Backport 4304

src/comment.rs

@@ -607,16 +607,22 @@ impl<'a> CommentRewrite<'a> {
         cr
     }
 
-    fn join_block(s: &str, sep: &str) -> String {
+    fn join_block(s: &str, sep: &str, add_sep_prefix: bool) -> String {
         let mut result = String::with_capacity(s.len() + 128);
         let mut iter = s.lines().peekable();
-        while let Some(line) = iter.next() {
-            result.push_str(line);
-            result.push_str(match iter.peek() {
-                Some(next_line) if next_line.is_empty() => sep.trim_end(),
+        let get_sep = |line: Option<&&str>| -> &str {
+            match line {
+                Some(&"") => sep.trim_end(),
                 Some(..) => sep,
                 None => "",
-            });
+            }
+        };
+        if add_sep_prefix {
+            result.push_str(get_sep(iter.peek()));
+        }
+        while let Some(line) = iter.next() {
+            result.push_str(line);
+            result.push_str(get_sep(iter.peek()));
         }
         result
     }
@@ -634,10 +640,10 @@ impl<'a> CommentRewrite<'a> {
         if !self.code_block_buffer.is_empty() {
             // There is a code block that is not properly enclosed by backticks.
             // We will leave them untouched.
-            self.result.push_str(&self.comment_line_separator);
             self.result.push_str(&Self::join_block(
                 &trim_custom_comment_prefix(&self.code_block_buffer),
                 &self.comment_line_separator,
+                true, /* add_sep_prefix */
             ));
         }
 
@@ -660,10 +666,12 @@ impl<'a> CommentRewrite<'a> {
                 Some(s) => self.result.push_str(&Self::join_block(
                     &s,
                     &format!("{}{}", self.comment_line_separator, ib.line_start),
+                    false, /* add_sep_prefix */
                 )),
                 None => self.result.push_str(&Self::join_block(
                     &ib.original_block_as_string(),
                     &self.comment_line_separator,
+                    false, /* add_sep_prefix */
                 )),
             };
         }
@@ -715,10 +723,12 @@ impl<'a> CommentRewrite<'a> {
                 Some(s) => self.result.push_str(&Self::join_block(
                     &s,
                     &format!("{}{}", self.comment_line_separator, ib.line_start),
+                    false, /* add_sep_prefix */
                 )),
                 None => self.result.push_str(&Self::join_block(
                     &ib.original_block_as_string(),
                     &self.comment_line_separator,
+                    false, /* add_sep_prefix */
                 )),
             };
         } else if self.code_block_attr.is_some() {
@@ -741,9 +751,11 @@ impl<'a> CommentRewrite<'a> {
                     _ => trim_custom_comment_prefix(&self.code_block_buffer),
                 };
                 if !code_block.is_empty() {
-                    self.result.push_str(&self.comment_line_separator);
-                    self.result
-                        .push_str(&Self::join_block(&code_block, &self.comment_line_separator));
+                    self.result.push_str(&Self::join_block(
+                        &code_block,
+                        &self.comment_line_separator,
+                        true, /* add_sep_prefix */
+                    ));
                 }
                 self.code_block_buffer.clear();
                 self.result.push_str(&self.comment_line_separator);

tests/source/issue-4251.rs

@@ -0,0 +1,6 @@
+// rustfmt-wrap_comments: true
+
+//! ```
+//! 
+//! use something;
+//! ```

tests/target/issue-4251.rs

@@ -0,0 +1,6 @@
+// rustfmt-wrap_comments: true
+
+//! ```
+//!
+//! use something;
+//! ```