fix: wrap_comments creating invalid code blocks

Author: grant0417

src/comment.rs

@@ -885,6 +885,9 @@ impl<'a> CommentRewrite<'a> {
                 // Remove space if this is an empty comment or a doc comment.
                 self.result.pop();
             }
+            if self.code_block_attr.is_some() && self.is_prev_line_multi_line {
+                self.result.push_str(&self.comment_line_separator);
+            }
             self.result.push_str(line);
             self.fmt.shape = Shape::legacy(self.max_width, self.fmt_indent);
             self.is_prev_line_multi_line = false;

tests/target/issue-6416/unwrapped.rs

@@ -0,0 +1,7 @@
+// rustfmt-wrap_comments: false
+
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam
+/// ```
+/// let foo = 0;
+/// ```
+fn bar() {}

tests/target/issue-6416/wrapped.rs

@@ -0,0 +1,8 @@
+// rustfmt-wrap_comments: true
+
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam
+/// lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam
+/// ```
+/// let foo = 0;
+/// ```
+fn bar() {}