` in RFC 2535 and allows or-pattern at the top level.
-    pub(super) fn parse_top_pat(&mut self, gate_or: bool) -> PResult<'a, P<Pat>> {
+    pub(super) fn parse_top_pat(&mut self, gate_or: GateOr) -> PResult<'a, P<Pat>> {
         // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
-        if self.eat_or_separator() && gate_or {
+        if self.eat_or_separator() && gate_or == GateOr::Yes {
             self.sess.gated_spans.or_patterns.borrow_mut().push(self.prev_span);
         }
 
@@ -50,7 +54,7 @@ impl<'a> Parser<'a> {
     fn parse_pat_with_or(
         &mut self,
         expected: Expected,
-        gate_or: bool,
+        gate_or: GateOr,
         top_level: bool
     ) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
@@ -73,7 +77,7 @@ impl<'a> Parser<'a> {
         let or_pattern_span = lo.to(self.prev_span);
 
         // Feature gate the or-pattern if instructed:
-        if gate_or {
+        if gate_or == GateOr::Yes {
             self.sess.gated_spans.or_patterns.borrow_mut().push(or_pattern_span);
         }
 
@@ -171,7 +175,7 @@ impl<'a> Parser<'a> {
             self.bump();
         }
 
-        self.parse_pat_with_or(expected, true, false)
+        self.parse_pat_with_or(expected, GateOr::Yes, false)
     }
 
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
diff --git a/src/libsyntax/parse/parser/stmt.rs b/src/libsyntax/parse/parser/stmt.rs
index 3c8cb4ea5a582..651ebf6342e71 100644
--- a/src/libsyntax/parse/parser/stmt.rs
+++ b/src/libsyntax/parse/parser/stmt.rs
@@ -1,6 +1,7 @@
 use super::{Parser, PResult, Restrictions, PrevTokenKind, SemiColonMode, BlockMode};
 use super::expr::LhsExpr;
 use super::path::PathStyle;
+use super::pat::GateOr;
 
 use crate::ptr::P;
 use crate::{maybe_whole, ThinVec};
@@ -207,7 +208,7 @@ impl<'a> Parser<'a> {
     /// Parses a local variable declaration.
     fn parse_local(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Local>> {
         let lo = self.prev_span;
-        let pat = self.parse_top_pat(true)?;
+        let pat = self.parse_top_pat(GateOr::Yes)?;
 
         let (err, ty) = if self.eat(&token::Colon) {
             // Save the state of the parser before parsing type normally, in case there is a `:`

From a9ef8592e47808539ffd9237c22ce5518aa7b188 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 24 Aug 2019 22:12:19 +0200
Subject: [PATCH 25/34] parser: bool -> TopLevel.

---
 src/libsyntax/parse/parser/pat.rs | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 3d89ec56ffafa..c168d03378133 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -18,6 +18,10 @@ type Expected = Option<&'static str>;
 #[derive(PartialEq)]
 pub enum GateOr { Yes, No }
 
+/// Whether or not this is the top level pattern context.
+#[derive(PartialEq, Copy, Clone)]
+enum TopLevel { Yes, No }
+
 impl<'a> Parser<'a> {
     /// Parses a pattern.
     ///
@@ -46,7 +50,7 @@ impl<'a> Parser<'a> {
             self.sess.gated_spans.or_patterns.borrow_mut().push(self.prev_span);
         }
 
-        self.parse_pat_with_or(None, gate_or, true)
+        self.parse_pat_with_or(None, gate_or, TopLevel::Yes)
     }
 
     /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
@@ -55,7 +59,7 @@ impl<'a> Parser<'a> {
         &mut self,
         expected: Expected,
         gate_or: GateOr,
-        top_level: bool
+        top_level: TopLevel,
     ) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
         let first_pat = self.parse_pat(expected)?;
@@ -112,8 +116,8 @@ impl<'a> Parser<'a> {
 
     /// Some special error handling for the "top-level" patterns in a match arm,
     /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
-    fn maybe_recover_unexpected_comma(&mut self, lo: Span, top_level: bool) -> PResult<'a, ()> {
-        if !top_level || self.token != token::Comma {
+    fn maybe_recover_unexpected_comma(&mut self, lo: Span, top_level: TopLevel) -> PResult<'a, ()> {
+        if top_level == TopLevel::No || self.token != token::Comma {
             return Ok(());
         }
 
@@ -175,7 +179,7 @@ impl<'a> Parser<'a> {
             self.bump();
         }
 
-        self.parse_pat_with_or(expected, GateOr::Yes, false)
+        self.parse_pat_with_or(expected, GateOr::Yes, TopLevel::No)
     }
 
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are

From 3a405421e7c1437416e225ea8d2f0fdfb501df7b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 24 Aug 2019 22:46:17 +0200
Subject: [PATCH 26/34] parse_top_pat: silence leading vert gating sometimes.

---
 src/libsyntax/parse/parser/pat.rs             | 18 ++++++--
 .../feature-gate-or_patterns-leading-for.rs   |  8 ++++
 ...eature-gate-or_patterns-leading-for.stderr | 12 +++++
 .../feature-gate-or_patterns-leading-let.rs   |  8 ++++
 ...eature-gate-or_patterns-leading-let.stderr | 12 +++++
 .../feature-gate-or_patterns-leading.stderr   | 12 +++++
 .../or-patterns/feature-gate-or_patterns.rs   |  2 -
 .../feature-gate-or_patterns.stderr           | 44 ++++++-------------
 8 files changed, 80 insertions(+), 36 deletions(-)
 create mode 100644 src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.rs
 create mode 100644 src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr
 create mode 100644 src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.rs
 create mode 100644 src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr
 create mode 100644 src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index c168d03378133..724edbcfaed79 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -46,11 +46,23 @@ impl<'a> Parser<'a> {
     /// Corresponds to `top_pat` in RFC 2535 and allows or-pattern at the top level.
     pub(super) fn parse_top_pat(&mut self, gate_or: GateOr) -> PResult<'a, P<Pat>> {
         // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
-        if self.eat_or_separator() && gate_or == GateOr::Yes {
-            self.sess.gated_spans.or_patterns.borrow_mut().push(self.prev_span);
+        let gated_leading_vert = self.eat_or_separator() && gate_or == GateOr::Yes;
+
+        // Parse the possibly-or-pattern.
+        let pat = self.parse_pat_with_or(None, gate_or, TopLevel::Yes)?;
+
+        // If we parsed a leading `|` which should be gated,
+        // and no other gated or-pattern has been parsed thus far,
+        // then we should really gate the leading `|`.
+        // This complicated procedure is done purely for diagnostics UX.
+        if gated_leading_vert {
+            let mut or_pattern_spans = self.sess.gated_spans.or_patterns.borrow_mut();
+            if or_pattern_spans.is_empty() {
+                or_pattern_spans.push(self.prev_span);
+            }
         }
 
-        self.parse_pat_with_or(None, gate_or, TopLevel::Yes)
+        Ok(pat)
     }
 
     /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.rs b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.rs
new file mode 100644
index 0000000000000..de8e1bba5576c
--- /dev/null
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.rs
@@ -0,0 +1,8 @@
+// Test feature gating for a sole leading `|` in `let`.
+
+fn main() {}
+
+#[cfg(FALSE)]
+fn gated_leading_vert_in_let() {
+    for | A in 0 {} //~ ERROR or-patterns syntax is experimental
+}
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr
new file mode 100644
index 0000000000000..83804d564f3e4
--- /dev/null
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr
@@ -0,0 +1,12 @@
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns-leading-for.rs:7:11
+   |
+LL |     for | A in 0 {}
+   |           ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.rs b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.rs
new file mode 100644
index 0000000000000..a4ea4e25d861e
--- /dev/null
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.rs
@@ -0,0 +1,8 @@
+// Test feature gating for a sole leading `|` in `let`.
+
+fn main() {}
+
+#[cfg(FALSE)]
+fn gated_leading_vert_in_let() {
+    let | A; //~ ERROR or-patterns syntax is experimental
+}
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr
new file mode 100644
index 0000000000000..f7954ad7a8ce1
--- /dev/null
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr
@@ -0,0 +1,12 @@
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns-leading-let.rs:7:11
+   |
+LL |     let | A;
+   |           ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr
new file mode 100644
index 0000000000000..8b18082fca7df
--- /dev/null
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr
@@ -0,0 +1,12 @@
+error[E0658]: or-patterns syntax is experimental
+  --> $DIR/feature-gate-or_patterns-leading.rs:7:11
+   |
+LL |     let | A;
+   |           ^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
+   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns.rs b/src/test/ui/or-patterns/feature-gate-or_patterns.rs
index 560db20e66862..e638838147a4d 100644
--- a/src/test/ui/or-patterns/feature-gate-or_patterns.rs
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns.rs
@@ -26,10 +26,8 @@ fn or_patterns() {
     // Gated:
 
