error: this function taking a value by value, but only using them by reference
 --> $DIR/needless_take_by_value.rs:9:23
  |
9 | fn foo<T: Default>(v: Vec<T>, w: Vec<T>, mut x: Vec<T>, y: Vec<T>) -> Vec<T> {
  |                       ^^^^^^
  |
note: lint level defined here
 --> $DIR/needless_take_by_value.rs:4:9
  |
4 | #![deny(needless_take_by_value)]
  |         ^^^^^^^^^^^^^^^^^^^^^^
help: consider taking a reference instead
  | fn foo<T: Default>(v: &Vec<T>, w: Vec<T>, mut x: Vec<T>, y: Vec<T>) -> Vec<T> {

error: aborting due to previous error