Tweak "expected ident" parse error to avoid talking about doc comments

When encountering a doc comment without an identifier after, we'd unconditionally state "this doc comment doesn't document anything", swallowing the *actual* error which is that the thing *after* the doc comment wasn't expected. Added a check that the found token is something that "conceptually" closes the previous item before emitting that error, otherwise just complain about the missing identifier.

In both of the following cases, the syntax error follows a doc comment:
```
error: expected identifier, found keyword `Self`
  --> $DIR/doc-before-bad-variant.rs:4:5
   |
LL | enum TestEnum {
   |      -------- while parsing this enum
...
LL |     Self,
   |     ^^^^ expected identifier, found keyword
   |
   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
```
```
error: expected identifier, found `<`
  --> $DIR/doc-before-syntax-error.rs:2:1
   |
LL | <>
   | ^ expected identifier
```

Fix #71982.

5 files changed, 38 insertions, 7 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 72aebb5d121..67abc2d5394 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -301,13 +301,6 @@ impl<'a> Parser<'a> {
         &mut self,
         recover: bool,
     ) -> PResult<'a, (Ident, IdentIsRaw)> {
-        if let TokenKind::DocComment(..) = self.prev_token.kind {
-            return Err(self.dcx().create_err(DocCommentDoesNotDocumentAnything {
-                span: self.prev_token.span,
-                missing_comma: None,
-            }));
-        }
-
         let valid_follow = &[
             TokenKind::Eq,
             TokenKind::Colon,
@@ -319,6 +312,15 @@ impl<'a> Parser<'a> {
             TokenKind::CloseDelim(Delimiter::Brace),
             TokenKind::CloseDelim(Delimiter::Parenthesis),
         ];
+        if let TokenKind::DocComment(..) = self.prev_token.kind
+            && valid_follow.contains(&self.token.kind)
+        {
+            let err = self.dcx().create_err(DocCommentDoesNotDocumentAnything {
+                span: self.prev_token.span,
+                missing_comma: None,
+            });
+            return Err(err);
+        }
 
         let mut recovered_ident = None;
         // we take this here so that the correct original token is retained in
diff --git a/tests/ui/parser/doc-before-bad-variant.rs b/tests/ui/parser/doc-before-bad-variant.rs
new file mode 100644
index 00000000000..bfede28c108
--- /dev/null
+++ b/tests/ui/parser/doc-before-bad-variant.rs
@@ -0,0 +1,6 @@
+enum TestEnum {
+    Works,
+    /// Some documentation
+    Self, //~ ERROR expected identifier, found keyword `Self`
+    //~^ HELP enum variants can be
+}
diff --git a/tests/ui/parser/doc-before-bad-variant.stderr b/tests/ui/parser/doc-before-bad-variant.stderr
new file mode 100644
index 00000000000..5e4d4116f25
--- /dev/null
+++ b/tests/ui/parser/doc-before-bad-variant.stderr
@@ -0,0 +1,13 @@
+error: expected identifier, found keyword `Self`
+  --> $DIR/doc-before-bad-variant.rs:4:5
+   |
+LL | enum TestEnum {
+   |      -------- while parsing this enum
+...
+LL |     Self,
+   |     ^^^^ expected identifier, found keyword
+   |
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/parser/doc-before-syntax-error.rs b/tests/ui/parser/doc-before-syntax-error.rs
new file mode 100644
index 00000000000..435f497d186
--- /dev/null
+++ b/tests/ui/parser/doc-before-syntax-error.rs
@@ -0,0 +1,2 @@
+/// Some documentation
+<> //~ ERROR expected identifier
diff --git a/tests/ui/parser/doc-before-syntax-error.stderr b/tests/ui/parser/doc-before-syntax-error.stderr
new file mode 100644
index 00000000000..93e39d63000
--- /dev/null
+++ b/tests/ui/parser/doc-before-syntax-error.stderr
@@ -0,0 +1,8 @@
+error: expected identifier, found `<`
+  --> $DIR/doc-before-syntax-error.rs:2:1
+   |
+LL | <>
+   | ^ expected identifier
+
+error: aborting due to 1 previous error
+