Propagate errors rather than using return_if_err

clippy_utils/src/usage.rs

@@ -21,7 +21,8 @@ pub fn mutated_variables<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) ->
         expr.hir_id.owner.def_id,
         &mut delegate,
     )
-    .walk_expr(expr);
+    .walk_expr(expr)
+    .into_ok();
 
     if delegate.skip {
         return None;