Fix instability in trailing clause body comments (#7556)

## Summary

When we format the trailing comments on a clause body, we check if there
are any newlines after the last statement; if not, we insert one.

This logic didn't take into account that the last statement could itself
have trailing comments, as in:

```python
if True:
    pass

    # comment
else:
    pass
```

We were thus inserting a newline after the comment, like:

```python
if True:
    pass

    # comment

else:
    pass
```

In the context of function definitions, this led to an instability,
since we insert a newline _after_ a function, which would in turn lead
to the bug above appearing in the second formatting pass.

Closes https://github.com/astral-sh/ruff/issues/7465.

## Test Plan

`cargo test`

Small improvement in `transformers`, but no regressions.

Before:

| project | similarity index | total files | changed files |

|--------------|------------------:|------------------:|------------------:|
| cpython | 0.76083 | 1789 | 1631 |
| django | 0.99983 | 2760 | 36 |
| transformers | 0.99956 | 2587 | 404 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99983 | 3496 | 18 |
| warehouse | 0.99967 | 648 | 15 |
| zulip | 0.99972 | 1437 | 21 |

After:

| project | similarity index | total files | changed files |

|--------------|------------------:|------------------:|------------------:|
| cpython | 0.76083 | 1789 | 1631 |
| django | 0.99983 | 2760 | 36 |
| **transformers** | **0.99957** | **2587** | **402** |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99983 | 3496 | 18 |
| warehouse | 0.99967 | 648 | 15 |
| zulip | 0.99972 | 1437 | 21 |

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/function.py

@@ -376,3 +376,11 @@ def f(  # first
 def this_is_unusual() -> (please := no): ...
 
 def this_is_unusual(x) -> (please := no): ...
+
+# Regression test for: https://github.com/astral-sh/ruff/issues/7465
+try:
+    def test():
+        pass
+    #comment
+except ImportError:
+    pass

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/if.py

@@ -104,6 +104,40 @@ if True: print("a") # 1
 elif True: print("b") # 2
 else: print("c") # 3
 
+# Regression test for: https://github.com/astral-sh/ruff/issues/7465
+if True:
+    pass
+
+    # comment
+else:
+    pass
+
+if True:
+    pass
+
+    # comment
+
+else:
+    pass
+
+if True:
+    pass
+    # comment
+
+else:
+    pass
+
+if True:
+    pass
+    # comment
+else:
+    pass
+
+if True:
+    pass  # comment
+else:
+    pass
+
 # Regression test for https://github.com/astral-sh/ruff/issues/5337
 if parent_body:
     if current_body:

crates/ruff_python_formatter/src/comments/format.rs

@@ -3,7 +3,7 @@ use std::borrow::Cow;
 use ruff_formatter::{format_args, write, FormatError, FormatOptions, SourceCode};
 use ruff_python_ast::node::{AnyNodeRef, AstNode};
 use ruff_python_ast::PySourceType;
-use ruff_python_trivia::{lines_after, lines_after_ignoring_trivia, lines_before};
+use ruff_python_trivia::{lines_after, lines_before};
 use ruff_text_size::{Ranged, TextLen, TextRange};
 
 use crate::comments::{CommentLinePosition, SourceComment};
@@ -94,7 +94,14 @@ impl Format<PyFormatContext<'_>> for FormatLeadingAlternateBranchComments<'_> {
         } else if let Some(last_preceding) = self.last_node {
             // The leading comments formatting ensures that it preserves the right amount of lines after
             // We need to take care of this ourselves, if there's no leading `else` comment.
-            if lines_after_ignoring_trivia(last_preceding.end(), f.context().source()) > 1 {
+            let end = if let Some(last_trailing) =
+                f.context().comments().trailing(last_preceding).last()
+            {
+                last_trailing.end()
+            } else {
+                last_preceding.end()
+            };
+            if lines_after(end, f.context().source()) > 1 {
                 write!(f, [empty_line()])?;
             }
         }

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

@@ -382,6 +382,14 @@ def f(  # first
 def this_is_unusual() -> (please := no): ...
 
 def this_is_unusual(x) -> (please := no): ...
+
+# Regression test for: https://github.com/astral-sh/ruff/issues/7465
+try:
+    def test():
+        pass
+    #comment
+except ImportError:
+    pass
 ```
 
 ## Output
@@ -921,6 +929,17 @@ def this_is_unusual() -> (please := no):
 
 def this_is_unusual(x) -> (please := no):
     ...
+
+
+# Regression test for: https://github.com/astral-sh/ruff/issues/7465
+try:
+
+    def test():
+        pass
+
+    # comment
+except ImportError:
+    pass
 ```
 
 

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

@@ -110,6 +110,40 @@ if True: print("a") # 1
 elif True: print("b") # 2
 else: print("c") # 3
 
+# Regression test for: https://github.com/astral-sh/ruff/issues/7465
+if True:
+    pass
+
+    # comment
+else:
+    pass
+
+if True:
+    pass
+
+    # comment
+
+else:
+    pass
+
+if True:
+    pass
+    # comment
+
+else:
+    pass
+
+if True:
+    pass
+    # comment
+else:
+    pass
+
+if True:
+    pass  # comment
+else:
+    pass
+
 # Regression test for https://github.com/astral-sh/ruff/issues/5337
 if parent_body:
     if current_body:
@@ -237,6 +271,40 @@ elif True:
 else:
     print("c")  # 3
 
+# Regression test for: https://github.com/astral-sh/ruff/issues/7465
+if True:
+    pass
+
+    # comment
+else:
+    pass
+
+if True:
+    pass
+
+    # comment
+
+else:
+    pass
+
+if True:
+    pass
+    # comment
+
+else:
+    pass
+
+if True:
+    pass
+    # comment
+else:
+    pass
+
+if True:
+    pass  # comment
+else:
+    pass
+
 # Regression test for https://github.com/astral-sh/ruff/issues/5337
 if parent_body:
     if current_body: