Don't suggest converting /// to // when expecting ,

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -686,23 +686,34 @@ impl<'a> Parser<'a> {
         }
 
         if let token::DocComment(kind, style, _) = self.token.kind {
-            // We have something like `expr //!val` where the user likely meant `expr // !val`
-            let pos = self.token.span.lo() + BytePos(2);
-            let span = self.token.span.with_lo(pos).with_hi(pos);
-            err.span_suggestion_verbose(
-                span,
-                format!(
-                    "add a space before {} to write a regular comment",
-                    match (kind, style) {
-                        (token::CommentKind::Line, ast::AttrStyle::Inner) => "`!`",
-                        (token::CommentKind::Block, ast::AttrStyle::Inner) => "`!`",
-                        (token::CommentKind::Line, ast::AttrStyle::Outer) => "the last `/`",
-                        (token::CommentKind::Block, ast::AttrStyle::Outer) => "the last `*`",
-                    },
-                ),
-                " ".to_string(),
-                Applicability::MachineApplicable,
-            );
+            // This is to avoid suggesting converting a doc comment to a regular comment
+            // when missing a comma before the doc comment in lists (#142311):
+            //
+            // ```
+            // enum Foo{
+            //     A /// xxxxxxx
+            //     B,
+            // }
+            // ```
+            if !expected.contains(&TokenType::Comma) {
+                // We have something like `expr //!val` where the user likely meant `expr // !val`
+                let pos = self.token.span.lo() + BytePos(2);
+                let span = self.token.span.with_lo(pos).with_hi(pos);
+                err.span_suggestion_verbose(
+                    span,
+                    format!(
+                        "add a space before {} to write a regular comment",
+                        match (kind, style) {
+                            (token::CommentKind::Line, ast::AttrStyle::Inner) => "`!`",
+                            (token::CommentKind::Block, ast::AttrStyle::Inner) => "`!`",
+                            (token::CommentKind::Line, ast::AttrStyle::Outer) => "the last `/`",
+                            (token::CommentKind::Block, ast::AttrStyle::Outer) => "the last `*`",
+                        },
+                    ),
+                    " ".to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+            }
         }
 
         let sp = if self.token == token::Eof {

tests/ui/parser/doc-comment-after-missing-comma-issue-142311.rs

@@ -0,0 +1,34 @@
+//! Check that if the parser suggests converting `///` to a regular comment
+//! when it appears after a missing comma in an list (e.g. `enum` variants).
+//!
+//! Related issue
+//! - https://github.com/rust-lang/rust/issues/142311
+
+enum Foo {
+    /// Like the noise a sheep makes
+    Bar
+    /// Like where people drink
+    //~^ ERROR expected one of `(`, `,`, `=`, `{`, or `}`, found doc comment `/// Like where people drink`
+    Baa///xxxxxx
+    //~^ ERROR expected one of `(`, `,`, `=`, `{`, or `}`, found doc comment `///xxxxxx`
+    Baz///xxxxxx
+    //~^ ERROR expected one of `(`, `,`, `=`, `{`, or `}`, found doc comment `///xxxxxx`
+}
+
+fn foo() {
+    let a = [
+        1///xxxxxx
+        //~^ ERROR expected one of `,`, `.`, `;`, `?`, `]`, or an operator, found doc comment `///xxxxxx`
+        2
+    ];
+}
+
+fn bar() {
+    let a = [
+        1,
+        2///xxxxxx
+        //~^ ERROR expected one of `,`, `.`, `?`, `]`, or an operator, found doc comment `///xxxxxx`
+    ];
+}
+
+fn main() {}

tests/ui/parser/doc-comment-after-missing-comma-issue-142311.stderr

@@ -0,0 +1,43 @@
+error: expected one of `(`, `,`, `=`, `{`, or `}`, found doc comment `/// Like where people drink`
+  --> $DIR/doc-comment-after-missing-comma-issue-142311.rs:10:5
+   |
+LL |     Bar
+   |        -
+   |        |
+   |        expected one of `(`, `,`, `=`, `{`, or `}`
+   |        help: missing `,`
+LL |     /// Like where people drink
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ unexpected token
+
+error: expected one of `(`, `,`, `=`, `{`, or `}`, found doc comment `///xxxxxx`
+  --> $DIR/doc-comment-after-missing-comma-issue-142311.rs:12:8
+   |
+LL |     Baa///xxxxxx
+   |        -^^^^^^^^
+   |        |
+   |        expected one of `(`, `,`, `=`, `{`, or `}`
+   |        help: missing `,`
+
+error: expected one of `(`, `,`, `=`, `{`, or `}`, found doc comment `///xxxxxx`
+  --> $DIR/doc-comment-after-missing-comma-issue-142311.rs:14:8
+   |
+LL |     Baz///xxxxxx
+   |        ^^^^^^^^^ expected one of `(`, `,`, `=`, `{`, or `}`
+   |
+   = help: doc comments must come before what they document, if a comment was intended use `//`
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
+
+error: expected one of `,`, `.`, `;`, `?`, `]`, or an operator, found doc comment `///xxxxxx`
+  --> $DIR/doc-comment-after-missing-comma-issue-142311.rs:20:10
+   |
+LL |         1///xxxxxx
+   |          ^^^^^^^^^ expected one of `,`, `.`, `;`, `?`, `]`, or an operator
+
+error: expected one of `,`, `.`, `?`, `]`, or an operator, found doc comment `///xxxxxx`
+  --> $DIR/doc-comment-after-missing-comma-issue-142311.rs:29:10
+   |
+LL |         2///xxxxxx
+   |          ^^^^^^^^^ expected one of `,`, `.`, `?`, `]`, or an operator
+
+error: aborting due to 5 previous errors
+