Rollup merge of #126697 - vincenzopalazzo:macros/find_the_expression_tok, r=eholk,compiler-errors

[RFC] mbe: consider the `_` in 2024 an expression

This commit is adding the possibility to parse the `_` as an expression inside the esition 2024.

Link: https://rust-lang.zulipchat.com/#narrow/stream/404510-wg-macros/topic/supporting.20.60_.60.20expressions

Issue https://github.com/rust-lang/rust/issues/123742

r? `@eholk`

1 files changed, 10 insertions, 1 deletions

diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index bd9d96ba573..999f6f0eeb0 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -39,6 +39,7 @@ impl<'a> Parser<'a> {
         }
 
         match kind {
+            // `expr_2021` and earlier
             NonterminalKind::Expr(Expr2021 { .. }) => {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
@@ -46,8 +47,16 @@ impl<'a> Parser<'a> {
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Const)
             }
+            // Current edition expressions
             NonterminalKind::Expr(Expr) => {
-                token.can_begin_expr()
+                // In Edition 2024, `_` is considered an expression, so we
+                // need to allow it here because `token.can_begin_expr()` does
+                // not consider `_` to be an expression.
+                //
+                // Because `can_begin_expr` is used elsewhere, we need to reduce
+                // the scope of where the `_` is considered an expression to
+                // just macro parsing code.
+                (token.can_begin_expr() || token.is_keyword(kw::Underscore))
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
             }