Stabilize let chains in the 2024 edition

compiler/rustc_parse/src/parser/expr.rs

@@ -26,6 +26,7 @@ use rustc_macros::Subdiagnostic;
 use rustc_session::errors::{ExprParenthesesNeeded, report_lit_error};
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::BREAK_WITH_LABEL_AND_LOOP;
+use rustc_span::edition::Edition;
 use rustc_span::source_map::{self, Spanned};
 use rustc_span::{BytePos, ErrorGuaranteed, Ident, Pos, Span, Symbol, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
@@ -2605,7 +2606,10 @@ impl<'a> Parser<'a> {
     /// Parses an `if` expression (`if` token already eaten).
     fn parse_expr_if(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
-        let cond = self.parse_expr_cond()?;
+        // Scoping code checks the top level edition of the `if`; let's match it here.
+        // The `CondChecker` also checks the edition of the `let` itself, just to make sure.
+        let let_chains_policy = LetChainsPolicy::EditionDependent { current_edition: lo.edition() };
+        let cond = self.parse_expr_cond(let_chains_policy)?;
         self.parse_if_after_cond(lo, cond)
     }
 
@@ -2714,18 +2718,17 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses the condition of a `if` or `while` expression.
+    ///
+    /// The specified `edition` in `let_chains_policy` should be that of the whole `if` construct,
+    /// i.e. the same span we use to later decide whether the drop behaviour should be that of
+    /// edition `..=2021` or that of `2024..`.
     // Public because it is used in rustfmt forks such as https://github.com/tucant/rustfmt/blob/30c83df9e1db10007bdd16dafce8a86b404329b2/src/parse/macros/html.rs#L57 for custom if expressions.
-    pub fn parse_expr_cond(&mut self) -> PResult<'a, P<Expr>> {
+    pub fn parse_expr_cond(&mut self, let_chains_policy: LetChainsPolicy) -> PResult<'a, P<Expr>> {
         let attrs = self.parse_outer_attributes()?;
         let (mut cond, _) =
             self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL | Restrictions::ALLOW_LET, attrs)?;
 
-        CondChecker::new(self).visit_expr(&mut cond);
-
-        if let ExprKind::Let(_, _, _, Recovered::No) = cond.kind {
-            // Remove the last feature gating of a `let` expression since it's stable.
-            self.psess.gated_spans.ungate_last(sym::let_chains, cond.span);
-        }
+        CondChecker::new(self, let_chains_policy).visit_expr(&mut cond);
 
         Ok(cond)
     }
@@ -3020,7 +3023,8 @@ impl<'a> Parser<'a> {
 
     /// Parses a `while` or `while let` expression (`while` token already eaten).
     fn parse_expr_while(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
-        let cond = self.parse_expr_cond().map_err(|mut err| {
+        let policy = LetChainsPolicy::EditionDependent { current_edition: lo.edition() };
+        let cond = self.parse_expr_cond(policy).map_err(|mut err| {
             err.span_label(lo, "while parsing the condition of this `while` expression");
             err
         })?;
@@ -3404,17 +3408,17 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_match_arm_guard(&mut self) -> PResult<'a, Option<P<Expr>>> {
-        // Used to check the `let_chains` and `if_let_guard` features mostly by scanning
+        // Used to check the `if_let_guard` feature mostly by scanning
         // `&&` tokens.
-        fn check_let_expr(expr: &Expr) -> (bool, bool) {
+        fn has_let_expr(expr: &Expr) -> bool {
             match &expr.kind {
                 ExprKind::Binary(BinOp { node: BinOpKind::And, .. }, lhs, rhs) => {
-                    let lhs_rslt = check_let_expr(lhs);
-                    let rhs_rslt = check_let_expr(rhs);
-                    (lhs_rslt.0 || rhs_rslt.0, false)
+                    let lhs_rslt = has_let_expr(lhs);
+                    let rhs_rslt = has_let_expr(rhs);
+                    lhs_rslt || rhs_rslt
                 }
-                ExprKind::Let(..) => (true, true),
-                _ => (false, true),
+                ExprKind::Let(..) => true,
+                _ => false,
             }
         }
         if !self.eat_keyword(exp!(If)) {
@@ -3425,14 +3429,9 @@ impl<'a> Parser<'a> {
         let if_span = self.prev_token.span;
         let mut cond = self.parse_match_guard_condition()?;
 
-        CondChecker::new(self).visit_expr(&mut cond);
+        CondChecker::new(self, LetChainsPolicy::AlwaysAllowed).visit_expr(&mut cond);
 
