Move trait into attr so it's greppable

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -61,8 +61,8 @@ enum GoalEvaluationKind {
     Nested { is_normalizes_to_hack: IsNormalizesToHack },
 }
 
-#[extension]
-impl<'tcx> CanonicalResponseExt for Canonical<'tcx, Response<'tcx>> {
+#[extension(trait CanonicalResponseExt)]
+impl<'tcx> Canonical<'tcx, Response<'tcx>> {
     fn has_no_inference_or_external_constraints(&self) -> bool {
         self.value.external_constraints.region_constraints.is_empty()
             && self.value.var_values.is_identity()