keep const blocks around

BoxyUwU · BoxyUwU · commit a86cfbbaabb6 · 2026-02-06T19:36:39.000Z
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
@@ -827,7 +827,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir_id,
             def_id: self.local_def_id(v.id),
             data: self.lower_variant_data(hir_id, item_kind, &v.data),
-            disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const_to_anon_const(e)),
+            disr_expr: v
+                .disr_expr
+                .as_ref()
+                .map(|e| self.lower_anon_const_to_anon_const(e, e.value.span)),
             ident: self.lower_ident(v.ident),
             span: self.lower_span(v.span),
         }
@@ -917,7 +920,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
             },
             vis_span: self.lower_span(f.vis.span),
-            default: f.default.as_ref().map(|v| self.lower_anon_const_to_anon_const(v)),
+            default: f
+                .default
+                .as_ref()
+                .map(|v| self.lower_anon_const_to_anon_const(v, v.value.span)),
             ty,
             safety: self.lower_safety(f.safety, hir::Safety::Safe),
         }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
@@ -2425,15 +2425,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 );
 
                 let lowered_args = self.arena.alloc_from_iter(args.iter().map(|arg| {
-                    let const_arg = if let ExprKind::ConstBlock(anon_const) = &arg.kind {
-                        let def_id = self.local_def_id(anon_const.id);
-                        let def_kind = self.tcx.def_kind(def_id);
-                        assert_eq!(DefKind::AnonConst, def_kind);
-                        self.lower_anon_const_to_const_arg(anon_const)
-                    } else {
-                        self.lower_expr_to_const_arg_direct(arg)
-                    };
-
+                    let const_arg = self.lower_expr_to_const_arg_direct(arg);
                     &*self.arena.alloc(const_arg)
                 }));
 
@@ -2445,16 +2437,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
             ExprKind::Tup(exprs) => {
                 let exprs = self.arena.alloc_from_iter(exprs.iter().map(|expr| {
-                    let expr = if let ExprKind::ConstBlock(anon_const) = &expr.kind {
-                        let def_id = self.local_def_id(anon_const.id);
-                        let def_kind = self.tcx.def_kind(def_id);
-                        assert_eq!(DefKind::AnonConst, def_kind);
-
-                        self.lower_anon_const_to_const_arg(anon_const)
-                    } else {
-                        self.lower_expr_to_const_arg_direct(&expr)
-                    };
-
+                    let expr = self.lower_expr_to_const_arg_direct(&expr);
                     &*self.arena.alloc(expr)
                 }));
 
@@ -2494,16 +2477,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // then go unused as the `Target::ExprField` is not actually
                     // corresponding to `Node::ExprField`.
                     self.lower_attrs(hir_id, &f.attrs, f.span, Target::ExprField);
-
-                    let expr = if let ExprKind::ConstBlock(anon_const) = &f.expr.kind {
-                        let def_id = self.local_def_id(anon_const.id);
-                        let def_kind = self.tcx.def_kind(def_id);
-                        assert_eq!(DefKind::AnonConst, def_kind);
-
-                        self.lower_anon_const_to_const_arg(anon_const)
-                    } else {
-                        self.lower_expr_to_const_arg_direct(&f.expr)
-                    };
+                    let expr = self.lower_expr_to_const_arg_direct(&f.expr);
 
