avoid many &str to String conversions with MultiSpan::push_span_label

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

@@ -2204,8 +2204,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             _ => true,
                         };
                     if !ident.span.overlaps(span) && !same_line {
-                        multispan
-                            .push_span_label(ident.span, "required by a bound in this".to_string());
+                        multispan.push_span_label(ident.span, "required by a bound in this");
                     }
                 }
                 let descr = format!("required by a bound in `{}`", item_name);