Improve StringMatchCandidate::new interface (#22011)

Release Notes:

- N/A

crates/assistant/src/slash_command/diagnostics_command.rs

@@ -218,10 +218,7 @@ impl Options {
     }
 
     fn match_candidates_for_args() -> [StringMatchCandidate; 1] {
-        [StringMatchCandidate::new(
-            0,
-            INCLUDE_WARNINGS_ARGUMENT.to_string(),
-        )]
+        [StringMatchCandidate::new(0, INCLUDE_WARNINGS_ARGUMENT)]
     }
 }