-        let (has_let_expr, does_not_have_bin_op) = check_let_expr(&cond);
-        if has_let_expr {
-            if does_not_have_bin_op {
-                // Remove the last feature gating of a `let` expression since it's stable.
-                self.psess.gated_spans.ungate_last(sym::let_chains, cond.span);
-            }
+        if has_let_expr(&cond) {
             let span = if_span.to(cond.span);
             self.psess.gated_spans.gate(sym::if_let_guard, span);
         }
@@ -3459,7 +3458,7 @@ impl<'a> Parser<'a> {
                     unreachable!()
                 };
                 self.psess.gated_spans.ungate_last(sym::guard_patterns, cond.span);
-                CondChecker::new(self).visit_expr(&mut cond);
+                CondChecker::new(self, LetChainsPolicy::AlwaysAllowed).visit_expr(&mut cond);
                 let right = self.prev_token.span;
                 self.dcx().emit_err(errors::ParenthesesInMatchPat {
                     span: vec![left, right],
@@ -4038,7 +4037,14 @@ pub(crate) enum ForbiddenLetReason {
     NotSupportedParentheses(#[primary_span] Span),
 }
 
-/// Visitor to check for invalid/unstable use of `ExprKind::Let` that can't
+/// Whether let chains are allowed on all editions, or it's edition dependent (allowed only on
+/// 2024 and later). In case of edition dependence, specify the currently present edition.
+pub enum LetChainsPolicy {
+    AlwaysAllowed,
+    EditionDependent { current_edition: Edition },
+}
+
+/// Visitor to check for invalid use of `ExprKind::Let` that can't
 /// easily be caught in parsing. For example:
 ///
 /// ```rust,ignore (example)
@@ -4049,19 +4055,29 @@ pub(crate) enum ForbiddenLetReason {
 /// ```
 struct CondChecker<'a> {
     parser: &'a Parser<'a>,
+    let_chains_policy: LetChainsPolicy,
+    depth: u32,
     forbid_let_reason: Option<ForbiddenLetReason>,
     missing_let: Option<errors::MaybeMissingLet>,
     comparison: Option<errors::MaybeComparison>,
 }
 
 impl<'a> CondChecker<'a> {
-    fn new(parser: &'a Parser<'a>) -> Self {
-        CondChecker { parser, forbid_let_reason: None, missing_let: None, comparison: None }
+    fn new(parser: &'a Parser<'a>, let_chains_policy: LetChainsPolicy) -> Self {
+        CondChecker {
+            parser,
+            forbid_let_reason: None,
+            missing_let: None,
+            comparison: None,
+            let_chains_policy,
+            depth: 0,
+        }
     }
 }
 
 impl MutVisitor for CondChecker<'_> {
     fn visit_expr(&mut self, e: &mut P<Expr>) {
+        self.depth += 1;
         use ForbiddenLetReason::*;
 
         let span = e.span;
@@ -4076,8 +4092,16 @@ impl MutVisitor for CondChecker<'_> {
                             comparison: self.comparison,
                         },
                     ));
-                } else {
-                    self.parser.psess.gated_spans.gate(sym::let_chains, span);
+                } else if self.depth > 1 {
+                    // Top level `let` is always allowed; only gate chains
+                    match self.let_chains_policy {
+                        LetChainsPolicy::AlwaysAllowed => (),
+                        LetChainsPolicy::EditionDependent { current_edition } => {
+                            if !current_edition.at_least_rust_2024() || !span.at_least_rust_2024() {
+                                self.parser.psess.gated_spans.gate(sym::let_chains, span);
+                            }
+                        }
+                    }
                 }
             }
             ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, _, _) => {
@@ -4179,5 +4203,6 @@ impl MutVisitor for CondChecker<'_> {
                 // These would forbid any let expressions they contain already.
             }
         }
+        self.depth -= 1;
     }
 }

tests/mir-opt/building/logical_or_in_conditional.rs

@@ -1,6 +1,6 @@
 // skip-filecheck
 //@ compile-flags: -Z validate-mir
-#![feature(let_chains)]
+//@ edition: 2024
 struct Droppy(u8);
 impl Drop for Droppy {
     fn drop(&mut self) {

tests/mir-opt/building/logical_or_in_conditional.test_complex.built.after.mir

@@ -19,7 +19,7 @@ fn test_complex() -> () {
     bb0: {
         StorageLive(_1);
         StorageLive(_2);
-        _2 = E::f() -> [return: bb1, unwind: bb34];
+        _2 = E::f() -> [return: bb1, unwind: bb35];
     }
 
     bb1: {
@@ -42,7 +42,7 @@ fn test_complex() -> () {
 
     bb5: {
         StorageLive(_4);
-        _4 = always_true() -> [return: bb6, unwind: bb34];
+        _4 = always_true() -> [return: bb6, unwind: bb35];
     }
 
     bb6: {
@@ -64,7 +64,7 @@ fn test_complex() -> () {
     }
 
     bb9: {
-        drop(_7) -> [return: bb11, unwind: bb34];
+        drop(_7) -> [return: bb11, unwind: bb35];
     }
 
     bb10: {
@@ -78,7 +78,7 @@ fn test_complex() -> () {
     }
 
     bb12: {
-        drop(_7) -> [return: bb13, unwind: bb34];
+        drop(_7) -> [return: bb13, unwind: bb35];
     }
 
     bb13: {
@@ -98,7 +98,7 @@ fn test_complex() -> () {
     }
 
     bb15: {
-        drop(_10) -> [return: bb17, unwind: bb34];
+        drop(_10) -> [return: bb17, unwind: bb35];
     }
 
