error[E0373]: closure may outlive the current function, but it borrows `x`, which is owned by the current function
  --> $DIR/async-borrowck-escaping-block-error.rs:6:20
   |
LL |     Box::new(async { x } )
   |                    ^^-^^
   |                    | |
   |                    | `x` is borrowed here
   |                    may outlive borrowed value `x`
   |
note: generator is returned here
  --> $DIR/async-borrowck-escaping-block-error.rs:4:13
   |
LL | fn foo() -> Box<impl std::future::Future<Output = u32>> {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to force the async block to take ownership of `x` (and any other referenced variables), use the `move` keyword
   |
LL |     Box::new(async move { x } )
   |                    ^^^^^^^^^^

error: aborting due to previous error

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