Auto merge of rust-lang#107478 - compiler-errors:anon-enum-tys-are-ambiguous, r=estebank

Revert "Teach parser to understand fake anonymous enum syntax" and related commits

anonymous enum types are currently ambiguous in positions like:

* `|` operator: `a as fn() -> B | C`
* closure args: `|_: as fn() -> A | B`

I first tried to thread around `RecoverAnonEnum` into all these positions, but the resulting complexity in the compiler is IMO not worth it, or at least worth a bit more thinking time. In the mean time, let's revert this syntax for now, so we can go back to the drawing board.

Fixes rust-lang#107461

cc: `@estebank` `@cjgillot` rust-lang#106960

---
### Squashed revert commits:

Revert "review comment: Remove AST AnonTy"

This reverts commit 020cca8.

Revert "Ensure macros are not affected"

This reverts commit 12d18e4.

Revert "Emit fewer errors on patterns with possible type ascription"

This reverts commit c847a01.

Revert "Teach parser to understand fake anonymous enum syntax"

This reverts commit 2d82420.

Author: bors

compiler/rustc_ast/src/visit.rs

@@ -403,8 +403,8 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
             walk_list!(visitor, visit_lifetime, opt_lifetime, LifetimeCtxt::Ref);
             visitor.visit_ty(&mutable_type.ty)
         }
-        TyKind::Tup(tys) => {
-            walk_list!(visitor, visit_ty, tys);
+        TyKind::Tup(tuple_element_types) => {
+            walk_list!(visitor, visit_ty, tuple_element_types);
         }
         TyKind::BareFn(function_declaration) => {
             walk_list!(visitor, visit_generic_param, &function_declaration.generic_params);

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -2394,7 +2394,7 @@ 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).
-    pub(crate) fn maybe_recover_colon_colon_in_pat_typo_or_anon_enum(
+    pub(crate) fn maybe_recover_colon_colon_in_pat_typo(
         &mut self,
         mut first_pat: P<Pat>,
         expected: Option<Expected>,
@@ -2405,41 +2405,26 @@ impl<'a> Parser<'a> {
         if !matches!(first_pat.kind, PatKind::Ident(_, _, None) | PatKind::Path(..))
             || !self.look_ahead(1, |token| token.is_ident() && !token.is_reserved_ident())
         {
-            let mut snapshot_type = self.create_snapshot_for_diagnostic();
-            snapshot_type.bump(); // `:`
-            match snapshot_type.parse_ty() {
-                Err(inner_err) => {
-                    inner_err.cancel();
-                }
-                Ok(ty) => {
-                    let Err(mut err) = self.expected_one_of_not_found(&[], &[]) else {
-                        return first_pat;
-                    };
-                    err.span_label(ty.span, "specifying the type of a pattern isn't supported");
-                    self.restore_snapshot(snapshot_type);
-                    let span = first_pat.span.to(ty.span);
-                    first_pat = self.mk_pat(span, PatKind::Wild);
-                    err.emit();
-                }
-            }
             return first_pat;
         }
         // The pattern looks like it might be a path with a `::` -> `:` typo:
         // `match foo { bar:baz => {} }`
-        let colon_span = self.token.span;
+        let span = self.token.span;
         // We only emit "unexpected `:`" error here if we can successfully parse the
         // whole pattern correctly in that case.
-        let mut snapshot_pat = self.create_snapshot_for_diagnostic();
-        let mut snapshot_type = self.create_snapshot_for_diagnostic();
+        let snapshot = self.create_snapshot_for_diagnostic();
 
