Fix failure with => in comment after match => (#6092)

* Find arrow with find_last_uncommented
* Add version gate for arrow finding fix

src/matches.rs

@@ -5,7 +5,7 @@ use std::iter::repeat;
 use rustc_ast::{ast, ptr};
 use rustc_span::{BytePos, Span};
 
-use crate::comment::{combine_strs_with_missing_comments, rewrite_comment};
+use crate::comment::{combine_strs_with_missing_comments, rewrite_comment, FindUncommented};
 use crate::config::lists::*;
 use crate::config::{Config, ControlBraceStyle, IndentStyle, MatchArmLeadingPipe, Version};
 use crate::expr::{
@@ -402,7 +402,11 @@ 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 arrow_index = if context.config.version() == Version::One {
+            arrow_snippet.rfind("=>").unwrap()
+        } else {
+            arrow_snippet.find_last_uncommented("=>").unwrap()
+        };
         // 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-version: Two
+fn main() {
+    match a {
+        _ =>
+        // comment with =>
+                {
+            println!("A")
+        }
+    }
+}

tests/source/arrow_in_comments/multiple_arrows.rs

@@ -0,0 +1,14 @@
+// rustfmt-version: Two
+fn main() {
+    match a {
+        _ => // comment with => 
+        match b {
+            // one goes to =>
+            one => {
+                println("1");
+            }
+            // two goes to =>
+            two => { println("2"); }
+        } 
+    }
+}

tests/target/arrow_in_comments/arrow_in_single_comment.rs

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

tests/target/arrow_in_comments/multiple_arrows.rs

@@ -0,0 +1,19 @@
+// rustfmt-version: Two
+fn main() {
+    match a {
+        _ =>
+        // comment with =>
+        {
+            match b {
+                // one goes to =>
+                one => {
+                    println("1");
+                }
+                // two goes to =>
+                two => {
+                    println("2");
+                }
+            }
+        }
+    }
+}