     bb16: {
@@ -113,11 +113,12 @@ fn test_complex() -> () {
 
     bb18: {
         _1 = const ();
+        StorageDead(_2);
         goto -> bb22;
     }
 
     bb19: {
-        drop(_10) -> [return: bb20, unwind: bb34];
+        drop(_10) -> [return: bb20, unwind: bb35];
     }
 
     bb20: {
@@ -127,6 +128,7 @@ fn test_complex() -> () {
     }
 
     bb21: {
+        StorageDead(_2);
         _1 = const ();
         goto -> bb22;
     }
@@ -135,18 +137,17 @@ fn test_complex() -> () {
         StorageDead(_8);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_2);
         StorageDead(_1);
         StorageLive(_11);
-        _11 = always_true() -> [return: bb23, unwind: bb34];
+        _11 = always_true() -> [return: bb23, unwind: bb35];
     }
 
     bb23: {
         switchInt(move _11) -> [0: bb25, otherwise: bb24];
     }
 
     bb24: {
-        goto -> bb32;
+        goto -> bb33;
     }
 
     bb25: {
@@ -155,7 +156,7 @@ fn test_complex() -> () {
 
     bb26: {
         StorageLive(_12);
-        _12 = E::f() -> [return: bb27, unwind: bb34];
+        _12 = E::f() -> [return: bb27, unwind: bb35];
     }
 
     bb27: {
@@ -178,21 +179,26 @@ fn test_complex() -> () {
 
     bb31: {
         _0 = const ();
-        goto -> bb33;
+        StorageDead(_12);
+        goto -> bb34;
     }
 
     bb32: {
-        _0 = const ();
+        StorageDead(_12);
         goto -> bb33;
     }
 
     bb33: {
+        _0 = const ();
+        goto -> bb34;
+    }
+
+    bb34: {
         StorageDead(_11);
-        StorageDead(_12);
         return;
     }
 
-    bb34 (cleanup): {
+    bb35 (cleanup): {
         resume;
     }
 }

tests/ui/deriving/auxiliary/malicious-macro.rs

@@ -1,4 +1,4 @@
-#![feature(let_chains)]
+//@ edition: 2024
 
 extern crate proc_macro;
 

tests/ui/drop/drop-order-comparisons.e2021.fixed

@@ -24,7 +24,7 @@
 //@ [e2024] edition: 2024
 //@ run-pass
 
-#![feature(let_chains)]
+#![cfg_attr(e2021, feature(let_chains))]
 #![cfg_attr(e2021, warn(rust_2024_compatibility))]
 
 fn t_bindings() {

tests/ui/drop/drop-order-comparisons.rs

@@ -24,7 +24,7 @@
 //@ [e2024] edition: 2024
 //@ run-pass
 
-#![feature(let_chains)]
+#![cfg_attr(e2021, feature(let_chains))]
 #![cfg_attr(e2021, warn(rust_2024_compatibility))]
 
 fn t_bindings() {

tests/ui/drop/drop_order.rs

@@ -2,9 +2,10 @@
 //@ compile-flags: -Z validate-mir
 //@ revisions: edition2021 edition2024
 //@ [edition2021] edition: 2021
+//@ [edition2024] compile-flags: -Z lint-mir
 //@ [edition2024] edition: 2024
 
-#![feature(let_chains)]
+#![cfg_attr(edition2021, feature(let_chains))]
 
 use std::cell::RefCell;
 use std::convert::TryInto;

tests/ui/drop/drop_order_if_let_rescope.rs

@@ -1,8 +1,6 @@
 //@ run-pass
 //@ edition:2024
-//@ compile-flags: -Z validate-mir
-
-#![feature(let_chains)]
+//@ compile-flags: -Z validate-mir -Z lint-mir
 
 use std::cell::RefCell;
 use std::convert::TryInto;

tests/ui/drop/issue-100276.rs

@@ -1,6 +1,11 @@
 //@ check-pass
 //@ compile-flags: -Z validate-mir
-#![feature(let_chains)]
+//@ revisions: edition2021 edition2024
+//@ [edition2021] edition: 2021
+//@ [edition2024] compile-flags: -Z lint-mir
+//@ [edition2024] edition: 2024
+
+#![cfg_attr(edition2021, feature(let_chains))]
 
 fn let_chains(entry: std::io::Result<std::fs::DirEntry>) {
     if let Ok(entry) = entry

tests/ui/expr/if/attrs/let-chains-attr.rs

@@ -1,6 +1,5 @@
 //@ check-pass
-
-#![feature(let_chains)]
+//@ edition:2024
 
 #[cfg(false)]
 fn foo() {

tests/ui/lint/issue-121070-let-range.rs

@@ -1,6 +1,6 @@
 //@ check-pass
+//@ edition:2024
 
-#![feature(let_chains)]
 #![allow(irrefutable_let_patterns)]
 fn main() {
     let _a = 0..1;

tests/ui/mir/issue-99852.rs

@@ -1,6 +1,6 @@
 //@ check-pass
 //@ compile-flags: -Z validate-mir
-#![feature(let_chains)]
+//@ edition: 2024
 
 fn lambda<T, U>() -> U
 where