Auto merge of #123865 - eholk:expr_2021, r=fmease

Update `expr` matcher for Edition 2024 and add `expr_2021` nonterminal

This commit adds a new nonterminal `expr_2021` in macro patterns, and `expr_fragment_specifier_2024` feature flag.

This change also updates `expr` so that on Edition 2024 it will also match `const { ... }` blocks, while `expr_2021` preserves the current behavior of `expr`, matching expressions without `const` blocks.

Joint work with `@vincenzopalazzo.`

Issue #123742

1 files changed, 10 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index a6f0ab78b5c..619c4c63e51 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -36,13 +36,19 @@ impl<'a> Parser<'a> {
         }
 
         match kind {
-            NonterminalKind::Expr => {
+            NonterminalKind::Expr2021 => {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Const)
             }
+            NonterminalKind::Expr => {
+                token.can_begin_expr()
+                // This exception is here for backwards compatibility.
+                && !token.is_keyword(kw::Let)
+                && (token.span.edition().at_least_rust_2024() || !token.is_keyword(kw::Const))
+            }
             NonterminalKind::Ty => token.can_begin_type(),
             NonterminalKind::Ident => get_macro_ident(token).is_some(),
             NonterminalKind::Literal => token.can_begin_literal_maybe_minus(),
@@ -143,7 +149,9 @@ impl<'a> Parser<'a> {
                 })?)
             }
 
-            NonterminalKind::Expr => NtExpr(self.parse_expr_force_collect()?),
+            NonterminalKind::Expr | NonterminalKind::Expr2021 => {
+                NtExpr(self.parse_expr_force_collect()?)
+            }
             NonterminalKind::Literal => {
                 // The `:literal` matcher does not support attributes
                 NtLiteral(self.collect_tokens_no_attrs(|this| this.parse_literal_maybe_minus())?)