1 error[E0373]: closure may outlive the current function, but it borrows `books`, which is owned by the current function
2 --> $DIR/borrowck-escaping-closure-error-2.rs:21:14
4 LL | Box::new(|| books.push(4))
5 | ^^ ----- `books` is borrowed here
7 | may outlive borrowed value `books`
8 help: to force the closure to take ownership of `books` (and any other referenced variables), use the `move` keyword
10 LL | Box::new(move || books.push(4))
13 error: aborting due to previous error
15 For more information about this error, try `rustc --explain E0373`.