rustc_parser: 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
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

compiler/rustc_ast/src/token.rs

@@ -214,6 +214,9 @@ pub fn ident_can_begin_expr(name: Symbol, span: Span, is_raw: IdentIsRaw) -> boo
             kw::Static,
         ]
         .contains(&name)
+        // Please note the order of the check here, the edition is not check on every
+        // token by only on the possible one.
+        || (kw::Underscore == name && span.edition().at_least_rust_2024())
 }
 
 fn ident_can_begin_type(name: Symbol, span: Span, is_raw: IdentIsRaw) -> bool {

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -42,6 +42,8 @@ impl<'a> Parser<'a> {
                 && !token.is_keyword(kw::Let)
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Const)
+                // FIXME(vincenzopalazzo): query less time the edition
+                && (token.span.edition().at_least_rust_2024() && !token.is_keyword(kw::Underscore) )
             }
             NonterminalKind::Expr => {
                 token.can_begin_expr()

tests/ui/macros/expr_2024_underscore_expr.edi2021.stderr

@@ -0,0 +1,32 @@
+error: no rules expected the token `_`
+  --> $DIR/expr_2024_underscore_expr.rs:23:12
+   |
+LL | macro_rules! m2021 {
+   | ------------------ when calling this macro
+...
+LL |     m2021!(_);
+   |            ^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$e:expr_2021`
+  --> $DIR/expr_2024_underscore_expr.rs:11:6
+   |
+LL |     ($e:expr_2021) => {
+   |      ^^^^^^^^^^^^
+
+error: no rules expected the token `_`
+  --> $DIR/expr_2024_underscore_expr.rs:24:12
+   |
+LL | macro_rules! m2024 {
+   | ------------------ when calling this macro
+...
+LL |     m2024!(_);
+   |            ^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$e:expr`
+  --> $DIR/expr_2024_underscore_expr.rs:17:6
+   |
+LL |     ($e:expr) => {
+   |      ^^^^^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/macros/expr_2024_underscore_expr.edi2024.stderr

@@ -0,0 +1,17 @@
+error: no rules expected the token `_`
+  --> $DIR/expr_2024_underscore_expr.rs:23:12
+   |
+LL | macro_rules! m2021 {
+   | ------------------ when calling this macro
+...
+LL |     m2021!(_);
+   |            ^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$e:expr_2021`
+  --> $DIR/expr_2024_underscore_expr.rs:11:6
+   |
+LL |     ($e:expr_2021) => {
+   |      ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/macros/expr_2024_underscore_expr.rs

@@ -0,0 +1,26 @@
+//@ revisions: edi2021 edi2024
+//@[edi2024]compile-flags: --edition=2024 -Z unstable-options
+//@[edi2021]compile-flags: --edition=2021
+
+// This test ensures that the `_` tok is considered an
+// expression on edition 2024.
+#![feature(expr_fragment_specifier_2024)]
+#![allow(incomplete_features)]
+
+macro_rules! m2021 {
+    ($e:expr_2021) => {
+        $e = 1;
+    };
+}
+
+macro_rules! m2024 {
+    ($e:expr) => {
+        $e = 1;
+    };
+}
+
+fn main() {
+    m2021!(_); //~ ERROR: no rules expected the token `_`
+    m2024!(_); //[edi2021]~ ERROR: no rules expected the token `_`
+}
+