Handle $_ in mbe

crates/mbe/src/mbe_expander/transcriber.rs

@@ -97,7 +97,7 @@ fn expand_subtree(
                 err = err.or(e);
                 arena.push(tt.into());
             }
-            Op::Var { name, kind: _ } => {
+            Op::Var { name, .. } => {
                 let ExpandResult { value: fragment, err: e } = expand_var(ctx, name);
                 err = err.or(e);
                 push_fragment(arena, fragment);

crates/mbe/src/parser.rs

@@ -101,7 +101,9 @@ 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(_) => {
+                        return Err(ExpandError::UnexpectedToken);
+                    }
                     tt::Leaf::Ident(ident) => {
                         let name = &ident.text;
                         let kind = eat_fragment_kind(src, mode)?;

crates/mbe/src/syntax_bridge.rs

@@ -313,7 +313,7 @@ trait TokenConvertor {
             return;
         }
 
-        result.push(if k.is_punct() {
+        result.push(if k.is_punct() && k != UNDERSCORE {
             assert_eq!(range.len(), TextSize::of('.'));
             let delim = match k {
                 T!['('] => Some((tt::DelimiterKind::Parenthesis, T![')'])),
@@ -378,6 +378,7 @@ trait TokenConvertor {
             let leaf: tt::Leaf = match k {
                 T![true] | T![false] => make_leaf!(Ident),
                 IDENT => make_leaf!(Ident),
+                UNDERSCORE => make_leaf!(Ident),
                 k if k.is_keyword() => make_leaf!(Ident),
                 k if k.is_literal() => make_leaf!(Literal),
                 LIFETIME_IDENT => {

crates/mbe/src/tests.rs

@@ -991,6 +991,18 @@ fn test_tt_composite2() {
     );
 }
 
+#[test]
+fn test_underscore() {
+    parse_macro(
+        r#"
+            macro_rules! foo {
+                 ($_:tt) => { 0 }
+            }
+    "#,
+    )
+    .assert_expand_items(r#"foo! { => }"#, r#"0"#);
+}
+
 #[test]
 fn test_lifetime() {
     parse_macro(