error[E0597]: `books` does not live long enough
  --> $DIR/borrowck-escaping-closure-error-2.rs:21:17
   |
LL |     Box::new(|| books.push(4))
   |              -- ^^^^^ borrowed value does not live long enough
   |              |
   |              value captured here
LL |     //~^ ERROR E0373
LL | }
   | - `books` dropped here while still borrowed
   |
note: borrowed value must be valid for the lifetime 'a as defined on the function body at 19:8...
  --> $DIR/borrowck-escaping-closure-error-2.rs:19:8
   |
LL | fn foo<'a>(x: &'a i32) -> Box<FnMut()+'a> {
   |        ^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0597`.