Fix formatter instability with half-indented comment (#6460)

## Summary
The bug was happening in this
[loop](75f402eb82/crates/ruff_python_formatter/src/comments/placement.rs (L545)).

Basically, In the first iteration of the loop, the `comment_indentation`
is bigger than `child_indentation` (`comment_indentation` is 7 and
`child_indentation` is 4) making the `Ordering::Greater` branch execute.
Inside the `Ordering::Greater` branch, the `if` block gets executed,
resulting in the update of these variables.
```rust
parent_body = current_body;                    
current_body = Some(last_child_in_current_body);
last_child_in_current_body = nested_child;
```
In the second iteration of the loop, `comment_indentation` is smaller
than `child_indentation` (`comment_indentation` is 7 and
`child_indentation` is 8) making the `Ordering::Less` branch execute.
Inside the `Ordering::Less` branch, the `if` block gets executed, this
is where the bug was happening. At this point `parent_body` should be a
`StmtFunctionDef` but it was a `StmtClassDef`. Causing the comment to be
incorrectly formatted.

That happened for the following code:
```python
class A:
    def f():
        pass
       # strangely indented comment

print()
```

There is only one problem that I couldn't figure it out a solution, the
variable `current_body` in this
[line](75f402eb82/crates/ruff_python_formatter/src/comments/placement.rs (L542C5-L542C49))
now gives this warning _"value assigned to `current_body` is never read
maybe it is overwritten before being read?"_
Any tips on how to solve that?

Closes #5337

## Test Plan

Add new test case.

---------

Co-authored-by: konstin <konstin@mailbox.org>

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

@@ -103,3 +103,15 @@ else:
 if True: print("a") # 1
 elif True: print("b") # 2
 else: print("c") # 3
+
+# Regression test for https://github.com/astral-sh/ruff/issues/5337
+if parent_body:
+    if current_body:
+        child_in_body()
+        last_child_in_current_body()  # may or may not have children on its own
+# a
+  # b
+    # c
+      # d
+        # e
+          #f

crates/ruff_python_formatter/src/comments/mod.rs

@@ -600,8 +600,13 @@ def test(x, y):
 def other(y, z):
     if y == z:
         pass
-            # Trailing `if` comment
+            # Trailing `pass` comment
-      # Trailing `other` function comment
+      # Trailing `if` statement comment
+
+class Test:
+    def func():
+        pass
+       # Trailing `func` function comment
 
 test(10, 20)
 "#;

crates/ruff_python_formatter/src/comments/placement.rs

@@ -451,12 +451,11 @@ fn handle_own_line_comment_after_branch<'a>(
         return CommentPlacement::Default(comment);
     }
 
-    let mut parent_body = None;
+    let mut parent = None;
-    let mut current_body = Some(preceding_node);
+    let mut last_child_in_parent = last_child;
-    let mut last_child_in_current_body = last_child;
 
     loop {
-        let child_indentation = indentation(locator, &last_child_in_current_body)
+        let child_indentation = indentation(locator, &last_child_in_parent)
             .unwrap_or_default()
             .len();
 
