config: add new docstring-code-format knob (#8854)

This PR does the plumbing to make a new formatting option,
`docstring-code-format`, available in the configuration for end users.
It is disabled by default (opt-in). It is opt-in at least initially to
reflect a conservative posture. The intent is to make it opt-out at some
point in the future.

This was split out from #8811 in order to make #8811 easier to merge.
Namely, once this is merged, docstring code snippet formatting will
become available to end users. (See comments below for how we arrived at
the name.)

Closes #7146

## Test Plan

Other than the standard test suite, I ran the formatter over the CPython
and polars projects to ensure both that the result looked sensible and
that tests still passed. At time of writing, one issue that currently
appears is that reformatting code snippets trips the long line lint:
1905886802

crates/ruff_workspace/src/options.rs

@@ -27,7 +27,7 @@ use ruff_linter::settings::types::{
 };
 use ruff_linter::{warn_user_once, RuleSelector};
 use ruff_macros::{CombineOptions, OptionsMetadata};
-use ruff_python_formatter::QuoteStyle;
+use ruff_python_formatter::{DocstringCodeLineWidth, QuoteStyle};
 
 use crate::settings::LineEnding;
 
@@ -2948,6 +2948,156 @@ pub struct FormatOptions {
         "#
     )]
     pub line_ending: Option<LineEnding>,
+
+    /// Whether to format code snippets in docstrings.
+    ///
+    /// When this is enabled, Python code examples within docstrings are
+    /// automatically reformatted.
+    ///
+    /// For example, when this is enabled, the following code:
+    ///
+    /// ```python
+    /// def f(x):
+    ///     """
+    ///     Something about `f`. And an example in doctest format:
+    ///
+    ///     >>> f(  x  )
+    ///
+    ///     Markdown is also supported:
+    ///
+    ///     ```py
+    ///     f(  x  )
+    ///     ```
+    ///
+    ///     As are reStructuredText literal blocks::
+    ///
+    ///         f(  x  )
+    ///
+    ///
+    ///     And reStructuredText code blocks:
+    ///
+    ///     .. code-block:: python
+    ///
+    ///         f(  x  )
+    ///     """
+    ///     pass
+    /// ```
+    ///
+    /// ... will be reformatted (assuming the rest of the options are set to
+    /// their defaults) as:
+    ///
+    /// ```python
+    /// def f(x):
+    ///     """
+    ///     Something about `f`. And an example in doctest format:
+    ///
+    ///     >>> f(x)
+    ///
+    ///     Markdown is also supported:
+    ///
+    ///     ```py
+    ///     f(x)
+    ///     ```
+    ///
+    ///     As are reStructuredText literal blocks::
+    ///
+    ///         f(x)
+    ///
+    ///
+    ///     And reStructuredText code blocks:
+    ///
+    ///     .. code-block:: python
+    ///
+    ///         f(x)
+    ///     """
+    ///     pass
+    /// ```
+    ///
+    /// If a code snippt in a docstring contains invalid Python code or if the
+    /// formatter would otherwise write invalid Python code, then the code
+    /// example is ignored by the formatter and kept as-is.
+    ///
+    /// Currently, doctest, Markdown, reStructuredText literal blocks, and
+    /// reStructuredText code blocks are all supported and automatically
+    /// recognized. In the case of unlabeled fenced code blocks in Markdown and
+    /// reStructuredText literal blocks, the contents are assumed to be Python
+    /// and reformatted. As with any other format, if the contents aren't valid
+    /// Python, then the block is left untouched automatically.
+    #[option(
+        default = "false",
+        value_type = "bool",
+        example = r#"
+            # Enable reformatting of code snippets in docstrings.
+            docstring-code-format = true
+        "#
+    )]
+    pub docstring_code_format: Option<bool>,
+
+    /// Set the line length used when formatting code snippets in docstrings.
+    ///
+    /// This only has an effect when the `docstring-code-format` setting is
+    /// enabled.
+    ///
+    /// The default value for this setting is `"dynamic"`, which has the effect
+    /// of ensuring that any reformatted code examples in docstrings adhere to
+    /// the global line length configuration that is used for the surrounding
+    /// Python code. The point of this setting is that it takes the indentation
+    /// of the docstring into account when reformatting code examples.
+    ///
+    /// Alternatively, this can be set to a fixed integer, which will result
+    /// in the same line length limit being applied to all reformatted code
+    /// examples in docstrings. When set to a fixed integer, the indent of the
+    /// docstring is not taken into account. That is, this may result in lines
+    /// in the reformatted code example that exceed the globally configured
+    /// line length limit.
+    ///
+    /// For example, when this is set to `20` and `docstring-code-format` is
+    /// enabled, then this code:
+    ///
+    /// ```python
+    /// def f(x):
+    ///     '''
+    ///     Something about `f`. And an example:
+    ///
+    ///     .. code-block:: python
+    ///
+    ///         foo, bar, quux = this_is_a_long_line(lion, hippo, lemur, bear)
+    ///     '''
+    ///     pass
+    /// ```
+    ///
+    /// ... will be reformatted (assuming the rest of the options are set
+    /// to their defaults) as:
+    ///
+    /// ```python
+    /// def f(x):
+    ///     """
+    ///     Something about `f`. And an example:
+    ///
+    ///     .. code-block:: python
+    ///
+    ///         (
+    ///             foo,
+    ///             bar,
+    ///             quux,
+    ///         ) = this_is_a_long_line(
+    ///             lion,
+    ///             hippo,
+    ///             lemur,
+    ///             bear,
+    ///         )
+    ///     """
+    ///     pass
+    /// ```
+    #[option(
+        default = r#""dynamic""#,
+        value_type = r#"int | "dynamic""#,
+        example = r#"
+            # Format all docstring code snippets with a line length of 60.
+            docstring-code-line-length = 60
+        "#
+    )]
+    pub docstring_code_line_length: Option<DocstringCodeLineWidth>,
 }
 
 #[cfg(test)]