     let | A | B; //~ ERROR or-patterns syntax is experimental
-    //~^ ERROR or-patterns syntax is experimental
     let A | B; //~ ERROR or-patterns syntax is experimental
     for | A | B in 0 {} //~ ERROR or-patterns syntax is experimental
-    //~^ ERROR or-patterns syntax is experimental
     for A | B in 0 {} //~ ERROR or-patterns syntax is experimental
     fn fun((A | B): _) {} //~ ERROR or-patterns syntax is experimental
     let _ = |(A | B): u8| (); //~ ERROR or-patterns syntax is experimental
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
index e2abfbfd5254d..aae6644dac2e0 100644
--- a/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns.stderr
@@ -7,15 +7,6 @@ LL |         Some(0 | 1 | 2) => {}
    = note: for more information, see https://github.com/rust-lang/rust/issues/54883
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
-error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:28:9
-   |
-LL |     let | A | B;
-   |         ^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
-   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
-
 error[E0658]: or-patterns syntax is experimental
   --> $DIR/feature-gate-or_patterns.rs:28:11
    |
@@ -26,7 +17,7 @@ LL |     let | A | B;
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:30:9
+  --> $DIR/feature-gate-or_patterns.rs:29:9
    |
 LL |     let A | B;
    |         ^^^^^
@@ -35,16 +26,7 @@ LL |     let A | B;
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:31:9
-   |
-LL |     for | A | B in 0 {}
-   |         ^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
-   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
-
-error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:31:11
+  --> $DIR/feature-gate-or_patterns.rs:30:11
    |
 LL |     for | A | B in 0 {}
    |           ^^^^^
@@ -53,7 +35,7 @@ LL |     for | A | B in 0 {}
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:33:9
+  --> $DIR/feature-gate-or_patterns.rs:31:9
    |
 LL |     for A | B in 0 {}
    |         ^^^^^
@@ -62,7 +44,7 @@ LL |     for A | B in 0 {}
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:34:13
+  --> $DIR/feature-gate-or_patterns.rs:32:13
    |
 LL |     fn fun((A | B): _) {}
    |             ^^^^^
@@ -71,7 +53,7 @@ LL |     fn fun((A | B): _) {}
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:35:15
+  --> $DIR/feature-gate-or_patterns.rs:33:15
    |
 LL |     let _ = |(A | B): u8| ();
    |               ^^^^^
@@ -80,7 +62,7 @@ LL |     let _ = |(A | B): u8| ();
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:36:10
+  --> $DIR/feature-gate-or_patterns.rs:34:10
    |
 LL |     let (A | B);
    |          ^^^^^
@@ -89,7 +71,7 @@ LL |     let (A | B);
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:37:10
+  --> $DIR/feature-gate-or_patterns.rs:35:10
    |
 LL |     let (A | B,);
    |          ^^^^^
@@ -98,7 +80,7 @@ LL |     let (A | B,);
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:38:11
+  --> $DIR/feature-gate-or_patterns.rs:36:11
    |
 LL |     let A(B | C);
    |           ^^^^^
@@ -107,7 +89,7 @@ LL |     let A(B | C);
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:39:14
+  --> $DIR/feature-gate-or_patterns.rs:37:14
    |
 LL |     let E::V(B | C);
    |              ^^^^^
@@ -116,7 +98,7 @@ LL |     let E::V(B | C);
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:40:17
+  --> $DIR/feature-gate-or_patterns.rs:38:17
    |
 LL |     let S { f1: B | C, f2 };
    |                 ^^^^^
@@ -125,7 +107,7 @@ LL |     let S { f1: B | C, f2 };
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:41:20
+  --> $DIR/feature-gate-or_patterns.rs:39:20
    |
 LL |     let E::V { f1: B | C, f2 };
    |                    ^^^^^
@@ -134,7 +116,7 @@ LL |     let E::V { f1: B | C, f2 };
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns.rs:42:10
+  --> $DIR/feature-gate-or_patterns.rs:40:10
    |
 LL |     let [A | B];
    |          ^^^^^
@@ -187,6 +169,6 @@ LL | accept_pat!([p | q]);
    = note: for more information, see https://github.com/rust-lang/rust/issues/54883
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
 
-error: aborting due to 21 previous errors
+error: aborting due to 19 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

From e3747722fbb8a44f9053922e4c39338a3a1f9597 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 24 Aug 2019 23:10:46 +0200
Subject: [PATCH 27/34] parser: extract recover_inner_leading_vert.

---
 src/libsyntax/parse/parser/pat.rs | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 724edbcfaed79..dc6632cf10da9 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -178,8 +178,13 @@ impl<'a> Parser<'a> {
     /// Recursive possibly-or-pattern parser with recovery for an erroneous leading `|`.
     /// See `parse_pat_with_or` for details on parsing or-patterns.
     fn parse_pat_with_or_inner(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
-        // Recover if `|` or `||` is here.
-        // The user is thinking that a leading `|` is allowed in this position.
+        self.recover_inner_leading_vert();
+        self.parse_pat_with_or(expected, GateOr::Yes, TopLevel::No)
+    }
+
+    /// Recover if `|` or `||` is here.
+    /// The user is thinking that a leading `|` is allowed in this position.
+    fn recover_inner_leading_vert(&mut self) {
         if let token::BinOp(token::Or) | token::OrOr = self.token.kind {
             let span = self.token.span;
             let rm_msg = format!("remove the `{}`", pprust::token_to_string(&self.token));
@@ -190,8 +195,6 @@ impl<'a> Parser<'a> {
 
             self.bump();
         }
-
-        self.parse_pat_with_or(expected, GateOr::Yes, TopLevel::No)
     }
 
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are

From 0ab84303326fff65d5d0a168fd47448e05135c9f Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 24 Aug 2019 23:44:28 +0200
Subject: [PATCH 28/34] parser: reword || recovery.

---
 src/libsyntax/parse/parser/pat.rs                  |  2 +-
 .../ui/or-patterns/multiple-pattern-typo.stderr    | 14 +++++++-------
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index dc6632cf10da9..0b3de57fd6f30 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -119,7 +119,7 @@ impl<'a> Parser<'a> {
         self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
             .span_suggestion(
                 self.token.span,
-                "use a single `|` to specify multiple patterns",
+                "use a single `|` to separate multiple alternative patterns",
                 "|".to_owned(),
                 Applicability::MachineApplicable
             )
diff --git a/src/test/ui/or-patterns/multiple-pattern-typo.stderr b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
index 765c7879b12ca..c61b5cb208251 100644
--- a/src/test/ui/or-patterns/multiple-pattern-typo.stderr
+++ b/src/test/ui/or-patterns/multiple-pattern-typo.stderr
@@ -2,43 +2,43 @@ error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:8:15
    |
 LL |         1 | 2 || 3 => (),
-   |               ^^ help: use a single `|` to specify multiple patterns: `|`
+   |               ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:13:16
    |
 LL |         (1 | 2 || 3) => (),
-   |                ^^ help: use a single `|` to specify multiple patterns: `|`
+   |                ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:18:16
    |
 LL |         (1 | 2 || 3,) => (),
-   |                ^^ help: use a single `|` to specify multiple patterns: `|`
+   |                ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:25:18
    |
 LL |         TS(1 | 2 || 3) => (),
-   |                  ^^ help: use a single `|` to specify multiple patterns: `|`
+   |                  ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:32:23
    |
 LL |         NS { f: 1 | 2 || 3 } => (),
-   |                       ^^ help: use a single `|` to specify multiple patterns: `|`
+   |                       ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:37:16
    |
 LL |         [1 | 2 || 3] => (),
-   |                ^^ help: use a single `|` to specify multiple patterns: `|`
+   |                ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:42:9
    |
 LL |         || 1 | 2 | 3 => (),
-   |         ^^ help: use a single `|` to specify multiple patterns: `|`
+   |         ^^ help: use a single `|` to separate multiple alternative patterns: `|`
 
 warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
   --> $DIR/multiple-pattern-typo.rs:1:12