@@ -467,13 +466,14 @@ fn handle_own_line_comment_after_branch<'a>(
         //         child_in_body()
         //         last_child_in_current_body  # may or may not have children on its own
         // # less: Comment belongs to the parent block.
-        //   # less
+        //   # less: Comment belongs to the parent block.
         //     # equal: The comment belongs to this block.
-        //        # greater
+        //       # greater (but less in the next iteration)
         //         # greater: The comment belongs to the inner block.
+        // ```
         match comment_indentation.cmp(&child_indentation) {
             Ordering::Less => {
-                return if let Some(parent_block) = parent_body {
+                return if let Some(parent_block) = parent {
                     // Comment belongs to the parent block.
                     CommentPlacement::trailing(parent_block, comment)
                 } else {
@@ -488,14 +488,13 @@ fn handle_own_line_comment_after_branch<'a>(
             }
             Ordering::Equal => {
                 // The comment belongs to this block.
-                return CommentPlacement::trailing(last_child_in_current_body, comment);
+                return CommentPlacement::trailing(last_child_in_parent, comment);
             }
             Ordering::Greater => {
-                if let Some(nested_child) = last_child_in_body(last_child_in_current_body) {
+                if let Some(nested_child) = last_child_in_body(last_child_in_parent) {
                     // The comment belongs to the inner block.
-                    parent_body = current_body;
+                    parent = Some(last_child_in_parent);
-                    current_body = Some(last_child_in_current_body);
+                    last_child_in_parent = nested_child;
-                    last_child_in_current_body = nested_child;
                 } else {
                     // The comment is overindented, we assign it to the most indented child we have.
                     // ```python
@@ -503,7 +502,7 @@ fn handle_own_line_comment_after_branch<'a>(
                     //     pass
                     //       # comment
                     // ```
-                    return CommentPlacement::trailing(last_child_in_current_body, comment);
+                    return CommentPlacement::trailing(last_child_in_parent, comment);
                 }
             }
         }
@@ -1346,7 +1345,7 @@ where
     right.is_some_and(|right| left.ptr_eq(right.into()))
 }
 
-/// The last child of the last branch, if the node hs multiple branches.
+/// The last child of the last branch, if the node has multiple branches.
 fn last_child_in_body(node: AnyNodeRef) -> Option<AnyNodeRef> {
     let body = match node {
         AnyNodeRef::StmtFunctionDef(ast::StmtFunctionDef { body, .. })
@@ -1504,9 +1503,7 @@ mod tests {
         );
 
         assert_eq!(
-            max_empty_lines(
+            max_empty_lines("# trailing comment\n\n# own line comment\n\n\n# an other own line comment\n# block"),
-                "# trailing comment\n\n# own line comment\n\n\n# an other own line comment\n# block"
-            ),
             2
         );
 

crates/ruff_python_formatter/src/comments/snapshots/ruff_python_formatter__comments__tests__trailing_function_comment.snap

@@ -34,15 +34,15 @@ expression: comments.debug(test_case.source_code)
         ],
     },
     Node {
-        kind: StmtFunctionDef,
+        kind: StmtIf,
-        range: 193..237,
+        range: 214..237,
-        source: `def other(y, z):⏎`,
+        source: `if y == z:⏎`,
     }: {
         "leading": [],
         "dangling": [],
         "trailing": [
             SourceComment {
-                text: "# Trailing `other` function comment",
+                text: "# Trailing `if` statement comment",
                 position: OwnLine,
                 formatted: false,
             },
@@ -57,7 +57,22 @@ expression: comments.debug(test_case.source_code)
         "dangling": [],
         "trailing": [
             SourceComment {
-                text: "# Trailing `if` comment",
+                text: "# Trailing `pass` comment",
+                position: OwnLine,
+                formatted: false,
+            },
+        ],
+    },
+    Node {
+        kind: StmtFunctionDef,
+        range: 333..357,
+        source: `def func():⏎`,
+    }: {
+        "leading": [],
+        "dangling": [],
+        "trailing": [
+            SourceComment {
+                text: "# Trailing `func` function comment",
                 position: OwnLine,
                 formatted: false,
             },

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

@@ -109,6 +109,18 @@ else:
 if True: print("a") # 1
 elif True: print("b") # 2
 else: print("c") # 3
+
+# Regression test for https://github.com/astral-sh/ruff/issues/5337
+if parent_body:
+    if current_body:
+        child_in_body()
+        last_child_in_current_body()  # may or may not have children on its own
+# a
+  # b
+    # c
+      # d
+        # e
+          #f
 ```
 
 ## Output
@@ -224,6 +236,18 @@ elif True:
     print("b")  # 2
 else:
     print("c")  # 3
+
+# Regression test for https://github.com/astral-sh/ruff/issues/5337
+if parent_body:
+    if current_body:
+        child_in_body()
+        last_child_in_current_body()  # may or may not have children on its own
+        # e
+        # f
+    # c
+    # d
+# a
+# b
 ```