Fix a number of formatter errors from the cpython repository (#5089)

## Summary

This fixes a number of problems in the formatter that showed up with
various files in the [cpython](https://github.com/python/cpython)
repository. These problems surfaced as unstable formatting and invalid
code. This is not the entirety of problems discovered through cpython,
but a big enough chunk to separate it. Individual fixes are generally
individual commits. They were discovered with #5055, which i update as i
work through the output

## Test Plan

I added regression tests with links to cpython for each entry, except
for the two stubs that also got comment stubs since they'll be
implemented properly later.

crates/ruff_python_formatter/src/expression/expr_constant.rs

@@ -39,6 +39,22 @@ impl FormatNodeRule<ExprConstant> for FormatExprConstant {
             }
         }
     }
+
+    fn fmt_dangling_comments(
+        &self,
+        _node: &ExprConstant,
+        _f: &mut PyFormatter,
+    ) -> FormatResult<()> {
+        // TODO(konstin): Reactivate when string formatting works, currently a source of unstable
+        // formatting, e.g.:
+        // magic_methods = (
+        //     "enter exit "
+        //     # we added divmod and rdivmod here instead of numerics
+        //     # because there is no idivmod
+        //     "divmod rdivmod neg pos abs invert "
+        // )
+        Ok(())
+    }
 }
 
 impl NeedsParentheses for ExprConstant {

crates/ruff_python_formatter/src/expression/expr_dict.rs

@@ -18,6 +18,17 @@ impl FormatNodeRule<ExprDict> for FormatExprDict {
             )]
         )
     }
+
+    fn fmt_dangling_comments(&self, _node: &ExprDict, _f: &mut PyFormatter) -> FormatResult<()> {
+        // TODO(konstin): Reactivate when string formatting works, currently a source of unstable
+        // formatting, e.g.
+        // ```python
+        // coverage_ignore_c_items = {
+        // #    'cfunction': [...]
+        // }
+        // ```
+        Ok(())
+    }
 }
 
 impl NeedsParentheses for ExprDict {

crates/ruff_python_formatter/src/expression/expr_list.rs

@@ -1,4 +1,4 @@
-use crate::comments::{dangling_comments, Comments};
+use crate::comments::{dangling_comments, CommentTextPosition, Comments};
 use crate::expression::parentheses::{
     default_expression_needs_parentheses, NeedsParentheses, Parentheses, Parenthesize,
 };
@@ -18,6 +18,39 @@ impl FormatNodeRule<ExprList> for FormatExprList {
             ctx: _,
         } = item;
 
+        let comments = f.context().comments().clone();
+        let dangling = comments.dangling_comments(item.into());
+
+        // The empty list is special because there can be dangling comments, and they can be in two
+        // positions:
+        // ```python
+        // a3 = [  # end-of-line
+        //     # own line
+        // ]
+        // ```
+        // In all other cases comments get assigned to a list element
+        if elts.is_empty() {
+            let end_of_line_split = dangling
+                .partition_point(|comment| comment.position() == CommentTextPosition::EndOfLine);
+            debug_assert!(dangling[end_of_line_split..]
+                .iter()
+                .all(|comment| comment.position() == CommentTextPosition::OwnLine));
+            return write!(
+                f,
+                [group(&format_args![
+                    text("["),
+                    dangling_comments(&dangling[..end_of_line_split]),
+                    soft_block_indent(&dangling_comments(&dangling[end_of_line_split..])),
+                    text("]")
+                ])]
+            );
+        }
+
+        debug_assert!(
+            dangling.is_empty(),
+            "A non-empty expression list has dangling comments"
+        );
+
         let items = format_with(|f| {
             let mut iter = elts.iter();
 
@@ -36,14 +69,10 @@ impl FormatNodeRule<ExprList> for FormatExprList {
             Ok(())
         });
 
-        let comments = f.context().comments().clone();
-        let dangling = comments.dangling_comments(item.into());
-
         write!(
             f,
             [group(&format_args![
                 text("["),
-                dangling_comments(dangling),
                 soft_block_indent(&items),
                 text("]")
             ])]