Merge #7059

7059: Special case $_ in meta var instead of treat it as ident in mbe  r=lnicola a=edwin0cheng

In #6929, we treat '_' as an ident but rustc is only allow it in some special places (e.g. meta var in mbe , type, pat etc). 

This PR rollback that and we only make '$_' works in meta var matching. 

Fixes #7056 

Co-authored-by: Edwin Cheng <[email protected]>

1 files changed, 9 insertions, 2 deletions

diff --git a/crates/mbe/src/parser.rs b/crates/mbe/src/parser.rs
index c3fdd4040..d681905f5 100644
--- a/crates/mbe/src/parser.rs
+++ b/crates/mbe/src/parser.rs
@@ -101,8 +101,15 @@ fn next_op<'a>(
                     Op::Repeat { subtree, separator, kind }
                 }
                 tt::TokenTree::Leaf(leaf) => match leaf {
-                    tt::Leaf::Punct(_) => {
-                        return Err(ExpandError::UnexpectedToken);
+                    tt::Leaf::Punct(punct) => {
+                        static UNDERSCORE: SmolStr = SmolStr::new_inline("_");
+
+                        if punct.char != '_' {
+                            return Err(ExpandError::UnexpectedToken);
+                        }
+                        let name = &UNDERSCORE;
+                        let kind = eat_fragment_kind(src, mode)?;
+                        Op::Var { name, kind }
                     }
                     tt::Leaf::Ident(ident) => {
                         let name = &ident.text;