From 1202cb0e2b168b0a913b33e3cb3c1d9339683e28 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 25 Aug 2019 01:00:19 +0200
Subject: [PATCH 29/34] parser: simplify parse_pat_with_or_{inner}

---
 src/libsyntax/parse/parser/pat.rs | 27 ++++++++++-----------------
 1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 0b3de57fd6f30..7c09dc4e56634 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -49,7 +49,7 @@ impl<'a> Parser<'a> {
         let gated_leading_vert = self.eat_or_separator() && gate_or == GateOr::Yes;
 
         // Parse the possibly-or-pattern.
-        let pat = self.parse_pat_with_or(None, gate_or, TopLevel::Yes)?;
+        let pat = self.parse_pat_with_or(gate_or, TopLevel::Yes)?;
 
         // If we parsed a leading `|` which should be gated,
         // and no other gated or-pattern has been parsed thus far,
@@ -67,14 +67,9 @@ impl<'a> Parser<'a> {
 
     /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
     /// Corresponds to `pat<allow_top_alt>` in RFC 2535.
-    fn parse_pat_with_or(
-        &mut self,
-        expected: Expected,
-        gate_or: GateOr,
-        top_level: TopLevel,
-    ) -> PResult<'a, P<Pat>> {
+    fn parse_pat_with_or(&mut self, gate_or: GateOr, top_level: TopLevel) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
-        let first_pat = self.parse_pat(expected)?;
+        let first_pat = self.parse_pat(None)?;
         self.maybe_recover_unexpected_comma(first_pat.span, top_level)?;
 
         // If the next token is not a `|`,
@@ -86,7 +81,7 @@ impl<'a> Parser<'a> {
         let lo = first_pat.span;
         let mut pats = vec![first_pat];
         while self.eat_or_separator() {
-            let pat = self.parse_pat(expected)?;
+            let pat = self.parse_pat(None)?;
             self.maybe_recover_unexpected_comma(pat.span, top_level)?;
             pats.push(pat);
         }
@@ -177,9 +172,9 @@ impl<'a> Parser<'a> {
 
     /// Recursive possibly-or-pattern parser with recovery for an erroneous leading `|`.
     /// See `parse_pat_with_or` for details on parsing or-patterns.
-    fn parse_pat_with_or_inner(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
+    fn parse_pat_with_or_inner(&mut self) -> PResult<'a, P<Pat>> {
         self.recover_inner_leading_vert();
-        self.parse_pat_with_or(expected, GateOr::Yes, TopLevel::No)
+        self.parse_pat_with_or(GateOr::Yes, TopLevel::No)
     }
 
     /// Recover if `|` or `||` is here.
@@ -215,7 +210,7 @@ impl<'a> Parser<'a> {
                 // Parse `[pat, pat,...]` as a slice pattern.
                 let (pats, _) = self.parse_delim_comma_seq(
                     token::Bracket,
-                    |p| p.parse_pat_with_or_inner(None),
+                    |p| p.parse_pat_with_or_inner(),
                 )?;
                 PatKind::Slice(pats)
             }
@@ -344,9 +339,7 @@ impl<'a> Parser<'a> {
 
     /// Parse a tuple or parenthesis pattern.
     fn parse_pat_tuple_or_parens(&mut self) -> PResult<'a, PatKind> {
-        let (fields, trailing_comma) = self.parse_paren_comma_seq(|p| {
-            p.parse_pat_with_or_inner(None)
-        })?;
+        let (fields, trailing_comma) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or_inner())?;
 
         // Here, `(pat,)` is a tuple pattern.
         // For backward compatibility, `(..)` is a tuple pattern as well.
@@ -589,7 +582,7 @@ impl<'a> Parser<'a> {
             err.span_label(self.token.span, msg);
             return Err(err);
         }
-        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or_inner(None))?;
+        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or_inner())?;
         Ok(PatKind::TupleStruct(path, fields))
     }
 
@@ -733,7 +726,7 @@ impl<'a> Parser<'a> {
             // Parsing a pattern of the form "fieldname: pat"
             let fieldname = self.parse_field_name()?;
             self.bump();
-            let pat = self.parse_pat_with_or_inner(None)?;
+            let pat = self.parse_pat_with_or_inner()?;
             hi = pat.span;
             (pat, fieldname, false)
         } else {

From 083963e58c752f1a51b67d65dc6a207bf69f1d64 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 25 Aug 2019 01:50:21 +0200
Subject: [PATCH 30/34] parser: 'while parsing this or-pattern...'

---
 src/libsyntax/parse/parser/pat.rs                      |  5 ++++-
 .../ui/or-patterns/while-parsing-this-or-pattern.rs    |  9 +++++++++
 .../or-patterns/while-parsing-this-or-pattern.stderr   | 10 ++++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/or-patterns/while-parsing-this-or-pattern.rs
 create mode 100644 src/test/ui/or-patterns/while-parsing-this-or-pattern.stderr

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 7c09dc4e56634..a0278fa407765 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -81,7 +81,10 @@ impl<'a> Parser<'a> {
         let lo = first_pat.span;
         let mut pats = vec![first_pat];
         while self.eat_or_separator() {
-            let pat = self.parse_pat(None)?;
+            let pat = self.parse_pat(None).map_err(|mut err| {
+                err.span_label(lo, "while parsing this or-pattern staring here");
+                err
+            })?;
             self.maybe_recover_unexpected_comma(pat.span, top_level)?;
             pats.push(pat);
         }
diff --git a/src/test/ui/or-patterns/while-parsing-this-or-pattern.rs b/src/test/ui/or-patterns/while-parsing-this-or-pattern.rs
new file mode 100644
index 0000000000000..4a9fae1406af7
--- /dev/null
+++ b/src/test/ui/or-patterns/while-parsing-this-or-pattern.rs
@@ -0,0 +1,9 @@
+// Test the parser for the "while parsing this or-pattern..." label here.
+
+fn main() {
+    match Some(42) {
+        Some(42) | .=. => {} //~ ERROR expected pattern, found `.`
+        //~^ while parsing this or-pattern staring here
+        //~| NOTE expected pattern
+    }
+}
diff --git a/src/test/ui/or-patterns/while-parsing-this-or-pattern.stderr b/src/test/ui/or-patterns/while-parsing-this-or-pattern.stderr
new file mode 100644
index 0000000000000..21fece6c64fe5
--- /dev/null
+++ b/src/test/ui/or-patterns/while-parsing-this-or-pattern.stderr
@@ -0,0 +1,10 @@
+error: expected pattern, found `.`
+  --> $DIR/while-parsing-this-or-pattern.rs:5:20
+   |
+LL |         Some(42) | .=. => {}
+   |         --------   ^ expected pattern
+   |         |
+   |         while parsing this or-pattern staring here
+
+error: aborting due to previous error
+

From 1caaa40768fecb91b322b1e1befc91c54b56817c Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 25 Aug 2019 04:39:28 +0200
Subject: [PATCH 31/34] parser: gracefully handle `fn foo(A | B: type)`.