         // Create error for "unexpected `:`".
         match self.expected_one_of_not_found(&[], &[]) {
             Err(mut err) => {
-                snapshot_pat.bump(); // Skip the `:`.
-                snapshot_type.bump(); // Skip the `:`.
-                match snapshot_pat.parse_pat_no_top_alt(expected) {
+                self.bump(); // Skip the `:`.
+                match self.parse_pat_no_top_alt(expected) {
                     Err(inner_err) => {
+                        // Carry on as if we had not done anything, callers will emit a
+                        // reasonable error.
                         inner_err.cancel();
+                        err.cancel();
+                        self.restore_snapshot(snapshot);
                     }
                     Ok(mut pat) => {
                         // We've parsed the rest of the pattern.
@@ -2503,33 +2488,22 @@ impl<'a> Parser<'a> {
                             _ => {}
                         }
                         if show_sugg {
-                            err.span_suggestion_verbose(
-                                colon_span.until(self.look_ahead(1, |t| t.span)),
+                            err.span_suggestion(
+                                span,
                                 "maybe write a path separator here",
                                 "::",
                                 Applicability::MaybeIncorrect,
                             );
                         } else {
                             first_pat = self.mk_pat(new_span, PatKind::Wild);
                         }
-                        self.restore_snapshot(snapshot_pat);
+                        err.emit();
                     }
                 }
-                match snapshot_type.parse_ty() {
-                    Err(inner_err) => {
-                        inner_err.cancel();
-                    }
-                    Ok(ty) => {
-                        err.span_label(ty.span, "specifying the type of a pattern isn't supported");
-                        self.restore_snapshot(snapshot_type);
-                        let new_span = first_pat.span.to(ty.span);
-                        first_pat = self.mk_pat(new_span, PatKind::Wild);
-                    }
-                }
-                err.emit();
             }
             _ => {
                 // Carry on as if we had not done anything. This should be unreachable.
+                self.restore_snapshot(snapshot);
             }
         };
         first_pat

compiler/rustc_parse/src/parser/pat.rs

@@ -148,8 +148,7 @@ impl<'a> Parser<'a> {
 
             // Check if the user wrote `foo:bar` instead of `foo::bar`.
             if ra == RecoverColon::Yes {
-                first_pat =
-                    self.maybe_recover_colon_colon_in_pat_typo_or_anon_enum(first_pat, expected);
+                first_pat = self.maybe_recover_colon_colon_in_pat_typo(first_pat, expected);
             }
 
             if let Some(leading_vert_span) = leading_vert_span {

compiler/rustc_parse/src/parser/ty.rs

@@ -17,7 +17,6 @@ use rustc_ast::{
     self as ast, BareFnTy, FnRetTy, GenericBound, GenericBounds, GenericParam, Generics, Lifetime,
     MacCall, MutTy, Mutability, PolyTraitRef, TraitBoundModifier, TraitObjectSyntax, Ty, TyKind,
 };
-use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, PResult};
 use rustc_span::source_map::Span;
 use rustc_span::symbol::{kw, sym, Ident};
@@ -50,24 +49,17 @@ pub(super) enum AllowPlus {
     No,
 }
 
-#[derive(PartialEq, Clone, Copy)]
+#[derive(PartialEq)]
 pub(super) enum RecoverQPath {
     Yes,
     No,
 }
 
-#[derive(PartialEq, Clone, Copy)]
 pub(super) enum RecoverQuestionMark {
     Yes,
     No,
 }
 
-#[derive(PartialEq, Clone, Copy)]
-pub(super) enum RecoverAnonEnum {
-    Yes,
-    No,
-}
-
 /// Signals whether parsing a type should recover `->`.
 ///
 /// More specifically, when parsing a function like:
@@ -100,7 +92,7 @@ impl RecoverReturnSign {
 }
 
 // Is `...` (`CVarArgs`) legal at this level of type parsing?
-#[derive(PartialEq, Clone, Copy)]
+#[derive(PartialEq)]
 enum AllowCVariadic {
     Yes,
     No,
@@ -125,7 +117,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::Yes,
             None,
             RecoverQuestionMark::Yes,
-            RecoverAnonEnum::No,
         )
     }
 
@@ -140,7 +131,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::Yes,
             Some(ty_params),
             RecoverQuestionMark::Yes,
-            RecoverAnonEnum::No,
         )
     }
 
@@ -155,7 +145,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::Yes,
             None,
             RecoverQuestionMark::Yes,
-            RecoverAnonEnum::Yes,
         )
     }
 
@@ -173,7 +162,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::Yes,
             None,
             RecoverQuestionMark::Yes,
-            RecoverAnonEnum::No,
         )
     }
 
@@ -187,7 +175,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::Yes,
             None,
             RecoverQuestionMark::No,
-            RecoverAnonEnum::No,
         )
     }
 
@@ -199,7 +186,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::Yes,
             None,
             RecoverQuestionMark::No,
-            RecoverAnonEnum::No,
         )
     }
 