                     &*self.arena.alloc(hir::ConstArgExprField {
                         hir_id,
@@ -2521,13 +2495,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
             ExprKind::Array(elements) => {
                 let lowered_elems = self.arena.alloc_from_iter(elements.iter().map(|element| {
-                    let const_arg = if let ExprKind::ConstBlock(anon_const) = &element.kind {
-                        let def_id = self.local_def_id(anon_const.id);
-                        assert_eq!(DefKind::AnonConst, self.tcx.def_kind(def_id));
-                        self.lower_anon_const_to_const_arg(anon_const)
-                    } else {
-                        self.lower_expr_to_const_arg_direct(element)
-                    };
+                    let const_arg = self.lower_expr_to_const_arg_direct(element);
                     &*self.arena.alloc(const_arg)
                 }));
                 let array_expr = self.arena.alloc(hir::ConstArgArrayExpr {
@@ -2557,6 +2525,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             | ExprKind::Call(..)
                             | ExprKind::Tup(..)
                             | ExprKind::Array(..)
+                            | ExprKind::ConstBlock(..)
                     )
                 {
                     return self.lower_expr_to_const_arg_direct(expr);
@@ -2574,6 +2543,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     span,
                 }
             }
+            ExprKind::ConstBlock(anon_const) => {
+                let def_id = self.local_def_id(anon_const.id);
+                assert_eq!(DefKind::AnonConst, self.tcx.def_kind(def_id));
+                self.lower_anon_const_to_const_arg(anon_const, span)
+            }
             _ => overly_complex_const(self),
         }
     }
@@ -2584,11 +2558,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         anon: &AnonConst,
     ) -> &'hir hir::ConstArg<'hir> {
-        self.arena.alloc(self.lower_anon_const_to_const_arg(anon))
+        self.arena.alloc(self.lower_anon_const_to_const_arg(anon, anon.value.span))
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn lower_anon_const_to_const_arg(&mut self, anon: &AnonConst) -> hir::ConstArg<'hir> {
+    fn lower_anon_const_to_const_arg(
+        &mut self,
+        anon: &AnonConst,
+        span: Span,
+    ) -> hir::ConstArg<'hir> {
         let tcx = self.tcx;
 
         // We cannot change parsing depending on feature gates available,
@@ -2599,7 +2577,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         if tcx.features().min_generic_const_args() {
             return match anon.mgca_disambiguation {
                 MgcaDisambiguation::AnonConst => {
-                    let lowered_anon = self.lower_anon_const_to_anon_const(anon);
+                    let lowered_anon = self.lower_anon_const_to_anon_const(anon, span);
                     ConstArg {
                         hir_id: self.next_id(),
                         kind: hir::ConstArgKind::Anon(lowered_anon),
@@ -2645,7 +2623,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             };
         }
 
