Implement no_blank_line_before_class_docstring preview style (#9154)

## Summary

This PR implements the `no_blank_line_before_class_docstring` preview
style.

## Test Plan

Update existing snapshots.

### Formatter ecosystem

`main`

| project | similarity index | total files | changed files |
|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75804 | 1799 | 1648 |
| django | 0.99984 | 2772 | 34 |
| home-assistant | 0.99955 | 10596 | 213 |
| poetry | 0.99905 | 321 | 15 |
| transformers | 0.99967 | 2657 | 324 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99980 | 3669 | 18 |
| warehouse | 0.99976 | 654 | 14 |
| zulip | 0.99958 | 1459 | 36 |

`dhruv/no-blank-line-docstring`

| project | similarity index | total files | changed files |
|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75804 | 1799 | 1648 |
| django | 0.99984 | 2772 | 34 |
| home-assistant | 0.99955 | 10596 | 213 |
| poetry | 0.99905 | 321 | 15 |
| transformers | 0.99967 | 2657 | 324 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99980 | 3669 | 18 |
| warehouse | 0.99976 | 654 | 14 |
| zulip | 0.99958 | 1459 | 36 |

fixes: #8888

crates/ruff_python_formatter/resources/test/fixtures/ruff/blank_line_before_class_docstring.options.json

@@ -0,0 +1,5 @@
+[
+  {
+    "preview": "enabled"
+  }
+]

crates/ruff_python_formatter/resources/test/fixtures/ruff/blank_line_before_class_docstring.py

@@ -0,0 +1,38 @@
+class NormalDocstring:
+
+    """This is a docstring."""
+
+
+class DocstringWithComment0:
+    # This is a comment
+    """This is a docstring."""
+
+
+class DocstringWithComment1:
+    # This is a comment
+
+    """This is a docstring."""
+
+
+class DocstringWithComment2:
+
+    # This is a comment
+    """This is a docstring."""
+
+
+class DocstringWithComment3:
+
+    # This is a comment
+
+    """This is a docstring."""
+
+
+class DocstringWithComment4:
+
+
+    # This is a comment
+
+
+    """This is a docstring."""
+
+

crates/ruff_python_formatter/src/preview.rs

@@ -24,3 +24,12 @@ pub(crate) const fn is_prefer_splitting_right_hand_side_of_assignments_enabled(
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the [`no_blank_line_before_class_docstring`] preview style is enabled.
+///
+/// [`no_blank_line_before_class_docstring`]: https://github.com/astral-sh/ruff/issues/8888
+pub(crate) const fn is_no_blank_line_before_class_docstring_enabled(
+    context: &PyFormatContext,
+) -> bool {
+    context.is_preview()
+}

crates/ruff_python_formatter/src/statement/suite.rs

@@ -11,6 +11,7 @@ use crate::comments::{
 use crate::context::{NodeLevel, TopLevelStatementPosition, WithIndentLevel, WithNodeLevel};
 use crate::expression::expr_string_literal::ExprStringLiteralKind;
 use crate::prelude::*;
+use crate::preview::is_no_blank_line_before_class_docstring_enabled;
 use crate::statement::stmt_expr::FormatStmtExpr;
 use crate::verbatim::{
     suppressed_node, write_suppressed_statements_starting_with_leading_comment,
@@ -108,14 +109,24 @@ impl FormatRule<Suite, PyFormatContext<'_>> for FormatSuite {
                     if !comments.has_leading(first)
                         && lines_before(first.start(), source) > 1
                         && !source_type.is_stub()
+                        && !is_no_blank_line_before_class_docstring_enabled(f.context())
                     {
                         // Allow up to one empty line before a class docstring, e.g., this is
                         // stable formatting:
+                        //
                         // ```python
                         // class Test:
                         //
                         //     """Docstring"""
                         // ```
+                        //
+                        // But, in preview mode, we don't want to allow any empty lines before a
+                        // class docstring, e.g., this is preview formatting:
+                        //
+                        // ```python
+                        // class Test:
+                        //   """Docstring"""
+                        // ```
                         empty_line().fmt(f)?;
                     }
 

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

@@ -27,30 +27,21 @@ class UpperCaseR:
 ```diff
 --- Black
 +++ Ruff
-@@ -1,4 +1,5 @@
- class C:
-+
-     r"""Raw"""
- 
- 
-@@ -7,8 +8,9 @@
+@@ -7,7 +7,7 @@
  
  
  class SingleQuotes:
 -    r'''Raw'''
- 
 +    r"""Raw"""
-+
+ 
  
  class UpperCaseR:
-     R"""Raw"""
 ```
 
 ## Ruff Output
 
 ```python
 class C:
-
     r"""Raw"""
 
 
@@ -59,7 +50,6 @@ def f():
 
 
 class SingleQuotes:
-
     r"""Raw"""
 
 

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

@@ -0,0 +1,95 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/blank_line_before_class_docstring.py
+---
+## Input
+```python
+class NormalDocstring:
+
+    """This is a docstring."""
+
+
+class DocstringWithComment0:
+    # This is a comment
+    """This is a docstring."""
+
+
+class DocstringWithComment1:
+    # This is a comment
+
+    """This is a docstring."""
+
+
+class DocstringWithComment2:
+
+    # This is a comment
+    """This is a docstring."""
+
+
+class DocstringWithComment3:
+
+    # This is a comment
+
+    """This is a docstring."""
+
+
+class DocstringWithComment4:
+
+
+    # This is a comment
+
+
+    """This is a docstring."""
+
+
+```
+
+## Outputs
+### Output 1
+```
+indent-style               = space
+line-width                 = 88
+indent-width               = 4
+quote-style                = Double
+line-ending                = LineFeed
+magic-trailing-comma       = Respect
+docstring-code             = Disabled
+docstring-code-line-width  = "dynamic"
+preview                    = Enabled
+```
+
+```python
+class NormalDocstring:
+    """This is a docstring."""
+
+
+class DocstringWithComment0:
+    # This is a comment
+    """This is a docstring."""
+
+
+class DocstringWithComment1:
+    # This is a comment
+
+    """This is a docstring."""
+
+
+class DocstringWithComment2:
+    # This is a comment
+    """This is a docstring."""
+
+
+class DocstringWithComment3:
+    # This is a comment
+
+    """This is a docstring."""
+
+
+class DocstringWithComment4:
+    # This is a comment
+
+    """This is a docstring."""
+```
+
+
+

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

@@ -198,7 +198,6 @@ def reference_docstring_newlines():
 
 
 class RemoveNewlineBeforeClassDocstring:
-
     """Black's `Preview.no_blank_line_before_class_docstring`"""