Fix failure with => in comment after match =>

Author: ding-young

src/matches.rs

@@ -402,7 +402,16 @@ fn rewrite_match_body(
         let arrow_snippet = context.snippet(arrow_span).trim();
         // search for the arrow starting from the end of the snippet since there may be a match
         // expression within the guard
-        let arrow_index = arrow_snippet.rfind("=>").unwrap();
+        let mut arrow_index = arrow_snippet.rfind("=>").unwrap();
+        // check whether `=>` is included in the comment
+        if arrow_index != 0 {
+            let prev_arrow = arrow_snippet[..arrow_index].trim();
+            let single_line_comment_index = prev_arrow.rfind("//").unwrap_or(0);
+            let new_line_index = prev_arrow.rfind("\n").unwrap_or(0);
+            if single_line_comment_index > new_line_index {
+                arrow_index = 0;
+            }
+        }
         // 2 = `=>`
         let comment_str = arrow_snippet[arrow_index + 2..].trim();
         if comment_str.is_empty() {

tests/source/arrow_in_comments/arrow_in_single_comment.rs

@@ -0,0 +1,10 @@
+// rustfmt-wrap_comments: true
+fn main() {
+    match a {
+        _ =>
+        // comment with =>
+                {
+            println!("A")
+        }
+    }
+}

tests/target/arrow_in_comments/arrow_in_single_comment.rs

@@ -0,0 +1,10 @@
+// rustfmt-wrap_comments: true
+fn main() {
+    match a {
+        _ =>
+        // comment with =>
+        {
+            println!("A")
+        }
+    }
+}