error[E0373]: closure may outlive the current function, but it borrows `books`, which is owned by the current function
  --> $DIR/borrowck-escaping-closure-error-1.rs:13:11
   |
LL |     spawn(|| books.push(4));
   |           ^^ ----- `books` is borrowed here
   |           |
   |           may outlive borrowed value `books`
   |
note: function requires argument type to outlive `'static`
  --> $DIR/borrowck-escaping-closure-error-1.rs:13:5
   |
LL |     spawn(|| books.push(4));
   |     ^^^^^^^^^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `books` (and any other referenced variables), use the `move` keyword
   |
LL |     spawn(move || books.push(4));
   |           ++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0373`.