working for Micha's exact example

crates/ruff_python_formatter/src/comments/placement.rs

@@ -1884,14 +1884,19 @@ fn handle_lambda_comment<'a>(
             return CommentPlacement::dangling(comment.enclosing_node(), comment);
         }
     } else {
-        // Comments between the lambda and the body are dangling on the lambda:
+        // End-of-line comments between the lambda and the body are dangling on the lambda:
         // ```python
         // (
         //     lambda:  # comment
         //     y
         // )
         // ```
+        // But own-line comments after `:` are leading on the body.
         if comment.start() < lambda.body.start() {
+            if comment.line_position().is_own_line() {
+                return CommentPlacement::leading(&*lambda.body, comment);
+            }
+
             // If the value is parenthesized, and the comment is within the parentheses, it should
             // be a leading comment on the value, not a dangling comment in the lambda, as in:
             // ```python

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -377,6 +377,32 @@ impl Format<PyFormatContext<'_>> for FormatBody<'_> {
         );
 
         let fmt_body = format_with(|f: &mut PyFormatter| {
+            let body_comments = f.context().comments().leading_dangling_trailing(&**body);
+            if body_comments.has_leading() {
+                if body_comments
+                    .leading
+                    .iter()
+                    .any(|comment| comment.line_position().is_own_line())
+                {
+                    write!(
+                        f,
+                        [
+                            trailing_comments(dangling),
+                            hard_line_break(),
+                            body.format().with_options(Parentheses::Always),
+                        ]
+                    )
+                } else {
+                    write!(
+                        f,
+                        [
+                            space(),
+                            trailing_comments(dangling),
+                            body.format().with_options(Parentheses::Always),
+                        ]
+                    )
+                }
+            } else {
                 write!(
                     f,
                     [
@@ -390,6 +416,7 @@ impl Format<PyFormatContext<'_>> for FormatBody<'_> {
                         token(")")
                     ]
                 )
+            }
         });
 
         match layout {