---
 src/libsyntax/parse/parser.rs                 |  2 +-
 src/libsyntax/parse/parser/expr.rs            |  4 +-
 src/libsyntax/parse/parser/pat.rs             | 46 +++++++++++++++----
 src/test/ui/anon-params-denied-2018.rs        |  2 +-
 src/test/ui/anon-params-denied-2018.stderr    | 16 +++----
 .../feature-gate-or_patterns-leading.stderr   | 12 -----
 .../or-patterns/or-patterns-syntactic-fail.rs |  6 ++-
 .../or-patterns-syntactic-fail.stderr         | 42 +++++++++++------
 src/test/ui/parser/inverted-parameters.rs     | 12 ++---
 src/test/ui/parser/inverted-parameters.stderr | 24 +++++-----
 src/test/ui/parser/issue-33413.rs             |  2 +-
 src/test/ui/parser/issue-33413.stderr         |  4 +-
 src/test/ui/parser/issue-63135.rs             |  2 +-
 src/test/ui/parser/issue-63135.stderr         | 12 ++---
 src/test/ui/parser/omitted-arg-in-item-fn.rs  |  2 +-
 .../ui/parser/omitted-arg-in-item-fn.stderr   |  4 +-
 src/test/ui/parser/pat-lt-bracket-2.rs        |  2 +-
 src/test/ui/parser/pat-lt-bracket-2.stderr    |  4 +-
 src/test/ui/parser/removed-syntax-mode.rs     |  2 +-
 src/test/ui/parser/removed-syntax-mode.stderr |  4 +-
 .../rfc-2565-param-attrs/param-attrs-2018.rs  |  2 +-
 .../param-attrs-2018.stderr                   |  4 +-
 src/test/ui/span/issue-34264.stderr           | 12 ++---
 23 files changed, 125 insertions(+), 97 deletions(-)
 delete mode 100644 src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 002e9bccec72a..25ad2d4404cac 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -988,7 +988,7 @@ impl<'a> Parser<'a> {
         let (pat, ty) = if is_name_required || self.is_named_argument() {
             debug!("parse_arg_general parse_pat (is_name_required:{})", is_name_required);
 
-            let pat = self.parse_pat(Some("argument name"))?;
+            let pat = self.parse_fn_param_pat()?;
             if let Err(mut err) = self.expect(&token::Colon) {
                 if let Some(ident) = self.argument_without_type(
                     &mut err,
diff --git a/src/libsyntax/parse/parser/expr.rs b/src/libsyntax/parse/parser/expr.rs
index 83e5a84a8c6ae..f7c090b5135e9 100644
--- a/src/libsyntax/parse/parser/expr.rs
+++ b/src/libsyntax/parse/parser/expr.rs
@@ -1,7 +1,7 @@
 use super::{Parser, PResult, Restrictions, PrevTokenKind, TokenType, PathStyle};
 use super::{BlockMode, SemiColonMode};
 use super::{SeqSep, TokenExpectType};
-use super::pat::GateOr;
+use super::pat::{GateOr, PARAM_EXPECTED};
 
 use crate::maybe_recover_from_interpolated_ty_qpath;
 use crate::ptr::P;
@@ -1176,7 +1176,7 @@ impl<'a> Parser<'a> {
     fn parse_fn_block_arg(&mut self) -> PResult<'a, Arg> {
         let lo = self.token.span;
         let attrs = self.parse_arg_attributes()?;
-        let pat = self.parse_pat(Some("argument name"))?;
+        let pat = self.parse_pat(PARAM_EXPECTED)?;
         let t = if self.eat(&token::Colon) {
             self.parse_ty()?
         } else {
diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index a0278fa407765..1541031ec2539 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -14,6 +14,9 @@ use errors::{Applicability, DiagnosticBuilder};
 
 type Expected = Option<&'static str>;
 
+/// `Expected` for function and lambda parameter patterns.
+pub(super) const PARAM_EXPECTED: Expected = Some("parameter name");
+
 /// Whether or not an or-pattern should be gated when occurring in the current context.
 #[derive(PartialEq)]
 pub enum GateOr { Yes, No }
@@ -49,7 +52,7 @@ impl<'a> Parser<'a> {
         let gated_leading_vert = self.eat_or_separator() && gate_or == GateOr::Yes;
 
         // Parse the possibly-or-pattern.
-        let pat = self.parse_pat_with_or(gate_or, TopLevel::Yes)?;
+        let pat = self.parse_pat_with_or(None, gate_or, TopLevel::Yes)?;
 
         // If we parsed a leading `|` which should be gated,
         // and no other gated or-pattern has been parsed thus far,
@@ -65,11 +68,38 @@ impl<'a> Parser<'a> {
         Ok(pat)
     }
 
+    /// Parse the pattern for a function or function pointer parameter.
+    /// Special recovery is provided for or-patterns and leading `|`.
+    pub(super) fn parse_fn_param_pat(&mut self) -> PResult<'a, P<Pat>> {
+        self.recover_leading_vert("not allowed in a parameter pattern");
+        let pat = self.parse_pat_with_or(PARAM_EXPECTED, GateOr::No, TopLevel::No)?;
+
+        if let PatKind::Or(..) = &pat.node {
+            self.ban_illegal_fn_param_or_pat(&pat);
+        }
+
+        Ok(pat)
+    }
+
+    /// Ban `A | B` immediately in a parameter pattern and suggest wrapping in parens.
+    fn ban_illegal_fn_param_or_pat(&self, pat: &Pat) {
+        let msg = "wrap the pattern in parenthesis";
+        let fix = format!("({})", pprust::pat_to_string(pat));
+        self.struct_span_err(pat.span, "an or-pattern parameter must be wrapped in parenthesis")
+            .span_suggestion(pat.span, msg, fix, Applicability::MachineApplicable)
+            .emit();
+    }
+
     /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
     /// Corresponds to `pat<allow_top_alt>` in RFC 2535.
-    fn parse_pat_with_or(&mut self, gate_or: GateOr, top_level: TopLevel) -> PResult<'a, P<Pat>> {
+    fn parse_pat_with_or(
+        &mut self,
+        expected: Expected,
+        gate_or: GateOr,
+        top_level: TopLevel,
+    ) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
-        let first_pat = self.parse_pat(None)?;
+        let first_pat = self.parse_pat(expected)?;
         self.maybe_recover_unexpected_comma(first_pat.span, top_level)?;
 
         // If the next token is not a `|`,
@@ -81,7 +111,7 @@ impl<'a> Parser<'a> {
         let lo = first_pat.span;
         let mut pats = vec![first_pat];
         while self.eat_or_separator() {
-            let pat = self.parse_pat(None).map_err(|mut err| {
+            let pat = self.parse_pat(expected).map_err(|mut err| {
                 err.span_label(lo, "while parsing this or-pattern staring here");
                 err
             })?;
@@ -176,18 +206,18 @@ impl<'a> Parser<'a> {
     /// Recursive possibly-or-pattern parser with recovery for an erroneous leading `|`.
     /// See `parse_pat_with_or` for details on parsing or-patterns.
     fn parse_pat_with_or_inner(&mut self) -> PResult<'a, P<Pat>> {
-        self.recover_inner_leading_vert();
-        self.parse_pat_with_or(GateOr::Yes, TopLevel::No)
+        self.recover_leading_vert("only allowed in a top-level pattern");
+        self.parse_pat_with_or(None, GateOr::Yes, TopLevel::No)
     }
 
     /// Recover if `|` or `||` is here.
     /// The user is thinking that a leading `|` is allowed in this position.
-    fn recover_inner_leading_vert(&mut self) {
+    fn recover_leading_vert(&mut self, ctx: &str) {
         if let token::BinOp(token::Or) | token::OrOr = self.token.kind {
             let span = self.token.span;
             let rm_msg = format!("remove the `{}`", pprust::token_to_string(&self.token));
 
-            self.struct_span_err(span, "a leading `|` is only allowed in a top-level pattern")
+            self.struct_span_err(span, &format!("a leading `|` is {}", ctx))
                 .span_suggestion(span, &rm_msg, String::new(), Applicability::MachineApplicable)
                 .emit();
 
diff --git a/src/test/ui/anon-params-denied-2018.rs b/src/test/ui/anon-params-denied-2018.rs
index abff8275064e2..5721f5d235783 100644
--- a/src/test/ui/anon-params-denied-2018.rs
+++ b/src/test/ui/anon-params-denied-2018.rs
@@ -3,7 +3,7 @@
 // edition:2018
 
 trait T {
-    fn foo(i32); //~ expected one of `:` or `@`, found `)`
+    fn foo(i32); //~ expected one of `:`, `@`, or `|`, found `)`
 
     fn bar_with_default_impl(String, String) {}
     //~^ ERROR expected one of `:`
diff --git a/src/test/ui/anon-params-denied-2018.stderr b/src/test/ui/anon-params-denied-2018.stderr
index 438bcf4274daa..a58998e4891e0 100644
--- a/src/test/ui/anon-params-denied-2018.stderr
+++ b/src/test/ui/anon-params-denied-2018.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:` or `@`, found `)`
+error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/anon-params-denied-2018.rs:6:15
    |
 LL |     fn foo(i32);
-   |               ^ expected one of `:` or `@` here
+   |               ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
@@ -14,11 +14,11 @@ help: if this is a type, explicitly ignore the parameter name
 LL |     fn foo(_: i32);
    |            ^^^^^^
 
