internal: Move TyBuilder::subst_for_def into a block

Author: davidbarsky

crates/hir-ty/src/method_resolution.rs

@@ -1439,15 +1439,15 @@ fn is_valid_fn_candidate(
             _ => unreachable!(),
         };
 
-        let fn_subst = TyBuilder::subst_for_def(db, fn_id, Some(impl_subst.clone()))
-            .fill_with_inference_vars(table)
-            .build();
-
         check_that!(table.unify(&expect_self_ty, self_ty));
 
         if let Some(receiver_ty) = receiver_ty {
             check_that!(data.has_self_param());
 
+            let fn_subst = TyBuilder::subst_for_def(db, fn_id, Some(impl_subst.clone()))
+                .fill_with_inference_vars(table)
+                .build();
+
             let sig = db.callable_item_signature(fn_id.into());
             let expected_receiver =
                 sig.map(|s| s.params()[0].clone()).substitute(Interner, &fn_subst);