fix(expand): prevent infinity loop in macro containing only "///"

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -249,6 +249,7 @@ pub(super) fn compute_locs(matcher: &[TokenTree]) -> Vec<MatcherLoc> {
 }
 
 /// A single matcher position, representing the state of matching.
+#[derive(Debug)]
 struct MatcherPos {
     /// The index into `TtParser::locs`, which represents the "dot".
     idx: usize,

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -647,6 +647,7 @@ fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[mbe::TokenTree]) -> bool {
                 if seq.separator.is_none()
                     && seq.tts.iter().all(|seq_tt| match seq_tt {
                         TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Vis)) => true,
+                        TokenTree::Token(t) => matches!(t, Token { kind: DocComment(..), .. }),
                         TokenTree::Sequence(_, sub_seq) => {
                             sub_seq.kleene.op == mbe::KleeneOp::ZeroOrMore
                                 || sub_seq.kleene.op == mbe::KleeneOp::ZeroOrOne

tests/ui/macros/issue-112342-1.rs

@@ -0,0 +1,36 @@
+// same as #95267, ignore doc comment although it's a bug.
+
+macro_rules! m1 {
+    (
+        $(
+            ///
+        )*
+        //~^^^ERROR repetition matches empty token tree
+    ) => {};
+}
+
+m1! {}
+
+macro_rules! m2 {
+    (
+        $(
+            ///
+        )+
+        //~^^^ERROR repetition matches empty token tree
+    ) => {};
+}
+
+m2! {}
+
+macro_rules! m3 {
+    (
+        $(
+            ///
+        )?
+        //~^^^ERROR repetition matches empty token tree
+    ) => {};
+}
+
+m3! {}
+
+fn main() {}

tests/ui/macros/issue-112342-1.stderr

@@ -0,0 +1,29 @@
+error: repetition matches empty token tree
+  --> $DIR/issue-112342-1.rs:5:10
+   |
+LL |           $(
+   |  __________^
+LL | |             ///
+LL | |         )*
+   | |_________^
+
+error: repetition matches empty token tree
+  --> $DIR/issue-112342-1.rs:16:10
+   |
+LL |           $(
+   |  __________^
+LL | |             ///
+LL | |         )+
+   | |_________^
+
+error: repetition matches empty token tree
+  --> $DIR/issue-112342-1.rs:27:10
+   |
+LL |           $(
+   |  __________^
+LL | |             ///
+LL | |         )?
+   | |_________^
+
+error: aborting due to 3 previous errors
+

tests/ui/macros/issue-112342-2.rs

@@ -0,0 +1,28 @@
+// check-pass
+
+// same as #95267, ignore doc comment although it's a bug.
+
+macro_rules! m1 {
+    (
+        $(
+            ///
+            $expr: expr,
+        )*
+    ) => {};
+}
+
+m1! {}
+
+macro_rules! m2 {
+    (
+        $(
+            ///
+            $expr: expr,
+            ///
+        )*
+    ) => {};
+}
+
+m2! {}
+
+fn main() {}