@@ -212,7 +198,6 @@ impl<'a> Parser<'a> {
             RecoverReturnSign::OnlyFatArrow,
             None,
             RecoverQuestionMark::Yes,
-            RecoverAnonEnum::No,
         )
     }
 
@@ -232,7 +217,6 @@ impl<'a> Parser<'a> {
                 recover_return_sign,
                 None,
                 RecoverQuestionMark::Yes,
-                RecoverAnonEnum::Yes,
             )?;
             FnRetTy::Ty(ty)
         } else if recover_return_sign.can_recover(&self.token.kind) {
@@ -247,7 +231,6 @@ impl<'a> Parser<'a> {
                 recover_return_sign,
                 None,
                 RecoverQuestionMark::Yes,
-                RecoverAnonEnum::Yes,
             )?;
             FnRetTy::Ty(ty)
         } else {
@@ -263,7 +246,6 @@ impl<'a> Parser<'a> {
         recover_return_sign: RecoverReturnSign,
         ty_generics: Option<&Generics>,
         recover_question_mark: RecoverQuestionMark,
-        recover_anon_enum: RecoverAnonEnum,
     ) -> PResult<'a, P<Ty>> {
         let allow_qpath_recovery = recover_qpath == RecoverQPath::Yes;
         maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
@@ -347,50 +329,9 @@ impl<'a> Parser<'a> {
             AllowPlus::Yes => self.maybe_recover_from_bad_type_plus(&ty)?,
             AllowPlus::No => self.maybe_report_ambiguous_plus(impl_dyn_multi, &ty),
         }
-        if RecoverQuestionMark::Yes == recover_question_mark {
+        if let RecoverQuestionMark::Yes = recover_question_mark {
             ty = self.maybe_recover_from_question_mark(ty);
         }
-        if recover_anon_enum == RecoverAnonEnum::Yes
-            && self.check_noexpect(&token::BinOp(token::Or))
-            && self.look_ahead(1, |t| t.can_begin_type())
-        {
-            let mut pipes = vec![self.token.span];
-            let mut types = vec![ty];
-            loop {
-                if !self.eat(&token::BinOp(token::Or)) {
-                    break;
-                }
-                pipes.push(self.prev_token.span);
-                types.push(self.parse_ty_common(
-                    allow_plus,
-                    allow_c_variadic,
-                    recover_qpath,
-                    recover_return_sign,
-                    ty_generics,
-                    recover_question_mark,
-                    RecoverAnonEnum::No,
-                )?);
-            }
-            let mut err = self.struct_span_err(pipes, "anonymous enums are not supported");
-            for ty in &types {
-                err.span_label(ty.span, "");
-            }
-            err.help(&format!(
-                "create a named `enum` and use it here instead:\nenum Name {{\n{}\n}}",
-                types
-                    .iter()
-                    .enumerate()
-                    .map(|(i, t)| format!(
-                        "    Variant{}({}),",
-                        i + 1, // Lets not confuse people with zero-indexing :)
-                        pprust::to_string(|s| s.print_type(&t)),
-                    ))
-                    .collect::<Vec<_>>()
-                    .join("\n"),
-            ));
-            err.emit();
-            return Ok(self.mk_ty(lo.to(self.prev_token.span), TyKind::Err));
-        }
         if allow_qpath_recovery { self.maybe_recover_from_bad_qpath(ty) } else { Ok(ty) }
     }
 

tests/ui/parser/anon-enums-are-ambiguous.rs

@@ -0,0 +1,26 @@
+// check-pass
+
+macro_rules! test_expr {
+    ($expr:expr) => {};
+}
+
+macro_rules! test_ty {
+    ($a:ty | $b:ty) => {};
+}
+
+fn main() {
+    test_expr!(a as fn() -> B | C);
+    // Do not break the `|` operator.
+
+    test_expr!(|_: fn() -> B| C | D);
+    // Do not break `-> Ret` in closure args.
+
+    test_ty!(A | B);
+    // We can't support anon enums in arbitrary positions.
+
+    test_ty!(fn() -> A | B);
+    // Don't break fn ptrs.
+
+    test_ty!(impl Fn() -> A | B);
+    // Don't break parenthesized generics.
+}