Insert necessary blank line between class and leading comments (#8224)

## Summary

Given:

```python
# comment

class A:
    def foo(self):
        pass
```

We need to insert an additional newline between `# comment` and `class
A`. We were missing this handling for the case in which `# comment` is a
leading comment on `class A`, as opposed to a trailing comment of some
preceding statement.

In practice, I think this only applies to the specific case in which a
class or function is the first statement in a module, and there's a
single empty line between a leading comment and that class or function.
If there are no empty lines, then the comment "sticks" to the
definition; if there are two or more, then `leading_comments` will
truncate appropriately. If the class or function is nested, then we only
need one empty line anyway.

Closes #8215.

## Test Plan

No change in similarity.

Before:

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

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75803 | 1799 | 1647 |
| django | 0.99983 | 2772 | 34 |
| home-assistant | 0.99953 | 10596 | 186 |
| poetry | 0.99891 | 317 | 17 |
| transformers | 0.99966 | 2657 | 330 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99978 | 3669 | 20 |
| warehouse | 0.99977 | 654 | 13 |
| zulip | 0.99970 | 1459 | 22 |

After:

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

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75803 | 1799 | 1648 |
| django | 0.99983 | 2772 | 34 |
| home-assistant | 0.99953 | 10596 | 186 |
| poetry | 0.99891 | 317 | 17 |
| transformers | 0.99966 | 2657 | 330 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99978 | 3669 | 20 |
| warehouse | 0.99977 | 654 | 13 |
| zulip | 0.99970 | 1459 | 22 |

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

@@ -1,3 +1,5 @@
+# comment
+
 class Test(
     Aaaaaaaaaaaaaaaaa,
     Bbbbbbbbbbbbbbbb,

crates/ruff_python_formatter/src/comments/format.rs

@@ -507,13 +507,12 @@ fn strip_comment_prefix(comment_text: &str) -> FormatResult<&str> {
 ///
 /// For example, given:
 /// ```python
-/// def func():
+/// class Class:
 ///     ...
 /// # comment
 /// ```
 ///
 /// This builder will insert two empty lines before the comment.
-/// ```
 pub(crate) fn empty_lines_before_trailing_comments<'a>(
     f: &PyFormatter,
     comments: &'a [SourceComment],
@@ -555,3 +554,69 @@ impl Format<PyFormatContext<'_>> for FormatEmptyLinesBeforeTrailingComments<'_>
         Ok(())
     }
 }
+
+/// Format the empty lines between a node and its leading comments.
+///
+/// For example, given:
+/// ```python
+/// # comment
+///
+/// class Class:
+///     ...
+/// ```
+///
+/// While `leading_comments` will preserve the existing empty line, this builder will insert an
+/// additional empty line before the comment.
+pub(crate) fn empty_lines_after_leading_comments<'a>(
+    f: &PyFormatter,
+    comments: &'a [SourceComment],
+) -> FormatEmptyLinesAfterLeadingComments<'a> {
+    // Black has different rules for stub vs. non-stub and top level vs. indented
+    let empty_lines = match (f.options().source_type(), f.context().node_level()) {
+        (PySourceType::Stub, NodeLevel::TopLevel) => 1,
+        (PySourceType::Stub, _) => 0,
+        (_, NodeLevel::TopLevel) => 2,
+        (_, _) => 1,
+    };
+
+    FormatEmptyLinesAfterLeadingComments {
+        comments,
+        empty_lines,
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+pub(crate) struct FormatEmptyLinesAfterLeadingComments<'a> {
+    /// The leading comments of the node.
+    comments: &'a [SourceComment],
+    /// The expected number of empty lines after the leading comments.
+    empty_lines: u32,
+}
+
+impl Format<PyFormatContext<'_>> for FormatEmptyLinesAfterLeadingComments<'_> {
+    fn fmt(&self, f: &mut Formatter<PyFormatContext>) -> FormatResult<()> {
+        if let Some(comment) = self
+            .comments
+            .iter()
+            .rev()
+            .find(|comment| comment.line_position().is_own_line())
+        {
+            let actual = lines_after(comment.end(), f.context().source()).saturating_sub(1);
+            // If there are no empty lines, keep the comment tight to the node.
+            if actual == 0 {
+                return Ok(());
+            }
+
+            // If there are more than enough empty lines already, `leading_comments` will
+            // trim them as necessary.
+            if actual >= self.empty_lines {
+                return Ok(());
+            }
+
+            for _ in actual..self.empty_lines {
+                write!(f, [empty_line()])?;
+            }
+        }
+        Ok(())
+    }
+}

crates/ruff_python_formatter/src/statement/stmt_class_def.rs

@@ -3,7 +3,9 @@ use ruff_python_ast::{Decorator, StmtClassDef};
 use ruff_python_trivia::lines_after_ignoring_end_of_line_trivia;
 use ruff_text_size::Ranged;
 
-use crate::comments::format::empty_lines_before_trailing_comments;
+use crate::comments::format::{
+    empty_lines_after_leading_comments, empty_lines_before_trailing_comments,
+};
 use crate::comments::{leading_comments, trailing_comments, SourceComment};
 use crate::prelude::*;
 use crate::statement::clause::{clause_body, clause_header, ClauseHeader};
@@ -32,6 +34,29 @@ impl FormatNodeRule<StmtClassDef> for FormatStmtClassDef {
         let (leading_definition_comments, trailing_definition_comments) =
             dangling_comments.split_at(trailing_definition_comments_start);
 
