Fix instable formatting for trailing subscribt end-of-line comment (#…

…10492)

## Summary

This PR fixes an instability where formatting a subscribt 
where the `slice` is not an `ExprSlice` and it has a trailing
end-of-line comment after its opening `[` required two formatting passes
to be stable.

The fix is to associate the trailing end-of-line comment as dangling
comment on `[` to preserve its position, similar to how Ruff does it for
other parenthesized expressions.
This also matches how trailing end-of-line subscript comments are
handled when the `slice` is an `ExprSlice`.

Fixes #10355

## Versioning

Shipping this as part of a patch release is fine because:

* It fixes a stability issue
* It doesn't impact already formatted code because Ruff would already
have moved to the comment to the end of the line (instead of preserving
it)

## Test Plan

Added tests

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/subscript.py

@@ -3,3 +3,29 @@
     f(111111111111111111111111111111111111111111111111111111111111111111111111111111111)
     + 1
 )[0]
+
+
+# Regression tests for: https://github.com/astral-sh/ruff/issues/10355
+repro(
+    "some long string that takes up some space"
+)[  # some long comment also taking up space
+    0
+]
+
+repro(
+    "some long string that takes up some space"
+)[0 # some long comment also taking up space
+]
+
+repro(
+    "some long string that takes up some space"
+)[0]  # some long comment also taking up space
+
+repro("some long string that takes up some space")[0]  # some long comment also taking up space
+
+
+repro(
+    "some long string that takes up some space"
+)[  # some long comment also taking up space
+0:-1
+]

crates/ruff_python_formatter/src/comments/placement.rs

@@ -251,10 +251,41 @@ fn handle_enclosed_comment<'a>(
         }
         AnyNodeRef::ExprSubscript(expr_subscript) => {
             if let Expr::Slice(expr_slice) = expr_subscript.slice.as_ref() {
-                handle_slice_comments(comment, expr_slice, comment_ranges, locator)
-            } else {
-                CommentPlacement::Default(comment)
+                return handle_slice_comments(comment, expr_slice, comment_ranges, locator);
+            }
+
+            // Handle non-slice subscript end-of-line comments coming after the `[`
+            // ```python
+            // repro(
+            //     "some long string that takes up some space"
+            //  )[  # some long comment also taking up space
+            //     0
+            // ]
+            // ```
+            if comment.line_position().is_end_of_line() {
+                // Ensure that there are no tokens between the open bracket and the comment.
+                let mut lexer = SimpleTokenizer::new(
+                    locator.contents(),
+                    TextRange::new(expr_subscript.value.end(), comment.start()),
+                )
+                .skip_trivia();
+
+                // Skip the opening parenthesis.
+                let Some(paren) = lexer.next() else {
+                    return CommentPlacement::Default(comment);
+                };
+
+                debug_assert_eq!(paren.kind(), SimpleTokenKind::LBracket);
+
+                // If there are no additional tokens between the open parenthesis and the comment, then
+                // it should be attached as a dangling comment on the brackets, rather than a leading
+                // comment on the first argument.
+                if lexer.next().is_none() {
+                    return CommentPlacement::dangling(expr_subscript, comment);
+                }
             }
+
+            CommentPlacement::Default(comment)
         }
         AnyNodeRef::ModModule(module) => {
             handle_trailing_module_comment(module, comment).or_else(|comment| {

crates/ruff_python_formatter/tests/snapshots/format@expression__subscript.py.snap

@@ -9,6 +9,32 @@ result = (
     f(111111111111111111111111111111111111111111111111111111111111111111111111111111111)
     + 1
 )[0]
+
+
+# Regression tests for: https://github.com/astral-sh/ruff/issues/10355
+repro(
+    "some long string that takes up some space"
+)[  # some long comment also taking up space
+    0
+]
+
+repro(
+    "some long string that takes up some space"
+)[0 # some long comment also taking up space
+]
+
+repro(
+    "some long string that takes up some space"
+)[0]  # some long comment also taking up space
+
+repro("some long string that takes up some space")[0]  # some long comment also taking up space
+
+
+repro(
+    "some long string that takes up some space"
+)[  # some long comment also taking up space
+0:-1
+]
 ```
 
 ## Output
@@ -18,7 +44,31 @@ result = (
     f(111111111111111111111111111111111111111111111111111111111111111111111111111111111)
     + 1
 )[0]
-```
 
 
+# Regression tests for: https://github.com/astral-sh/ruff/issues/10355
+repro(
+    "some long string that takes up some space"
+)[  # some long comment also taking up space
+    0
+]
+
+repro("some long string that takes up some space")[
+    0  # some long comment also taking up space
+]
+
+repro("some long string that takes up some space")[
+    0
+]  # some long comment also taking up space
 
+repro("some long string that takes up some space")[
+    0
+]  # some long comment also taking up space
+
+
+repro(
+    "some long string that takes up some space"
+)[  # some long comment also taking up space
+    0:-1
+]
+```