Add doc_comment_code_block_small_heuristics

This allows overriding `use_small_heuristics` in doc code. This is
particularly useful for code that wants to use
`use_small_heuristics = "Max"` internally but present doctests and
examples using `use_small_heuristics = "Default"`.

Author: joshtriplett

Configurations.md

@@ -1051,6 +1051,14 @@ Max width for code snippets included in doc comments. Only used if [`format_code
 - **Possible values**: any positive integer that is less than or equal to the value specified for [`max_width`](#max_width)
 - **Stable**: No (tracking issue: [#5359](https://github.com/rust-lang/rustfmt/issues/5359))
 
+## `doc_comment_code_block_small_heuristics`
+
+Value for [`use_small_heuristics`](#use_small_heuristics) for use in code blocks in doc comments. Only used if [`format_code_in_doc_comments`](#format_code_in_doc_comments) is true.
+
+- **Default value**: `"Default"`
+- **Possible values**: `"Default"`, `"Off"`, `"Max"`
+- **Stable**: No (tracking issue: [#FIXME](https://github.com/rust-lang/rustfmt/issues/FIXME))
+
 ## `format_generated_files`
 
 Format generated files. A file is considered generated if any of the first several lines contain a `@generated` comment marker. The number of lines to check is configured by `generated_marker_line_search_limit`.

src/comment.rs

@@ -764,6 +764,14 @@ impl<'a> CommentRewrite<'a> {
                             .doc_comment_code_block_width()
                             .min(config.max_width());
                         config.set().max_width(comment_max_width);
+                        if let Some(comment_use_small_heuristics) = config
+                            .doc_comment_code_block_small_heuristics()
+                            .to_heuristics()
+                        {
+                            config
+                                .set()
+                                .use_small_heuristics(comment_use_small_heuristics);
+                        }
                         if let Some(s) =
                             crate::format_code_block(&self.code_block_buffer, &config, false)
                         {

src/config/mod.rs

@@ -65,6 +65,9 @@ create_config! {
         doc comments.";
     doc_comment_code_block_width: DocCommentCodeBlockWidth, false, "Maximum width for code \
         snippets in doc comments. No effect unless format_code_in_doc_comments = true";
+    doc_comment_code_block_small_heuristics: DocUseSmallHeuristics, false,
+        "Value for use_small_heuristics for code blocks in doc comments. \
+        No effect unless format_code_in_doc_comments = true";
     comment_width: CommentWidth, false,
         "Maximum length of comments. No effect unless wrap_comments = true";
     normalize_comments: NormalizeComments, false, "Convert /* */ comments to // comments where \
@@ -772,6 +775,7 @@ single_line_let_else_max_width = 50
 wrap_comments = false
 format_code_in_doc_comments = false
 doc_comment_code_block_width = 100
+doc_comment_code_block_small_heuristics = "Inherit"
 comment_width = 80
 normalize_comments = false
 normalize_doc_attributes = false
@@ -864,6 +868,7 @@ single_line_let_else_max_width = 50
 wrap_comments = false
 format_code_in_doc_comments = false
 doc_comment_code_block_width = 100
+doc_comment_code_block_small_heuristics = "Inherit"
 comment_width = 80
 normalize_comments = false
 normalize_doc_attributes = false

src/config/options.rs

@@ -95,6 +95,31 @@ pub enum Heuristics {
     Default,
 }
 
+#[config_type]
+/// Heuristic settings for doc comments. Same as `Heuristics`, but `Inherit` will inherit the value
+/// from the top-level configuration.
+pub enum DocCodeHeuristics {
+    /// Inherit from the top-level configuration
+    Inherit,
+    /// Turn off any heuristics
+    Off,
+    /// Turn on max heuristics
+    Max,
+    /// Use scaled values based on the value of `max_width`
+    Default,
+}
+
+impl DocCodeHeuristics {
+    pub fn to_heuristics(self) -> Option<Heuristics> {
+        match self {
+            DocCodeHeuristics::Inherit => None,
+            DocCodeHeuristics::Off => Some(Heuristics::Off),
+            DocCodeHeuristics::Max => Some(Heuristics::Max),
+            DocCodeHeuristics::Default => Some(Heuristics::Default),
+        }
+    }
+}
+
 impl Density {
     pub fn to_list_tactic(self, len: usize) -> ListTactic {
         match self {
@@ -620,6 +645,7 @@ config_option_with_style_edition_default!(
     WrapComments, bool, _ => false;
     FormatCodeInDocComments, bool, _ => false;
     DocCommentCodeBlockWidth, usize, _ => 100;
+    DocUseSmallHeuristics, DocCodeHeuristics, _ => DocCodeHeuristics::Inherit;
     CommentWidth, usize, _ => 80;
     NormalizeComments, bool, _ => false;
     NormalizeDocAttributes, bool, _ => false;