-error: expected one of `:` or `@`, found `,`
+error: expected one of `:`, `@`, or `|`, found `,`
   --> $DIR/anon-params-denied-2018.rs:8:36
    |
 LL |     fn bar_with_default_impl(String, String) {}
-   |                                    ^ expected one of `:` or `@` here
+   |                                    ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
@@ -30,11 +30,11 @@ help: if this is a type, explicitly ignore the parameter name
 LL |     fn bar_with_default_impl(_: String, String) {}
    |                              ^^^^^^^^^
 
-error: expected one of `:` or `@`, found `)`
+error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/anon-params-denied-2018.rs:8:44
    |
 LL |     fn bar_with_default_impl(String, String) {}
-   |                                            ^ expected one of `:` or `@` here
+   |                                            ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
@@ -46,11 +46,11 @@ help: if this is a type, explicitly ignore the parameter name
 LL |     fn bar_with_default_impl(String, _: String) {}
    |                                      ^^^^^^^^^
 
-error: expected one of `:` or `@`, found `,`
+error: expected one of `:`, `@`, or `|`, found `,`
   --> $DIR/anon-params-denied-2018.rs:13:22
    |
 LL |     fn baz(a:usize, b, c: usize) -> usize {
-   |                      ^ expected one of `:` or `@` here
+   |                      ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr
deleted file mode 100644
index 8b18082fca7df..0000000000000
--- a/src/test/ui/or-patterns/feature-gate-or_patterns-leading.stderr
+++ /dev/null
@@ -1,12 +0,0 @@
-error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns-leading.rs:7:11
-   |
-LL |     let | A;
-   |           ^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/54883
-   = help: add `#![feature(or_patterns)]` to the crate attributes to enable
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs b/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
index 7959812f5b39c..b676ea851a3ba 100644
--- a/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-fail.rs
@@ -25,7 +25,11 @@ fn no_top_level_or_patterns() {
     //           -------- This looks like an or-pattern but is in fact `|A| (B: E | ())`.
 
     // ...and for now neither do we allow or-patterns at the top level of functions.
-    fn fun(A | B: E) {} //~ ERROR expected one of `:` or `@`, found `|`
+    fn fun1(A | B: E) {} //~ ERROR an or-pattern parameter must be wrapped in parenthesis
+
+    fn fun2(| A | B: E) {}
+    //~^ ERROR a leading `|` is not allowed in a parameter pattern
+    //~| ERROR an or-pattern parameter must be wrapped in parenthesis
 }
 
 // We also do not allow a leading `|` when not in a top level position:
diff --git a/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr b/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
index dd4c309ce85a3..2a3a6abfb7b62 100644
--- a/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
+++ b/src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr
@@ -1,59 +1,71 @@
-error: expected one of `:` or `@`, found `|`
-  --> $DIR/or-patterns-syntactic-fail.rs:28:14
+error: an or-pattern parameter must be wrapped in parenthesis
+  --> $DIR/or-patterns-syntactic-fail.rs:28:13
    |
-LL |     fn fun(A | B: E) {}
-   |              ^ expected one of `:` or `@` here
+LL |     fn fun1(A | B: E) {}
+   |             ^^^^^ help: wrap the pattern in parenthesis: `(A | B)`
+
+error: a leading `|` is not allowed in a parameter pattern
+  --> $DIR/or-patterns-syntactic-fail.rs:30:13
+   |
+LL |     fn fun2(| A | B: E) {}
+   |             ^ help: remove the `|`
+
+error: an or-pattern parameter must be wrapped in parenthesis
+  --> $DIR/or-patterns-syntactic-fail.rs:30:15
+   |
+LL |     fn fun2(| A | B: E) {}
+   |               ^^^^^ help: wrap the pattern in parenthesis: `(A | B)`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:37:11
+  --> $DIR/or-patterns-syntactic-fail.rs:41:11
    |
 LL |     let ( | A | B) = E::A;
    |           ^ help: remove the `|`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:38:11
+  --> $DIR/or-patterns-syntactic-fail.rs:42:11
    |
 LL |     let ( | A | B,) = (E::B,);
    |           ^ help: remove the `|`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:39:11
+  --> $DIR/or-patterns-syntactic-fail.rs:43:11
    |
 LL |     let [ | A | B ] = [E::A];
    |           ^ help: remove the `|`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:40:13
+  --> $DIR/or-patterns-syntactic-fail.rs:44:13
    |
 LL |     let TS( | A | B );
    |             ^ help: remove the `|`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:41:17
+  --> $DIR/or-patterns-syntactic-fail.rs:45:17
    |
 LL |     let NS { f: | A | B };
    |                 ^ help: remove the `|`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:43:11
+  --> $DIR/or-patterns-syntactic-fail.rs:47:11
    |
 LL |     let ( || A | B) = E::A;
    |           ^^ help: remove the `||`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:44:11
+  --> $DIR/or-patterns-syntactic-fail.rs:48:11
    |
 LL |     let [ || A | B ] = [E::A];
    |           ^^ help: remove the `||`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:45:13
+  --> $DIR/or-patterns-syntactic-fail.rs:49:13
    |
 LL |     let TS( || A | B );
    |             ^^ help: remove the `||`
 
 error: a leading `|` is only allowed in a top-level pattern
-  --> $DIR/or-patterns-syntactic-fail.rs:46:17
+  --> $DIR/or-patterns-syntactic-fail.rs:50:17
    |
 LL |     let NS { f: || A | B };
    |                 ^^ help: remove the `||`
@@ -95,7 +107,7 @@ LL |     let _ = |A | B: E| ();
    = note: an implementation of `std::ops::BitOr` might be missing for `E`
 
 error[E0308]: mismatched types
-  --> $DIR/or-patterns-syntactic-fail.rs:48:36
+  --> $DIR/or-patterns-syntactic-fail.rs:52:36
    |
 LL |     let recovery_witness: String = 0;
    |                                    ^
@@ -106,7 +118,7 @@ LL |     let recovery_witness: String = 0;
    = note: expected type `std::string::String`
               found type `{integer}`
 
-error: aborting due to 14 previous errors
+error: aborting due to 16 previous errors
 
 Some errors have detailed explanations: E0308, E0369.
 For more information about an error, try `rustc --explain E0308`.
diff --git a/src/test/ui/parser/inverted-parameters.rs b/src/test/ui/parser/inverted-parameters.rs
index f06b951041731..d6efc8be072bd 100644
--- a/src/test/ui/parser/inverted-parameters.rs
+++ b/src/test/ui/parser/inverted-parameters.rs
@@ -2,29 +2,29 @@ struct S;
 
 impl S {
     fn foo(&self, &str bar) {}
-    //~^ ERROR expected one of `:` or `@`
+    //~^ ERROR expected one of `:`, `@`
     //~| HELP declare the type after the parameter binding
     //~| SUGGESTION <identifier>: <type>
 }
 
 fn baz(S quux, xyzzy: i32) {}
-//~^ ERROR expected one of `:` or `@`
+//~^ ERROR expected one of `:`, `@`
 //~| HELP declare the type after the parameter binding
 //~| SUGGESTION <identifier>: <type>
 
 fn one(i32 a b) {}
-//~^ ERROR expected one of `:` or `@`
+//~^ ERROR expected one of `:`, `@`
 
 fn pattern((i32, i32) (a, b)) {}
-//~^ ERROR expected `:`
+//~^ ERROR expected one of `:`
 
 fn fizz(i32) {}
-//~^ ERROR expected one of `:` or `@`
+//~^ ERROR expected one of `:`, `@`
 //~| HELP if this was a parameter name, give it a type
 //~| HELP if this is a type, explicitly ignore the parameter name
 
 fn missing_colon(quux S) {}
-//~^ ERROR expected one of `:` or `@`
+//~^ ERROR expected one of `:`, `@`
 //~| HELP declare the type after the parameter binding
 //~| SUGGESTION <identifier>: <type>
 
diff --git a/src/test/ui/parser/inverted-parameters.stderr b/src/test/ui/parser/inverted-parameters.stderr
index fb48bd1fe9383..2bda4460031a0 100644
--- a/src/test/ui/parser/inverted-parameters.stderr
+++ b/src/test/ui/parser/inverted-parameters.stderr
@@ -1,38 +1,38 @@
-error: expected one of `:` or `@`, found `bar`
+error: expected one of `:`, `@`, or `|`, found `bar`
   --> $DIR/inverted-parameters.rs:4:24
    |
 LL |     fn foo(&self, &str bar) {}
    |                   -----^^^
    |                   |    |
-   |                   |    expected one of `:` or `@` here
+   |                   |    expected one of `:`, `@`, or `|` here
    |                   help: declare the type after the parameter binding: `<identifier>: <type>`
 
