Stabilize quote-style preserve

This PR removes the `--preview` requirement for
`quote-style: preserve` and changes the semantics of
the style to apply to all strings (including triple quoted and doc-strings).

The motivation for changing the semantic to apply to all strings is:

* The option is intended as a replacement for black's `--skip-string-normalization` that applies to all strings
* The option is intended for projects that can't enforce a consistent quote style. These are mainly projects that want to use a formatter but can't agree on a single quote style or changing all strings leads to too large diffs. Allowing these users to use a formatter is more important than being opinionated on the quote style
* We received reports that users were surprised that `preserve` didn't apply to all strings.

Fixes #9185

crates/ruff_python_formatter/src/options.rs

@@ -248,6 +248,12 @@ pub enum QuoteStyle {
     Preserve,
 }
 
+impl QuoteStyle {
+    pub const fn is_preserve(self) -> bool {
+        matches!(self, QuoteStyle::Preserve)
+    }
+}
+
 impl fmt::Display for QuoteStyle {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {

crates/ruff_python_formatter/src/other/string_literal.rs

@@ -50,11 +50,13 @@ impl Format<PyFormatContext<'_>> for FormatStringLiteral<'_> {
     fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
         let locator = f.context().locator();
 
-        let quote_style = if self.layout.is_docstring() {
-            // Per PEP 8 and PEP 257, always prefer double quotes for docstrings
+        let quote_style = f.options().quote_style();
+        let quote_style = if self.layout.is_docstring() && !quote_style.is_preserve() {
+            // Per PEP 8 and PEP 257, always prefer double quotes for docstrings,
+            // except when using quote-style=preserve
             QuoteStyle::Double
         } else {
-            f.options().quote_style()
+            quote_style
         };
 
         let normalized = StringPart::from_source(self.value.range(), &locator).normalize(

crates/ruff_python_formatter/src/string/mod.rs

@@ -306,6 +306,7 @@ impl StringPart {
         normalize_hex: bool,
     ) -> NormalizedString<'a> {
         // Per PEP 8, always prefer double quotes for triple-quoted strings.
+        // Except when using quote-style-preserve.
         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
@@ -354,6 +355,8 @@ impl StringPart {
             // if it doesn't have perfect alignment with PEP8.
             if let Some(quote) = parent_docstring_quote_char {
                 QuoteStyle::from(quote.invert())
+            } else if configured_style.is_preserve() {
+                QuoteStyle::Preserve
             } else {
                 QuoteStyle::Double
             }

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

@@ -263,21 +263,21 @@ rb"rb double"
 rb'br single'
 rb"br double"
 
-"""single triple"""
+'''single triple'''
 """double triple"""
-r"""r single triple"""
+r'''r single triple'''
 r"""r double triple"""
-f"""f single triple"""
+f'''f single triple'''
 f"""f double triple"""
-rf"""fr single triple"""
+rf'''fr single triple'''
 rf"""fr double triple"""
-rf"""rf single triple"""
+rf'''rf single triple'''
 rf"""rf double triple"""
-b"""b single triple"""
+b'''b single triple'''
 b"""b double triple"""
-rb"""rb single triple"""
+rb'''rb single triple'''
 rb"""rb double triple"""
-rb"""br single triple"""
+rb'''br single triple'''
 rb"""br double triple"""
 
 'single1' 'single2'
@@ -287,7 +287,7 @@ rb"""br double triple"""
 
 
 def docstring_single_triple():
-    """single triple"""
+    '''single triple'''
 
 
 def docstring_double_triple():
@@ -299,7 +299,7 @@ def docstring_double():
 
 
 def docstring_single():
-    "single"
+    'single'
 ```
 
 
@@ -308,8 +308,7 @@ def docstring_single():
 --- Stable
 +++ Preview
 @@ -1,4 +1,5 @@
--'single'  # this string is treated as a docstring
-+"single"  # this string is treated as a docstring
+ 'single'  # this string is treated as a docstring
 +
  "double"
  r'r single'

crates/ruff_workspace/src/configuration.rs

@@ -166,12 +166,6 @@ impl Configuration {
             PreviewMode::Enabled => ruff_python_formatter::PreviewMode::Enabled,
         };
 
-        if quote_style == QuoteStyle::Preserve && !format_preview.is_enabled() {
-            return Err(anyhow!(
-                "'quote-style = preserve' is a preview only feature. Run with '--preview' to enable it."
-            ));
-        }
-
         let formatter = FormatterSettings {
             exclude: FilePatternSet::try_from_iter(format.exclude.unwrap_or_default())?,
             extension: self.extension.clone().unwrap_or_default(),

crates/ruff_workspace/src/options.rs

@@ -2942,28 +2942,30 @@ pub struct FormatOptions {
     )]
     pub indent_style: Option<IndentStyle>,
 
-    /// Configures the preferred quote character for strings. Valid options are:
-    ///
+    /// Configures the preferred quote character for strings. The recommended options are
     /// * `double` (default): Use double quotes `"`
     /// * `single`: Use single quotes `'`
-    /// * `preserve` (preview only): Keeps the existing quote character. We don't recommend using this option except for projects
-    ///    that already use a mixture of single and double quotes and can't migrate to using double or single quotes.
     ///
     /// In compliance with [PEP 8](https://peps.python.org/pep-0008/) and [PEP 257](https://peps.python.org/pep-0257/),
-    /// Ruff prefers double quotes for multiline strings and docstrings, regardless of the
+    /// Ruff prefers double quotes for triple quoted strings and docstrings, regardless of the
     /// configured quote style.
     ///
-    /// Ruff may also deviate from using the configured quotes if doing so requires
-    /// escaping quote characters within the string. For example, given:
+    /// Ruff deviates from using the configured quotes if doing so prevents the need for
+    /// escaping quote characters inside the string:
     ///
     /// ```python
     /// a = "a string without any quotes"
     /// b = "It's monday morning"
     /// ```
     ///
-    /// Ruff will change `a` to use single quotes when using `quote-style = "single"`. However,
-    /// `b` remains unchanged, as converting to single quotes requires escaping the inner `'`,
-    /// which leads to less readable code: `'It\'s monday morning'`. This does not apply when using `preserve`.
+    /// Ruff will change the quotes of the string assigned to `a` to single quotes when using `quote-style = "single"`.
+    /// However, ruff uses double quotes for he string assigned to `b` because using single quotes would require escaping the `'`,
+    /// which leads to the less readable code: `'It\'s monday morning'`.
+    ///
+    /// In addition, Ruff supports the quote style `preserve` for projects that already use
+    /// a mixture of single and double quotes and can't migrate to the `double` or `single` style.
+    /// The quote style `preserve` leaves the quotes of all strings unchanged.
+    /// We don't recommend using `preserve` for new project because it doesn't enforce consistent quotes.
     #[option(
         default = r#"double"#,
         value_type = r#""double" | "single" | "preserve""#,