Add width for codeblocks in comments

Configurations.md

@@ -926,6 +926,14 @@ fn add_one(x: i32) -> i32 {
 }
 ```
 
+## `doc_comment_code_block_width`
+
+Max width for code snippets included in doc comments. Only used if `format_code_in_doc_comments` is true.
+
+- **Default value**: `100`
+- **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))
+
 ## `format_generated_files`
 
 Format generated files. A file is considered generated

src/comment.rs

@@ -730,6 +730,10 @@ impl<'a> CommentRewrite<'a> {
                     {
                         let mut config = self.fmt.config.clone();
                         config.set().wrap_comments(false);
+                        let comment_max_width = config
+                            .doc_comment_code_block_width()
+                            .min(config.max_width());
+                        config.set().max_width(comment_max_width);
                         if let Some(s) =
                             crate::format_code_block(&self.code_block_buffer, &config, false)
                         {

src/config/mod.rs

@@ -57,6 +57,8 @@ create_config! {
     // Comments. macros, and strings
     wrap_comments: bool, false, false, "Break comments to fit on the line";
     format_code_in_doc_comments: bool, false, false, "Format the code snippet in doc comments.";
+    doc_comment_code_block_width: usize, 100, false, "Maximum width for code snippets in doc \
+        comments. No effect unless format_code_in_doc_comments = true";
     comment_width: usize, 80, false,
         "Maximum length of comments. No effect unless wrap_comments = true";
     normalize_comments: bool, false, false, "Convert /* */ comments to // comments where possible";
@@ -532,6 +534,7 @@ chain_width = 60
 single_line_if_else_max_width = 50
 wrap_comments = false
 format_code_in_doc_comments = false
+doc_comment_code_block_width = 100
 comment_width = 80
 normalize_comments = false
 normalize_doc_attributes = false

tests/source/issue-5359/code_comment_width_greater_max_width.rs

@@ -0,0 +1,17 @@
+// rustfmt-max_width: 50
+// rustfmt-format_code_in_doc_comments: true
+// rustfmt-doc_comment_code_block_width: 100
+
+/// ```rust
+/// impl Test {
+///     pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+///         Self::from_bytes_manual_slice(v, 0, v.len()      )
+///     }
+/// }
+/// ```
+
+impl Test {
+    pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+        Self::from_bytes_manual_slice(v, 0, v.len()       )
+    }
+}

tests/source/issue-5359/with_code_comment_max_width.rs

@@ -0,0 +1,16 @@
+// rustfmt-format_code_in_doc_comments: true
+// rustfmt-doc_comment_code_block_width: 50
+
+/// ```rust
+/// impl Test {
+///     pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+///         Self::from_bytes_manual_slice(v, 0, v.len()      )
+///     }
+/// }
+/// ```
+
+impl Test {
+    pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+        Self::from_bytes_manual_slice(v, 0, v.len()       )
+    }
+}

tests/source/issue-5359/wo_code_comment_max_width.rs

@@ -0,0 +1,15 @@
+// rustfmt-format_code_in_doc_comments: true
+
+/// ```rust
+/// impl Test {
+///     pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+///         Self::from_bytes_manual_slice(v, 0, v.len()      )
+///     }
+/// }
+/// ```
+
+impl Test {
+    pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+        Self::from_bytes_manual_slice(v, 0, v.len()       )
+    }
+}

tests/target/issue-5359/code_comment_width_greater_max_width.rs

@@ -0,0 +1,29 @@
+// rustfmt-max_width: 50
+// rustfmt-format_code_in_doc_comments: true
+// rustfmt-doc_comment_code_block_width: 100
+
+/// ```rust
+/// impl Test {
+///     pub const fn from_bytes(
+///         v: &[u8],
+///     ) -> Result<Self, ParserError> {
+///         Self::from_bytes_manual_slice(
+///             v,
+///             0,
+///             v.len(),
+///         )
+///     }
+/// }
+/// ```
+
+impl Test {
+    pub const fn from_bytes(
+        v: &[u8],
+    ) -> Result<Self, ParserError> {
+        Self::from_bytes_manual_slice(
+            v,
+            0,
+            v.len(),
+        )
+    }
+}

tests/target/issue-5359/with_code_comment_max_width.rs

@@ -0,0 +1,22 @@
+// rustfmt-format_code_in_doc_comments: true
+// rustfmt-doc_comment_code_block_width: 50
+
+/// ```rust
+/// impl Test {
+///     pub const fn from_bytes(
+///         v: &[u8],
+///     ) -> Result<Self, ParserError> {
+///         Self::from_bytes_manual_slice(
+///             v,
+///             0,
+///             v.len(),
+///         )
+///     }
+/// }
+/// ```
+
+impl Test {
+    pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+        Self::from_bytes_manual_slice(v, 0, v.len())
+    }
+}

tests/target/issue-5359/wo_code_comment_max_width.rs

@@ -0,0 +1,15 @@
+// rustfmt-format_code_in_doc_comments: true
+
+/// ```rust
+/// impl Test {
+///     pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+///         Self::from_bytes_manual_slice(v, 0, v.len())
+///     }
+/// }
+/// ```
+
+impl Test {
+    pub const fn from_bytes(v: &[u8]) -> Result<Self, ParserError> {
+        Self::from_bytes_manual_slice(v, 0, v.len())
+    }
+}