-error: expected one of `:` or `@`, found `quux`
+error: expected one of `:`, `@`, or `|`, found `quux`
   --> $DIR/inverted-parameters.rs:10:10
    |
 LL | fn baz(S quux, xyzzy: i32) {}
    |        --^^^^
    |        | |
-   |        | expected one of `:` or `@` here
+   |        | expected one of `:`, `@`, or `|` here
    |        help: declare the type after the parameter binding: `<identifier>: <type>`
 
-error: expected one of `:` or `@`, found `a`
+error: expected one of `:`, `@`, or `|`, found `a`
   --> $DIR/inverted-parameters.rs:15:12
    |
 LL | fn one(i32 a b) {}
-   |            ^ expected one of `:` or `@` here
+   |            ^ expected one of `:`, `@`, or `|` here
 
-error: expected `:`, found `(`
+error: expected one of `:` or `|`, found `(`
   --> $DIR/inverted-parameters.rs:18:23
    |
 LL | fn pattern((i32, i32) (a, b)) {}
-   |                       ^ expected `:`
+   |                       ^ expected one of `:` or `|` here
 
-error: expected one of `:` or `@`, found `)`
+error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/inverted-parameters.rs:21:12
    |
 LL | fn fizz(i32) {}
-   |            ^ expected one of `:` or `@` here
+   |            ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
@@ -44,13 +44,13 @@ help: if this is a type, explicitly ignore the parameter name
 LL | fn fizz(_: i32) {}
    |         ^^^^^^
 
-error: expected one of `:` or `@`, found `S`
+error: expected one of `:`, `@`, or `|`, found `S`
   --> $DIR/inverted-parameters.rs:26:23
    |
 LL | fn missing_colon(quux S) {}
    |                  -----^
    |                  |    |
-   |                  |    expected one of `:` or `@` here
+   |                  |    expected one of `:`, `@`, or `|` here
    |                  help: declare the type after the parameter binding: `<identifier>: <type>`
 
 error: aborting due to 6 previous errors
diff --git a/src/test/ui/parser/issue-33413.rs b/src/test/ui/parser/issue-33413.rs
index 2ec86958174a4..22f80a8aae866 100644
--- a/src/test/ui/parser/issue-33413.rs
+++ b/src/test/ui/parser/issue-33413.rs
@@ -2,7 +2,7 @@ struct S;
 
 impl S {
     fn f(*, a: u8) -> u8 {}
-    //~^ ERROR expected argument name, found `*`
+    //~^ ERROR expected parameter name, found `*`
 }
 
 fn main() {}
diff --git a/src/test/ui/parser/issue-33413.stderr b/src/test/ui/parser/issue-33413.stderr
index f6f096b1b9a44..9e1178e8ac1f3 100644
--- a/src/test/ui/parser/issue-33413.stderr
+++ b/src/test/ui/parser/issue-33413.stderr
@@ -1,8 +1,8 @@
-error: expected argument name, found `*`
+error: expected parameter name, found `*`
   --> $DIR/issue-33413.rs:4:10
    |
 LL |     fn f(*, a: u8) -> u8 {}
-   |          ^ expected argument name
+   |          ^ expected parameter name
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/issue-63135.rs b/src/test/ui/parser/issue-63135.rs
index d5f5f1469f35a..a5a8de85466bb 100644
--- a/src/test/ui/parser/issue-63135.rs
+++ b/src/test/ui/parser/issue-63135.rs
@@ -1,3 +1,3 @@
-// error-pattern: aborting due to 6 previous errors
+// error-pattern: aborting due to 5 previous errors
 
 fn i(n{...,f #
diff --git a/src/test/ui/parser/issue-63135.stderr b/src/test/ui/parser/issue-63135.stderr
index c0286d90af744..a077ad454a9df 100644
--- a/src/test/ui/parser/issue-63135.stderr
+++ b/src/test/ui/parser/issue-63135.stderr
@@ -28,17 +28,11 @@ error: expected `[`, found `}`
 LL | fn i(n{...,f #
    |               ^ expected `[`
 
-error: expected `:`, found `)`
+error: expected one of `:` or `|`, found `)`
   --> $DIR/issue-63135.rs:3:15
    |
 LL | fn i(n{...,f #
-   |               ^ expected `:`
+   |               ^ expected one of `:` or `|` here
 
-error: expected one of `->`, `where`, or `{`, found `<eof>`
-  --> $DIR/issue-63135.rs:3:15
-   |
-LL | fn i(n{...,f #
-   |               ^ expected one of `->`, `where`, or `{` here
-
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
diff --git a/src/test/ui/parser/omitted-arg-in-item-fn.rs b/src/test/ui/parser/omitted-arg-in-item-fn.rs
index 5ee9daf464055..49cbc4d6bf40f 100644
--- a/src/test/ui/parser/omitted-arg-in-item-fn.rs
+++ b/src/test/ui/parser/omitted-arg-in-item-fn.rs
@@ -1,4 +1,4 @@
-fn foo(x) { //~ ERROR expected one of `:` or `@`, found `)`
+fn foo(x) { //~ ERROR expected one of `:`, `@`, or `|`, found `)`
 }
 
 fn main() {}
diff --git a/src/test/ui/parser/omitted-arg-in-item-fn.stderr b/src/test/ui/parser/omitted-arg-in-item-fn.stderr
index e501f235d6d3b..7feb15592c54f 100644
--- a/src/test/ui/parser/omitted-arg-in-item-fn.stderr
+++ b/src/test/ui/parser/omitted-arg-in-item-fn.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:` or `@`, found `)`
+error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/omitted-arg-in-item-fn.rs:1:9
    |
 LL | fn foo(x) {
-   |         ^ expected one of `:` or `@` here
+   |         ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
diff --git a/src/test/ui/parser/pat-lt-bracket-2.rs b/src/test/ui/parser/pat-lt-bracket-2.rs
index 6eb01c1c93337..3a778ed14f637 100644
--- a/src/test/ui/parser/pat-lt-bracket-2.rs
+++ b/src/test/ui/parser/pat-lt-bracket-2.rs
@@ -1,4 +1,4 @@
 fn a(B<) {}
-   //~^ error: expected one of `:` or `@`, found `<`
+   //~^ error: expected one of `:`, `@`, or `|`, found `<`
 
 fn main() {}
diff --git a/src/test/ui/parser/pat-lt-bracket-2.stderr b/src/test/ui/parser/pat-lt-bracket-2.stderr
index cce1a17e9e8df..dbc8d0f5865c6 100644
--- a/src/test/ui/parser/pat-lt-bracket-2.stderr
+++ b/src/test/ui/parser/pat-lt-bracket-2.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:` or `@`, found `<`
+error: expected one of `:`, `@`, or `|`, found `<`
   --> $DIR/pat-lt-bracket-2.rs:1:7
    |
 LL | fn a(B<) {}
-   |       ^ expected one of `:` or `@` here
+   |       ^ expected one of `:`, `@`, or `|` here
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/removed-syntax-mode.rs b/src/test/ui/parser/removed-syntax-mode.rs
index 23851b5f70b37..a438db3b0c18b 100644
--- a/src/test/ui/parser/removed-syntax-mode.rs
+++ b/src/test/ui/parser/removed-syntax-mode.rs
@@ -1,4 +1,4 @@
 fn f(+x: isize) {}
-//~^ ERROR expected argument name, found `+`
+//~^ ERROR expected parameter name, found `+`
 
