From 3d479a0447154850975f1a395451bda51ea1f144 Mon Sep 17 00:00:00 2001
From: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Date: Fri, 31 May 2024 16:47:30 +0000
Subject: [PATCH] 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               |  3 ++
 .../rustc_parse/src/parser/nonterminal.rs     |  2 ++
 .../expr_2024_underscore_expr.edi2021.stderr  | 32 +++++++++++++++++++
 .../expr_2024_underscore_expr.edi2024.stderr  | 17 ++++++++++
 tests/ui/macros/expr_2024_underscore_expr.rs  | 26 +++++++++++++++
 5 files changed, 80 insertions(+)
 create mode 100644 tests/ui/macros/expr_2024_underscore_expr.edi2021.stderr
 create mode 100644 tests/ui/macros/expr_2024_underscore_expr.edi2024.stderr
 create mode 100644 tests/ui/macros/expr_2024_underscore_expr.rs

diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index 109c401bb6a20..1e47e59beae54 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/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 {
diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index a0b704aeea5fb..a4eee91107eeb 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/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()
diff --git a/tests/ui/macros/expr_2024_underscore_expr.edi2021.stderr b/tests/ui/macros/expr_2024_underscore_expr.edi2021.stderr
new file mode 100644
index 0000000000000..09b7de2a5f19c
--- /dev/null
+++ b/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
+
diff --git a/tests/ui/macros/expr_2024_underscore_expr.edi2024.stderr b/tests/ui/macros/expr_2024_underscore_expr.edi2024.stderr
new file mode 100644
index 0000000000000..e6c2bef10e299
--- /dev/null
+++ b/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
+
diff --git a/tests/ui/macros/expr_2024_underscore_expr.rs b/tests/ui/macros/expr_2024_underscore_expr.rs
new file mode 100644
index 0000000000000..5456cc948cae0
--- /dev/null
+++ b/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 `_`
+}
+