Clean up mut keyword check

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1369,13 +1369,9 @@ fn suggest_change_mut(
                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
                     if points_at_arg && mutability.is_not() && refs_number > 0 {
                         // If we have a call like foo(&mut buf), then don't suggest foo(&mut mut buf)
-                        if "mut"
-                            == snippet
-                                .chars()
-                                .filter(|c| !c.is_whitespace())
-                                .skip(refs_number)
-                                .take(3)
-                                .collect::<String>()
+                        if snippet
+                            .trim_start_matches(|c: char| c.is_whitespace() || c == '&')
+                            .starts_with("mut")
                         {
                             return;
                         }