 fn main() {}
diff --git a/src/test/ui/parser/removed-syntax-mode.stderr b/src/test/ui/parser/removed-syntax-mode.stderr
index 5e7139d6bfd85..d0393b379f06d 100644
--- a/src/test/ui/parser/removed-syntax-mode.stderr
+++ b/src/test/ui/parser/removed-syntax-mode.stderr
@@ -1,8 +1,8 @@
-error: expected argument name, found `+`
+error: expected parameter name, found `+`
   --> $DIR/removed-syntax-mode.rs:1:6
    |
 LL | fn f(+x: isize) {}
-   |      ^ expected argument name
+   |      ^ expected parameter name
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.rs b/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.rs
index e900ccab4fd83..d71711336b06e 100644
--- a/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.rs
+++ b/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.rs
@@ -3,6 +3,6 @@
 #![feature(param_attrs)]
 
 trait Trait2015 { fn foo(#[allow(C)] i32); }
-//~^ ERROR expected one of `:` or `@`, found `)`
+//~^ ERROR expected one of `:`, `@`, or `|`, found `)`
 
 fn main() {}
diff --git a/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.stderr b/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.stderr
index d0ed65f288011..26b414e426805 100644
--- a/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.stderr
+++ b/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.stderr
@@ -1,8 +1,8 @@
-error: expected one of `:` or `@`, found `)`
+error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/param-attrs-2018.rs:5:41
    |
 LL | trait Trait2015 { fn foo(#[allow(C)] i32); }
-   |                                         ^ expected one of `:` or `@` here
+   |                                         ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
diff --git a/src/test/ui/span/issue-34264.stderr b/src/test/ui/span/issue-34264.stderr
index 5dd9895c6e4f4..cc0eccd37a26f 100644
--- a/src/test/ui/span/issue-34264.stderr
+++ b/src/test/ui/span/issue-34264.stderr
@@ -1,14 +1,14 @@
-error: expected one of `:` or `@`, found `<`
+error: expected one of `:`, `@`, or `|`, found `<`
   --> $DIR/issue-34264.rs:1:14
    |
 LL | fn foo(Option<i32>, String) {}
-   |              ^ expected one of `:` or `@` here
+   |              ^ expected one of `:`, `@`, or `|` here
 
-error: expected one of `:` or `@`, found `)`
+error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/issue-34264.rs:1:27
    |
 LL | fn foo(Option<i32>, String) {}
-   |                           ^ expected one of `:` or `@` here
+   |                           ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type
@@ -20,11 +20,11 @@ help: if this is a type, explicitly ignore the parameter name
 LL | fn foo(Option<i32>, _: String) {}
    |                     ^^^^^^^^^
 
-error: expected one of `:` or `@`, found `,`
+error: expected one of `:`, `@`, or `|`, found `,`
   --> $DIR/issue-34264.rs:3:9
    |
 LL | fn bar(x, y: usize) {}
-   |         ^ expected one of `:` or `@` here
+   |         ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
 help: if this was a parameter name, give it a type

From 6a73199da6c06c0b71ed6eeca578b00925137664 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 25 Aug 2019 05:45:51 +0200
Subject: [PATCH 32/34] or_patterns: add run-rustfix tests.

---
 .../ui/or-patterns/fn-param-wrap-parens.fixed | 14 ++++
 .../ui/or-patterns/fn-param-wrap-parens.rs    | 14 ++++
 .../or-patterns/fn-param-wrap-parens.stderr   |  8 +++
 .../ui/or-patterns/remove-leading-vert.fixed  | 23 +++++++
 .../ui/or-patterns/remove-leading-vert.rs     | 23 +++++++
 .../ui/or-patterns/remove-leading-vert.stderr | 68 +++++++++++++++++++
 6 files changed, 150 insertions(+)
 create mode 100644 src/test/ui/or-patterns/fn-param-wrap-parens.fixed
 create mode 100644 src/test/ui/or-patterns/fn-param-wrap-parens.rs
 create mode 100644 src/test/ui/or-patterns/fn-param-wrap-parens.stderr
 create mode 100644 src/test/ui/or-patterns/remove-leading-vert.fixed
 create mode 100644 src/test/ui/or-patterns/remove-leading-vert.rs
 create mode 100644 src/test/ui/or-patterns/remove-leading-vert.stderr

diff --git a/src/test/ui/or-patterns/fn-param-wrap-parens.fixed b/src/test/ui/or-patterns/fn-param-wrap-parens.fixed
new file mode 100644
index 0000000000000..08730fe8b07b7
--- /dev/null
+++ b/src/test/ui/or-patterns/fn-param-wrap-parens.fixed
@@ -0,0 +1,14 @@
+// Test the suggestion to wrap an or-pattern as a function parameter in parens.
+
+// run-rustfix
+
+#![feature(or_patterns)]
+#![allow(warnings)]
+
+fn main() {}
+
+enum E { A, B }
+use E::*;
+
+#[cfg(FALSE)]
+fn fun1((A | B): E) {} //~ ERROR an or-pattern parameter must be wrapped in parenthesis
diff --git a/src/test/ui/or-patterns/fn-param-wrap-parens.rs b/src/test/ui/or-patterns/fn-param-wrap-parens.rs
new file mode 100644
index 0000000000000..ed667e0e66067
--- /dev/null
+++ b/src/test/ui/or-patterns/fn-param-wrap-parens.rs
@@ -0,0 +1,14 @@
+// Test the suggestion to wrap an or-pattern as a function parameter in parens.
+
+// run-rustfix
+
+#![feature(or_patterns)]
+#![allow(warnings)]
+
+fn main() {}
+
+enum E { A, B }
+use E::*;
+
+#[cfg(FALSE)]
+fn fun1(A | B: E) {} //~ ERROR an or-pattern parameter must be wrapped in parenthesis
diff --git a/src/test/ui/or-patterns/fn-param-wrap-parens.stderr b/src/test/ui/or-patterns/fn-param-wrap-parens.stderr
new file mode 100644
index 0000000000000..2c6e4d9838ddc
--- /dev/null
+++ b/src/test/ui/or-patterns/fn-param-wrap-parens.stderr
@@ -0,0 +1,8 @@
+error: an or-pattern parameter must be wrapped in parenthesis
+  --> $DIR/fn-param-wrap-parens.rs:14:9
+   |
+LL | fn fun1(A | B: E) {}
+   |         ^^^^^ help: wrap the pattern in parenthesis: `(A | B)`
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/or-patterns/remove-leading-vert.fixed b/src/test/ui/or-patterns/remove-leading-vert.fixed
new file mode 100644
index 0000000000000..e96d76061ac28
--- /dev/null
+++ b/src/test/ui/or-patterns/remove-leading-vert.fixed
@@ -0,0 +1,23 @@
+// Test the suggestion to remove a leading `|`.
+
+// run-rustfix
+
+#![feature(or_patterns)]
+#![allow(warnings)]
+
+fn main() {}
+
+#[cfg(FALSE)]
+fn leading_vert() {
+    fn fun1(  A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
+    fn fun2(  A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
+    let (  A): E; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let (  A): (E); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let (  A,): (E,); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let [  A ]: [E; 1]; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let [  A ]: [E; 1]; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let TS(  A ): TS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let TS(  A ): TS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let NS { f:  A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let NS { f:  A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+}
diff --git a/src/test/ui/or-patterns/remove-leading-vert.rs b/src/test/ui/or-patterns/remove-leading-vert.rs
new file mode 100644
index 0000000000000..3790b17553fe3
--- /dev/null
+++ b/src/test/ui/or-patterns/remove-leading-vert.rs
@@ -0,0 +1,23 @@
+// Test the suggestion to remove a leading `|`.
+
+// run-rustfix
+
+#![feature(or_patterns)]
+#![allow(warnings)]
+
+fn main() {}
+
+#[cfg(FALSE)]
+fn leading_vert() {
+    fn fun1( | A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
+    fn fun2( || A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
+    let ( | A): E; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let ( || A): (E); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let ( | A,): (E,); //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let [ | A ]: [E; 1]; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let [ || A ]: [E; 1]; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let TS( | A ): TS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let TS( || A ): TS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let NS { f: | A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+    let NS { f: || A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
+}
diff --git a/src/test/ui/or-patterns/remove-leading-vert.stderr b/src/test/ui/or-patterns/remove-leading-vert.stderr
new file mode 100644
index 0000000000000..cbe06f997296a
--- /dev/null
+++ b/src/test/ui/or-patterns/remove-leading-vert.stderr
@@ -0,0 +1,68 @@
+error: a leading `|` is not allowed in a parameter pattern
+  --> $DIR/remove-leading-vert.rs:12:14
+   |
+LL |     fn fun1( | A: E) {}
+   |              ^ help: remove the `|`
+
+error: a leading `|` is not allowed in a parameter pattern
+  --> $DIR/remove-leading-vert.rs:13:14
+   |
+LL |     fn fun2( || A: E) {}
+   |              ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:14:11
+   |
+LL |     let ( | A): E;
+   |           ^ help: remove the `|`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:15:11
+   |
+LL |     let ( || A): (E);
+   |           ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:16:11
+   |
+LL |     let ( | A,): (E,);
+   |           ^ help: remove the `|`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:17:11
+   |
+LL |     let [ | A ]: [E; 1];
+   |           ^ help: remove the `|`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:18:11
+   |
+LL |     let [ || A ]: [E; 1];
+   |           ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:19:13
+   |
+LL |     let TS( | A ): TS;
+   |             ^ help: remove the `|`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:20:13
+   |
+LL |     let TS( || A ): TS;
+   |             ^^ help: remove the `||`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:21:17
+   |
+LL |     let NS { f: | A }: NS;
+   |                 ^ help: remove the `|`
+
+error: a leading `|` is only allowed in a top-level pattern
+  --> $DIR/remove-leading-vert.rs:22:17
+   |
+LL |     let NS { f: || A }: NS;
+   |                 ^^ help: remove the `||`
+
+error: aborting due to 11 previous errors
+

