Restrict the width of doc comments with comment_width

See the diff in tests/target/enum.rs for an example.

src/attr.rs

@@ -21,8 +21,6 @@ use rewrite::{Rewrite, RewriteContext};
 use shape::Shape;
 use utils::{count_newlines, mk_sp};
 
-use std::cmp;
-
 /// Returns attributes on the given statement.
 pub fn get_attrs_from_stmt(stmt: &ast::Stmt) -> &[ast::Attribute] {
     match stmt.node {
@@ -140,7 +138,7 @@ fn rewrite_first_group_attrs(
             .join("\n");
         return Some((
             sugared_docs.len(),
-            rewrite_doc_comment(&snippet, shape, context.config)?,
+            rewrite_doc_comment(&snippet, shape.comment(context.config), context.config)?,
         ));
     }
     // Rewrite `#[derive(..)]`s.
@@ -249,13 +247,7 @@ impl Rewrite for ast::Attribute {
         };
         let snippet = context.snippet(self.span);
         if self.is_sugared_doc {
-            let doc_shape = Shape {
-                width: cmp::min(shape.width, context.config.comment_width())
-                    .checked_sub(shape.indent.width())
-                    .unwrap_or(0),
-                ..shape
-            };
-            rewrite_doc_comment(snippet, doc_shape, context.config)
+            rewrite_doc_comment(snippet, shape.comment(context.config), context.config)
         } else {
             if contains_comment(snippet) {
                 return Some(snippet.to_owned());

src/shape.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 use std::borrow::Cow;
+use std::cmp::min;
 use std::ops::{Add, Sub};
 
 use Config;
@@ -276,6 +277,17 @@ impl Shape {
             .checked_sub(self.used_width() + self.width)
             .unwrap_or(0)
     }
+
+    pub fn comment(&self, config: &Config) -> Shape {
+        let width = min(
+            self.width,
+            config
+                .comment_width()
+                .checked_sub(self.indent.width())
+                .unwrap_or(0),
+        );
+        Shape { width, ..*self }
+    }
 }
 
 #[cfg(test)]

tests/target/enum.rs

@@ -145,8 +145,9 @@ pub enum Bencoding<'i> {
     Str(&'i [u8]),
     Int(i64),
     List(Vec<Bencoding<'i>>),
-    /// A bencoded dict value. The first element the slice of bytes in the source that the dict is
-    /// composed of. The second is the dict, decoded into an ordered map.
+    /// A bencoded dict value. The first element the slice of bytes in the
+    /// source that the dict is composed of. The second is the dict,
+    /// decoded into an ordered map.
     // TODO make Dict "structlike" AKA name the two values.
     Dict(&'i [u8], BTreeMap<&'i [u8], Bencoding<'i>>),
 }