-        let lowered_anon = self.lower_anon_const_to_anon_const(anon);
+        let lowered_anon = self.lower_anon_const_to_anon_const(anon, anon.value.span);
         ConstArg {
             hir_id: self.next_id(),
             kind: hir::ConstArgKind::Anon(lowered_anon),
@@ -2655,15 +2633,19 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     /// See [`hir::ConstArg`] for when to use this function vs
     /// [`Self::lower_anon_const_to_const_arg`].
-    fn lower_anon_const_to_anon_const(&mut self, c: &AnonConst) -> &'hir hir::AnonConst {
+    fn lower_anon_const_to_anon_const(
+        &mut self,
+        c: &AnonConst,
+        span: Span,
+    ) -> &'hir hir::AnonConst {
         self.arena.alloc(self.with_new_scopes(c.value.span, |this| {
             let def_id = this.local_def_id(c.id);
             let hir_id = this.lower_node_id(c.id);
             hir::AnonConst {
                 def_id,
                 hir_id,
                 body: this.lower_const_body(c.value.span, Some(&c.value)),
-                span: this.lower_span(c.value.span),
+                span: this.lower_span(span),
             }
         }))
     }
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
@@ -1627,16 +1627,8 @@ impl<'a> Parser<'a> {
             let first_expr = self.parse_expr()?;
             if self.eat(exp!(Semi)) {
                 // Repeating array syntax: `[ 0; 512 ]`
-                let count = if self.eat_keyword(exp!(Const)) {
-                    // While we could just disambiguate `Direct` from `AnonConst` by
-                    // treating all const block exprs as `AnonConst`, that would
-                    // complicate the DefCollector and likely all other visitors.
-                    // So we strip the const blockiness and just store it as a block
-                    // in the AST with the extra disambiguator on the AnonConst
-                    self.parse_mgca_const_block(false)?
-                } else {
-                    self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?
-                };
+                let count =
+                    self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?;
                 self.expect(close)?;
                 ExprKind::Repeat(first_expr, count)
             } else if self.eat(exp!(Comma)) {
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
@@ -33,9 +33,9 @@ use rustc_ast::tokenstream::{
 };
 use rustc_ast::util::case::Case;
 use rustc_ast::{
-    self as ast, AnonConst, AttrArgs, AttrId, BlockCheckMode, ByRef, Const, CoroutineKind,
-    DUMMY_NODE_ID, DelimArgs, Expr, ExprKind, Extern, HasAttrs, HasTokens, MgcaDisambiguation,
-    Mutability, Recovered, Safety, StrLit, Visibility, VisibilityKind,
+    self as ast, AnonConst, AttrArgs, AttrId, ByRef, Const, CoroutineKind, DUMMY_NODE_ID,
+    DelimArgs, Expr, ExprKind, Extern, HasAttrs, HasTokens, MgcaDisambiguation, Mutability,
+    Recovered, Safety, StrLit, Visibility, VisibilityKind,
 };
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::debug_assert_matches;
@@ -1269,19 +1269,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn parse_mgca_const_block(&mut self, gate_syntax: bool) -> PResult<'a, AnonConst> {
-        let kw_span = self.prev_token.span;
-        let value = self.parse_expr_block(None, kw_span, BlockCheckMode::Default)?;
-        if gate_syntax {
-            self.psess.gated_spans.gate(sym::min_generic_const_args, kw_span.to(value.span));
-        }
-        Ok(AnonConst {
-            id: ast::DUMMY_NODE_ID,
-            value,
-            mgca_disambiguation: MgcaDisambiguation::AnonConst,
-        })
-    }
-
     /// Parses inline const expressions.
     fn parse_const_block(&mut self, span: Span) -> PResult<'a, Box<Expr>> {
         self.expect_keyword(exp!(Const))?;
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
@@ -847,6 +847,7 @@ impl<'a> Parser<'a> {
     /// - A literal.
     /// - A numeric literal prefixed by `-`.
     /// - A single-segment path.
+    /// - A const block (under mGCA)
     pub(super) fn expr_is_valid_const_arg(&self, expr: &Box<rustc_ast::Expr>) -> bool {
         match &expr.kind {
             ast::ExprKind::Block(_, _)
@@ -863,6 +864,10 @@ impl<'a> Parser<'a> {
             {
                 true
             }
+            ast::ExprKind::ConstBlock(_) => {
+                self.psess.gated_spans.gate(sym::min_generic_const_args, expr.span);
+                true
+            }
             _ => false,
         }
     }
@@ -874,14 +879,6 @@ impl<'a> Parser<'a> {
         let (value, mgca_disambiguation) = if self.token.kind == token::OpenBrace {
             let value = self.parse_expr_block(None, self.token.span, BlockCheckMode::Default)?;
             (value, MgcaDisambiguation::Direct)
-        } else if self.eat_keyword(exp!(Const)) {
-            // While we could just disambiguate `Direct` from `AnonConst` by
-            // treating all const block exprs as `AnonConst`, that would
-            // complicate the DefCollector and likely all other visitors.
-            // So we strip the const blockiness and just store it as a block
-            // in the AST with the extra disambiguator on the AnonConst
-            let value = self.parse_mgca_const_block(true)?;
-            (value.value, MgcaDisambiguation::AnonConst)
         } else {
             self.parse_unambiguous_unbraced_const_arg()?
         };
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
@@ -658,16 +658,8 @@ impl<'a> Parser<'a> {
         };
 
         let ty = if self.eat(exp!(Semi)) {
-            let mut length = if self.eat_keyword(exp!(Const)) {
-                // While we could just disambiguate `Direct` from `AnonConst` by
-                // treating all const block exprs as `AnonConst`, that would
-                // complicate the DefCollector and likely all other visitors.
-                // So we strip the const blockiness and just store it as a block
-                // in the AST with the extra disambiguator on the AnonConst
-                self.parse_mgca_const_block(false)?
-            } else {
-                self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?
-            };
+            let mut length =
+                self.parse_expr_anon_const(|this, expr| this.mgca_direct_lit_hack(expr))?;
 
             if let Err(e) = self.expect(exp!(CloseBracket)) {
                 // Try to recover from `X<Y, ...>` when `X::<Y, ...>` works
diff --git a/src/tools/rustfmt/tests/source/issue-6788.rs b/src/tools/rustfmt/tests/source/issue-6788.rs
@@ -0,0 +1,7 @@
+fn foo() {
+    let a = [(); const { let x = 1; x }];
+}
+
+fn foo() {
+    let x = [(); const { 1 }];
+}
diff --git a/src/tools/rustfmt/tests/target/issue-6788.rs b/src/tools/rustfmt/tests/target/issue-6788.rs
@@ -0,0 +1,10 @@
+fn foo() {
+    let a = [(); const {
+        let x = 1;
+        x
+    }];
+}
+
+fn foo() {
+    let x = [(); const { 1 }];
+}
