exit status: 1 with stderr:
<stdin>:4:15: error: immutable value 'x' may only be initialized once async let x = foo() ^ <stdin>:4:19: note: initial value already provided in 'let' declaration async let x = foo() ^ <stdin>:4:15: note: change 'let' to 'var' to make it mutable async let x = foo() ^~~ var