Prevent wrapping markdown headers in doc comments

Fixes 5238

A markdown header is defined by a string that starts with `#`.

Previously, rustfmt would wrap long markdown headers when
`wrap_comments=true`. This lead to issues when rendering these headers
in HTML using rustdoc.

Now, rustfmt leaves markdown headers alone when wrapping comments.

src/comment.rs

@@ -683,6 +683,7 @@ impl<'a> CommentRewrite<'a> {
         i: usize,
         line: &'a str,
         has_leading_whitespace: bool,
+        is_doc_comment: bool,
     ) -> bool {
         let num_newlines = count_newlines(orig);
         let is_last = i == num_newlines;
@@ -789,10 +790,19 @@ impl<'a> CommentRewrite<'a> {
             }
         }
 
-        if self.fmt.config.wrap_comments()
+        let is_markdown_header_doc_comment = is_doc_comment && line.starts_with("#");
+
+        // We only want to wrap the comment if:
+        // 1) wrap_comments = true is configured
+        // 2) The comment is not the start of a markdown header doc comment
+        // 3) The comment width exceeds the shape's width
+        // 4) No URLS were found in the commnet
+        let should_wrap_comment = self.fmt.config.wrap_comments()
+            && !is_markdown_header_doc_comment
             && unicode_str_width(line) > self.fmt.shape.width
-            && !has_url(line)
-        {
+            && !has_url(line);
+
+        if should_wrap_comment {
             match rewrite_string(line, &self.fmt, self.max_width) {
                 Some(ref s) => {
                     self.is_prev_line_multi_line = s.contains('\n');
@@ -882,7 +892,7 @@ fn rewrite_comment_inner(
         });
 
     for (i, (line, has_leading_whitespace)) in lines.enumerate() {
-        if rewriter.handle_line(orig, i, line, has_leading_whitespace) {
+        if rewriter.handle_line(orig, i, line, has_leading_whitespace, is_doc_comment) {
             break;
         }
     }

tests/source/issue-5238/markdown_header_wrap_comments_false.rs

@@ -0,0 +1,11 @@
+// rustfmt-wrap_comments: false
+
+/// no markdown header so rustfmt should wrap this comment when `format_code_in_doc_comments = true` and `wrap_comments = true`
+fn not_documented_with_markdown_header() {
+    // This is just a normal inline comment so rustfmt should wrap this comment when `wrap_comments = true`
+}
+
+/// # We're using a markdown header here so rustfmt should refuse to wrap this comment in all circumstances
+fn documented_with_markdown_header() {
+    // # We're using a markdown header in an inline comment. rustfmt should be able to wrap this comment when `wrap_comments = true`
+}

tests/source/issue-5238/markdown_header_wrap_comments_true.rs

@@ -0,0 +1,11 @@
+// rustfmt-wrap_comments: true
+
+/// no markdown header so rustfmt should wrap this comment when `format_code_in_doc_comments = true` and `wrap_comments = true`
+fn not_documented_with_markdown_header() {
+    // This is just a normal inline comment so rustfmt should wrap this comment when `wrap_comments = true`
+}
+
+/// # We're using a markdown header here so rustfmt should refuse to wrap this comment in all circumstances
+fn documented_with_markdown_header() {
+    // # We're using a markdown header in an inline comment. rustfmt should be able to wrap this comment when `wrap_comments = true`
+}

tests/target/issue-5238/markdown_header_wrap_comments_false.rs

@@ -0,0 +1,11 @@
+// rustfmt-wrap_comments: false
+
+/// no markdown header so rustfmt should wrap this comment when `format_code_in_doc_comments = true` and `wrap_comments = true`
+fn not_documented_with_markdown_header() {
+    // This is just a normal inline comment so rustfmt should wrap this comment when `wrap_comments = true`
+}
+
+/// # We're using a markdown header here so rustfmt should refuse to wrap this comment in all circumstances
+fn documented_with_markdown_header() {
+    // # We're using a markdown header in an inline comment. rustfmt should be able to wrap this comment when `wrap_comments = true`
+}

tests/target/issue-5238/markdown_header_wrap_comments_true.rs

@@ -0,0 +1,14 @@
+// rustfmt-wrap_comments: true
+
+/// no markdown header so rustfmt should wrap this comment when
+/// `format_code_in_doc_comments = true` and `wrap_comments = true`
+fn not_documented_with_markdown_header() {
+    // This is just a normal inline comment so rustfmt should wrap this comment
+    // when `wrap_comments = true`
+}
+
+/// # We're using a markdown header here so rustfmt should refuse to wrap this comment in all circumstances
+fn documented_with_markdown_header() {
+    // # We're using a markdown header in an inline comment. rustfmt should be
+    // able to wrap this comment when `wrap_comments = true`
+}