Enforce double quotes for all docstrings

crates/ruff_python_formatter/resources/test/fixtures/black/cases/ignore_pyi.pyi

@@ -15,7 +15,6 @@ def g():
     # hi
     ...
 
-# FIXME(#8905): Uncomment, leads to unstable formatting
-# def h():
-#     ...
-#     # bye
+def h():
+    ...
+    # bye

crates/ruff_python_formatter/resources/test/fixtures/black/cases/line_ranges_fmt_off_decorator.py

@@ -9,3 +9,12 @@ class MyClass:
     # fmt: on
     def method():
         print   ( "str" )
+
+    @decor(
+        a=1,
+        # fmt: off
+        b=(2,   3),
+        # fmt: on
+    )
+    def func():
+          pass

crates/ruff_python_formatter/resources/test/fixtures/black/cases/line_ranges_fmt_off_decorator.py.expect

@@ -1,4 +1,4 @@
-# flags: --line-ranges=12-12
+# flags: --line-ranges=12-12 --line-ranges=21-21
 # NOTE: If you need to modify this file, pay special attention to the --line-ranges=
 # flag above as it's formatting specifically these lines.
 
@@ -10,3 +10,12 @@ class MyClass:
     # fmt: on
     def method():
         print("str")
+
+    @decor(
+        a=1,
+        # fmt: off
+        b=(2,   3),
+        # fmt: on
+    )
+    def func():
+        pass

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

@@ -14,5 +14,8 @@
   {
     "indent_style": "tab",
     "indent_width": 4
+  },
+  {
+    "quote_style": "single"
   }
 ]

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

@@ -150,3 +150,8 @@ def tabbed_indent(self):
 		Normal indented line
 		  	- autor
 		"""
+
+
+def single_quoted():
+    ' content\     '
+    return

crates/ruff_python_formatter/src/expression/string/mod.rs

@@ -213,7 +213,8 @@ impl<'a> Format<PyFormatContext<'_>> for FormatString<'a> {
                 let normalized = string_part.normalize(
                     Quoting::CanChange,
                     &locator,
-                    f.options().quote_style(),
+                    // Per PEP 8 and PEP 257, always prefer double quotes for docstrings
+                    QuoteStyle::Double,
                     parent_docstring_quote_style,
                 );
                 docstring::format(&normalized, f)
@@ -323,9 +324,7 @@ impl StringPart {
         configured_style: QuoteStyle,
         parent_docstring_quote_style: Option<QuoteStyle>,
     ) -> NormalizedString<'a> {
-        // Per PEP 8 and PEP 257, always prefer double quotes for docstrings
-        // and triple-quoted strings. (We assume docstrings are always
-        // triple-quoted.)
+        // Per PEP 8, always prefer double quotes for triple-quoted strings.
         let preferred_style = if self.quotes.triple {
             // ... unless we're formatting a code snippet inside a docstring,
             // then we specifically want to invert our quote style to avoid

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__ignore_pyi.pyi.snap

@@ -22,10 +22,9 @@ def g():
     # hi
     ...
 
-# FIXME(#8905): Uncomment, leads to unstable formatting
-# def h():
-#     ...
-#     # bye
+def h():
+    ...
+    # bye
 ```
 
 ## Black Differences
@@ -41,17 +40,6 @@ def g():
  class y: ...  # comment
  
  # whitespace doesn't matter (note the next line has a trailing space and tab)
-@@ -13,6 +12,7 @@
-     # hi
-     ...
- 
--def h():
--    ...
--    # bye
-+# FIXME(#8905): Uncomment, leads to unstable formatting
-+# def h():
-+#     ...
-+#     # bye
 ```
 
 ## Ruff Output
@@ -71,10 +59,9 @@ def g():
     # hi
     ...
 
-# FIXME(#8905): Uncomment, leads to unstable formatting
-# def h():
-#     ...
-#     # bye
+def h():
+    ...
+    # bye
 ```
 
 ## Black Output

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

@@ -16,15 +16,24 @@ class MyClass:
     # fmt: on
     def method():
         print   ( "str" )
+
+    @decor(
+        a=1,
+        # fmt: off
+        b=(2,   3),
+        # fmt: on
+    )
+    def func():
+          pass
 ```
 
 ## Black Differences
 
 ```diff
 --- Black
 +++ Ruff
-@@ -1,12 +1,10 @@
--# flags: --line-ranges=12-12
+@@ -1,15 +1,13 @@
+-# flags: --line-ranges=12-12 --line-ranges=21-21
  # NOTE: If you need to modify this file, pay special attention to the --line-ranges=
  # flag above as it's formatting specifically these lines.
  
@@ -37,6 +46,15 @@ class MyClass:
      def method():
 -        print("str")
 +        print   ( "str" )
+ 
+     @decor(
+         a=1,
+@@ -18,4 +16,4 @@
+         # fmt: on
+     )
+     def func():
+-        pass
++          pass
 ```
 
 ## Ruff Output
@@ -52,12 +70,21 @@ class MyClass:
     # fmt: on
     def method():
         print   ( "str" )
+
+    @decor(
+        a=1,
+        # fmt: off
+        b=(2,   3),
+        # fmt: on
+    )
+    def func():
+          pass
 ```
 
 ## Black Output
 
 ```python
-# flags: --line-ranges=12-12
+# flags: --line-ranges=12-12 --line-ranges=21-21
 # NOTE: If you need to modify this file, pay special attention to the --line-ranges=
 # flag above as it's formatting specifically these lines.
 
@@ -69,6 +96,15 @@ class MyClass:
     # fmt: on
     def method():
         print("str")
+
+    @decor(
+        a=1,
+        # fmt: off
+        b=(2,   3),
+        # fmt: on
+    )
+    def func():
+        pass
 ```