From acb11305e8d7298750797d45324c0ecb3cc6c256 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 25 Aug 2019 06:15:11 +0200
Subject: [PATCH 33/34] parser: TopLevel -> RecoverComma.

---
 src/libsyntax/parse/parser/pat.rs | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index 1541031ec2539..f2e269e03baf9 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -21,9 +21,9 @@ pub(super) const PARAM_EXPECTED: Expected = Some("parameter name");
 #[derive(PartialEq)]
 pub enum GateOr { Yes, No }
 
-/// Whether or not this is the top level pattern context.
+/// Whether or not to recover a `,` when parsing or-patterns.
 #[derive(PartialEq, Copy, Clone)]
-enum TopLevel { Yes, No }
+enum RecoverComma { Yes, No }
 
 impl<'a> Parser<'a> {
     /// Parses a pattern.
@@ -52,7 +52,7 @@ impl<'a> Parser<'a> {
         let gated_leading_vert = self.eat_or_separator() && gate_or == GateOr::Yes;
 
         // Parse the possibly-or-pattern.
-        let pat = self.parse_pat_with_or(None, gate_or, TopLevel::Yes)?;
+        let pat = self.parse_pat_with_or(None, gate_or, RecoverComma::Yes)?;
 
         // If we parsed a leading `|` which should be gated,
         // and no other gated or-pattern has been parsed thus far,
@@ -72,7 +72,7 @@ impl<'a> Parser<'a> {
     /// Special recovery is provided for or-patterns and leading `|`.
     pub(super) fn parse_fn_param_pat(&mut self) -> PResult<'a, P<Pat>> {
         self.recover_leading_vert("not allowed in a parameter pattern");
-        let pat = self.parse_pat_with_or(PARAM_EXPECTED, GateOr::No, TopLevel::No)?;
+        let pat = self.parse_pat_with_or(PARAM_EXPECTED, GateOr::No, RecoverComma::No)?;
 
         if let PatKind::Or(..) = &pat.node {
             self.ban_illegal_fn_param_or_pat(&pat);
@@ -96,11 +96,11 @@ impl<'a> Parser<'a> {
         &mut self,
         expected: Expected,
         gate_or: GateOr,
-        top_level: TopLevel,
+        rc: RecoverComma,
     ) -> PResult<'a, P<Pat>> {
         // Parse the first pattern.
         let first_pat = self.parse_pat(expected)?;
-        self.maybe_recover_unexpected_comma(first_pat.span, top_level)?;
+        self.maybe_recover_unexpected_comma(first_pat.span, rc)?;
 
         // If the next token is not a `|`,
         // this is not an or-pattern and we should exit here.
@@ -115,7 +115,7 @@ impl<'a> Parser<'a> {
                 err.span_label(lo, "while parsing this or-pattern staring here");
                 err
             })?;
-            self.maybe_recover_unexpected_comma(pat.span, top_level)?;
+            self.maybe_recover_unexpected_comma(pat.span, rc)?;
             pats.push(pat);
         }
         let or_pattern_span = lo.to(self.prev_span);
@@ -156,8 +156,8 @@ impl<'a> Parser<'a> {
 
     /// Some special error handling for the "top-level" patterns in a match arm,
     /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
-    fn maybe_recover_unexpected_comma(&mut self, lo: Span, top_level: TopLevel) -> PResult<'a, ()> {
-        if top_level == TopLevel::No || self.token != token::Comma {
+    fn maybe_recover_unexpected_comma(&mut self, lo: Span, rc: RecoverComma) -> PResult<'a, ()> {
+        if rc == RecoverComma::No || self.token != token::Comma {
             return Ok(());
         }
 
@@ -207,7 +207,7 @@ impl<'a> Parser<'a> {
     /// See `parse_pat_with_or` for details on parsing or-patterns.
     fn parse_pat_with_or_inner(&mut self) -> PResult<'a, P<Pat>> {
         self.recover_leading_vert("only allowed in a top-level pattern");
-        self.parse_pat_with_or(None, GateOr::Yes, TopLevel::No)
+        self.parse_pat_with_or(None, GateOr::Yes, RecoverComma::No)
     }
 
     /// Recover if `|` or `||` is here.

From 2bd27fbdfe309f3f6abd76f72f379247d49048b7 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 26 Aug 2019 22:14:31 +0200
Subject: [PATCH 34/34] parser: fix span for leading vert.

---
 src/libsyntax/parse/parser/pat.rs                             | 3 ++-
 .../or-patterns/feature-gate-or_patterns-leading-for.stderr   | 4 ++--
 .../or-patterns/feature-gate-or_patterns-leading-let.stderr   | 4 ++--
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/src/libsyntax/parse/parser/pat.rs b/src/libsyntax/parse/parser/pat.rs
index f2e269e03baf9..78c9a289b3702 100644
--- a/src/libsyntax/parse/parser/pat.rs
+++ b/src/libsyntax/parse/parser/pat.rs
@@ -50,6 +50,7 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_top_pat(&mut self, gate_or: GateOr) -> PResult<'a, P<Pat>> {
         // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
         let gated_leading_vert = self.eat_or_separator() && gate_or == GateOr::Yes;
+        let leading_vert_span = self.prev_span;
 
         // Parse the possibly-or-pattern.
         let pat = self.parse_pat_with_or(None, gate_or, RecoverComma::Yes)?;
@@ -61,7 +62,7 @@ impl<'a> Parser<'a> {
         if gated_leading_vert {
             let mut or_pattern_spans = self.sess.gated_spans.or_patterns.borrow_mut();
             if or_pattern_spans.is_empty() {
-                or_pattern_spans.push(self.prev_span);
+                or_pattern_spans.push(leading_vert_span);
             }
         }
 
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr
index 83804d564f3e4..f520409e8bad6 100644
--- a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-for.stderr
@@ -1,8 +1,8 @@
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns-leading-for.rs:7:11
+  --> $DIR/feature-gate-or_patterns-leading-for.rs:7:9
    |
 LL |     for | A in 0 {}
-   |           ^
+   |         ^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/54883
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable
diff --git a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr
index f7954ad7a8ce1..30fd6a1a95eff 100644
--- a/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr
+++ b/src/test/ui/or-patterns/feature-gate-or_patterns-leading-let.stderr
@@ -1,8 +1,8 @@
 error[E0658]: or-patterns syntax is experimental
-  --> $DIR/feature-gate-or_patterns-leading-let.rs:7:11
+  --> $DIR/feature-gate-or_patterns-leading-let.rs:7:9
    |
 LL |     let | A;
-   |           ^
+   |         ^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/54883
    = help: add `#![feature(or_patterns)]` to the crate attributes to enable