feat(expand): emit note for doc comment in macro matcher

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -628,6 +628,40 @@ fn check_lhs_nt_follows(sess: &ParseSess, def: &ast::Item, lhs: &mbe::TokenTree)
     // after parsing/expansion. we can report every error in every macro this way.
 }
 
+fn is_empty_token_tree(sess: &ParseSess, seq: &mbe::SequenceRepetition) -> bool {
+    if seq.separator.is_some() {
+        false
+    } else {
+        let mut is_empty = true;
+        let mut iter = seq.tts.iter().peekable();
+        while let Some(tt) = iter.next() {
+            match tt {
+                mbe::TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Vis)) => {}
+                mbe::TokenTree::Token(t @ Token { kind: DocComment(..), .. }) => {
+                    let mut now = t;
+                    while let Some(&mbe::TokenTree::Token(
+                        next @ Token { kind: DocComment(..), .. },
+                    )) = iter.peek()
+                    {
+                        now = next;
+                        iter.next();
+                    }
+                    let span = t.span.to(now.span);
+                    sess.span_diagnostic.span_note_without_error(
+                        span,
+                        "doc comments are ignored in matcher position",
+                    );
+                }
+                mbe::TokenTree::Sequence(_, sub_seq)
+                    if (sub_seq.kleene.op == mbe::KleeneOp::ZeroOrMore
+                        || sub_seq.kleene.op == mbe::KleeneOp::ZeroOrOne) => {}
+                _ => is_empty = false,
+            }
+        }
+        is_empty
+    }
+}
+
 /// Checks that the lhs contains no repetition which could match an empty token
 /// tree, because then the matcher would hang indefinitely.
 fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[mbe::TokenTree]) -> bool {
@@ -644,17 +678,7 @@ fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[mbe::TokenTree]) -> bool {
                 }
             }
             TokenTree::Sequence(span, seq) => {
-                if seq.separator.is_none()
+                if is_empty_token_tree(sess, seq) {
-                    && 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
-                        }
-                        _ => false,
-                    })
-                {
                     let sp = span.entire();
                     sess.span_diagnostic.span_err(sp, "repetition matches empty token tree");
                     return false;

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

@@ -33,4 +33,17 @@ macro_rules! m3 {
 
 m3! {}
 
+
+macro_rules! m4 {
+    (
+        $(
+            ///
+            ///
+        )*
+        //~^^^^ERROR repetition matches empty token tree
+    ) => {};
+}
+
+m4! {}
+
 fn main() {}

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

@@ -1,3 +1,9 @@
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-1.rs:6:13
+   |
+LL |             ///
+   |             ^^^
+
 error: repetition matches empty token tree
   --> $DIR/issue-112342-1.rs:5:10
    |
@@ -7,6 +13,12 @@ LL | |             ///
 LL | |         )*
    | |_________^
 
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-1.rs:17:13
+   |
+LL |             ///
+   |             ^^^
+
 error: repetition matches empty token tree
   --> $DIR/issue-112342-1.rs:16:10
    |
@@ -16,6 +28,12 @@ LL | |             ///
 LL | |         )+
    | |_________^
 
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-1.rs:28:13
+   |
+LL |             ///
+   |             ^^^
+
 error: repetition matches empty token tree
   --> $DIR/issue-112342-1.rs:27:10
    |
@@ -25,5 +43,22 @@ LL | |             ///
 LL | |         )?
    | |_________^
 
-error: aborting due to 3 previous errors
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-1.rs:40:13
+   |
+LL | /             ///
+LL | |             ///
+   | |_______________^
+
+error: repetition matches empty token tree
+  --> $DIR/issue-112342-1.rs:39:10
+   |
+LL |           $(
+   |  __________^
+LL | |             ///
+LL | |             ///
+LL | |         )*
+   | |_________^
+
+error: aborting due to 4 previous errors
 

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

@@ -25,4 +25,15 @@ macro_rules! m2 {
 
 m2! {}
 
+macro_rules! m3 {
+    (
+        $(
+            ///
+            $tt: tt,
+        )*
+    ) => {};
+}
+
+m3! {}
+
 fn main() {}

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

@@ -0,0 +1,24 @@
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-2.rs:8:13
+   |
+LL |             ///
+   |             ^^^
+
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-2.rs:19:13
+   |
+LL |             ///
+   |             ^^^
+
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-2.rs:21:13
+   |
+LL |             ///
+   |             ^^^
+
+note: doc comments are ignored in matcher position
+  --> $DIR/issue-112342-2.rs:31:13
+   |
+LL |             ///
+   |             ^^^
+