Preview Style: Format module level docstring (#9725)

Co-authored-by: Micha Reiser <micha@reiser.io>

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

@@ -1,4 +1,4 @@
-'single'
+'single' # this string is treated as a docstring
 "double"
 r'r single'
 r"r double"

crates/ruff_python_formatter/src/preview.rs

@@ -74,3 +74,10 @@ pub(crate) const fn is_hex_codes_in_unicode_sequences_enabled(context: &PyFormat
 pub(crate) const fn is_multiline_string_handling_enabled(context: &PyFormatContext) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the [`multiline_string_handling`](https://github.com/astral-sh/ruff/pull/9725) preview style is enabled.
+/// Black does not [`format docstrings`](https://github.com/psf/black/issues/3493) so we keep this
+/// preview for compatibility with Black.
+pub(crate) const fn is_format_module_docstring_enabled(context: &PyFormatContext) -> bool {
+    context.is_preview()
+}

crates/ruff_python_formatter/src/statement/suite.rs

@@ -15,7 +15,8 @@ use crate::expression::expr_string_literal::ExprStringLiteralKind;
 use crate::prelude::*;
 use crate::preview::{
     is_blank_line_after_nested_stub_class_enabled, is_dummy_implementations_enabled,
-    is_module_docstring_newlines_enabled, is_no_blank_line_before_class_docstring_enabled,
+    is_format_module_docstring_enabled, is_module_docstring_newlines_enabled,
+    is_no_blank_line_before_class_docstring_enabled,
 };
 use crate::statement::stmt_expr::FormatStmtExpr;
 use crate::verbatim::{
@@ -140,7 +141,17 @@ impl FormatRule<Suite, PyFormatContext<'_>> for FormatSuite {
                     SuiteChildStatement::Other(first)
                 }
             }
-            SuiteKind::TopLevel => SuiteChildStatement::Other(first),
+            SuiteKind::TopLevel => {
+                if is_format_module_docstring_enabled(f.context()) {
+                    if let Some(docstring) = DocstringStmt::try_from_statement(first, self.kind) {
+                        SuiteChildStatement::Docstring(docstring)
+                    } else {
+                        SuiteChildStatement::Other(first)
+                    }
+                } else {
+                    SuiteChildStatement::Other(first)
+                }
+            }
         };
 
         let first_comments = comments.leading_dangling_trailing(first);

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

@@ -4,7 +4,7 @@ input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/quote_styl
 ---
 ## Input
 ```python
-'single'
+'single' # this string is treated as a docstring
 "double"
 r'r single'
 r"r double"
@@ -73,7 +73,7 @@ source_type                = Python
 ```
 
 ```python
-'single'
+'single'  # this string is treated as a docstring
 'double'
 r'r single'
 r'r double'
@@ -135,7 +135,8 @@ def docstring_single():
 --- Stable
 +++ Preview
 @@ -1,4 +1,5 @@
- 'single'
+-'single'  # this string is treated as a docstring
++"single"  # this string is treated as a docstring
 +
  'double'
  r'r single'
@@ -159,7 +160,7 @@ source_type                = Python
 ```
 
 ```python
-"single"
+"single"  # this string is treated as a docstring
 "double"
 r"r single"
 r"r double"
@@ -221,7 +222,7 @@ def docstring_single():
 --- Stable
 +++ Preview
 @@ -1,4 +1,5 @@
- "single"
+ "single"  # this string is treated as a docstring
 +
  "double"
  r"r single"
@@ -245,7 +246,7 @@ source_type                = Python
 ```
 
 ```python
-'single'
+'single'  # this string is treated as a docstring
 "double"
 r'r single'
 r"r double"
@@ -307,7 +308,8 @@ def docstring_single():
 --- Stable
 +++ Preview
 @@ -1,4 +1,5 @@
- 'single'
+-'single'  # this string is treated as a docstring
++"single"  # this string is treated as a docstring
 +
  "double"
  r'r single'