Fix and test edge cases of _ as ident

crates/mbe/src/expander/matcher.rs

@@ -710,7 +710,6 @@ fn match_meta_var(kind: &str, input: &mut TtIter) -> ExpandResult<Option<Fragmen
             let tt_result = match kind {
                 "ident" => input
                     .expect_ident()
-                    .and_then(|ident| if ident.text == "_" { Err(()) } else { Ok(ident) })
                     .map(|ident| Some(tt::Leaf::from(ident.clone()).into()))
                     .map_err(|()| err!("expected ident")),
                 "tt" => input.expect_tt().map(Some).map_err(|()| err!()),
@@ -763,7 +762,7 @@ impl<'a> TtIter<'a> {
     fn expect_separator(&mut self, separator: &Separator, idx: usize) -> bool {
         let mut fork = self.clone();
         let ok = match separator {
-            Separator::Ident(lhs) if idx == 0 => match fork.expect_ident() {
+            Separator::Ident(lhs) if idx == 0 => match fork.expect_ident_or_underscore() {
                 Ok(rhs) => rhs.text == lhs.text,
                 _ => false,
             },
@@ -853,7 +852,7 @@ impl<'a> TtIter<'a> {
         if punct.char != '\'' {
             return Err(());
         }
-        let ident = self.expect_ident()?;
+        let ident = self.expect_ident_or_underscore()?;
 
         Ok(tt::Subtree {
             delimiter: None,

crates/mbe/src/tests/rule.rs

@@ -12,6 +12,9 @@ fn test_valid_arms() {
     }
 
     check("($i:ident) => ()");
+    check("($(x),*) => ()");
+    check("($(x)_*) => ()");
+    check("($(x)i*) => ()");
     check("($($i:ident)*) => ($_)");
     check("($($true:ident)*) => ($true)");
     check("($($false:ident)*) => ($false)");
@@ -32,6 +35,7 @@ fn test_invalid_arms() {
 
     check("($i) => ($i)", ParseError::UnexpectedToken("bad fragment specifier 1".into()));
     check("($i:) => ($i)", ParseError::UnexpectedToken("bad fragment specifier 1".into()));
+    check("($i:_) => ()", ParseError::UnexpectedToken("bad fragment specifier 1".into()));
 }
 
 fn parse_macro_arm(arm_definition: &str) -> Result<crate::MacroRules, ParseError> {

crates/mbe/src/tt_iter.rs

@@ -49,6 +49,13 @@ impl<'a> TtIter<'a> {
     }
 
     pub(crate) fn expect_ident(&mut self) -> Result<&'a tt::Ident, ()> {
+        match self.expect_leaf()? {
+            tt::Leaf::Ident(it) if it.text != "_" => Ok(it),
+            _ => Err(()),
+        }
+    }
+
+    pub(crate) fn expect_ident_or_underscore(&mut self) -> Result<&'a tt::Ident, ()> {
         match self.expect_leaf()? {
             tt::Leaf::Ident(it) => Ok(it),
             _ => Err(()),