+        // If the class contains leading comments, insert newlines before them.
+        // For example, given:
+        // ```python
+        // # comment
+        //
+        // class Class:
+        //     ...
+        // ```
+        //
+        // At the top-level in a non-stub file, reformat as:
+        // ```python
+        // # comment
+        //
+        //
+        // class Class:
+        //     ...
+        // ```
+        // Note that this is only really relevant for the specific case in which there's a single
+        // newline between the comment and the node, but we _require_ two newlines. If there are
+        // _no_ newlines between the comment and the node, we don't insert _any_ newlines; if there
+        // are more than two, then `leading_comments` will preserve the correct number of newlines.
+        empty_lines_after_leading_comments(f, comments.leading(item)).fmt(f)?;
+
         write!(
             f,
             [

crates/ruff_python_formatter/src/statement/stmt_function_def.rs

@@ -1,7 +1,9 @@
 use ruff_formatter::write;
 use ruff_python_ast::StmtFunctionDef;
 
-use crate::comments::format::empty_lines_before_trailing_comments;
+use crate::comments::format::{
+    empty_lines_after_leading_comments, empty_lines_before_trailing_comments,
+};
 use crate::comments::SourceComment;
 use crate::expression::maybe_parenthesize_expression;
 use crate::expression::parentheses::{Parentheses, Parenthesize};
@@ -30,6 +32,29 @@ impl FormatNodeRule<StmtFunctionDef> for FormatStmtFunctionDef {
         let (leading_definition_comments, trailing_definition_comments) =
             dangling_comments.split_at(trailing_definition_comments_start);
 
+        // If the class contains leading comments, insert newlines before them.
+        // For example, given:
+        // ```python
+        // # comment
+        //
+        // def func():
+        //     ...
+        // ```
+        //
+        // At the top-level in a non-stub file, reformat as:
+        // ```python
+        // # comment
+        //
+        //
+        // def func():
+        //     ...
+        // ```
+        // Note that this is only really relevant for the specific case in which there's a single
+        // newline between the comment and the node, but we _require_ two newlines. If there are
+        // _no_ newlines between the comment and the node, we don't insert _any_ newlines; if there
+        // are more than two, then `leading_comments` will preserve the correct number of newlines.
+        empty_lines_after_leading_comments(f, comments.leading(item)).fmt(f)?;
+
         write!(
             f,
             [

crates/ruff_python_formatter/tests/snapshots/black_compatibility@miscellaneous__decorators.py.snap

@@ -162,7 +162,7 @@ def f():
 ```diff
 --- Black
 +++ Ruff
-@@ -1,29 +1,205 @@
+@@ -1,29 +1,206 @@
 +# This file doesn't use the standard decomposition.
 +# Decorator syntax test cases are separated by double # comments.
 +# Those before the 'output' comment are valid under the old syntax.
@@ -172,6 +172,7 @@ def f():
 +
 +##
 +
++
 +@decorator
 +def f():
 +    ...
@@ -209,99 +210,99 @@ def f():
 +    ...
 +
 +
-+##
-+
+ ##
+ 
+-@decorator()()
 +
 +@decorator(**kwargs)
-+def f():
-+    ...
-+
-+
-+##
+ def f():
+     ...
+ 
 +
+ ##
+ 
+-@(decorator)
 +
 +@decorator(*args, **kwargs)
-+def f():
-+    ...
-+
-+
-+##
+ def f():
+     ...
+ 
 +
+ ##
+ 
+-@sequence["decorator"]
 +
 +@decorator(
 +    *args,
 +    **kwargs,
 +)
+ def f():
+     ...
+ 
++
+ ##
+ 
+-@decorator[List[str]]
++
++@dotted.decorator
+ def f():
+     ...
+ 
++
+ ##
+ 
+-@var := decorator
++
++@dotted.decorator(arg)
 +def f():
 +    ...
 +
 +
 +##
 +
 +
-+@dotted.decorator
++@dotted.decorator(kwarg=0)
 +def f():
 +    ...
 +
 +
 +##
 +
 +
-+@dotted.decorator(arg)
++@dotted.decorator(*args)
 +def f():
 +    ...
 +
 +
 +##
 +
 +
-+@dotted.decorator(kwarg=0)
++@dotted.decorator(**kwargs)
 +def f():
 +    ...
 +
 +
- ##
- 
--@decorator()()
++##
 +
-+@dotted.decorator(*args)
- def f():
-     ...
- 
 +
- ##
- 
--@(decorator)
++@dotted.decorator(*args, **kwargs)
++def f():
++    ...
 +
-+@dotted.decorator(**kwargs)
- def f():
-     ...
- 
 +
- ##
- 
--@sequence["decorator"]
-+
-+@dotted.decorator(*args, **kwargs)
- def f():
-     ...
- 
++##
 +
- ##
- 
--@decorator[List[str]]
 +
 +@dotted.decorator(
 +    *args,
 +    **kwargs,
 +)
- def f():
-     ...
- 
++def f():
++    ...
++
++
++##
 +
- ##
- 
--@var := decorator
 +
 +@double.dotted.decorator
 +def f():
@@ -387,6 +388,7 @@ def f():
 
 ##
 
+
 @decorator
 def f():
     ...