$ ./foo
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /Users/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-mac/build/src/libcore/option.rs:323
note: Run with `RUST_BACKTRACE=1` for a backtrace.
fatal runtime error: failed to initiate panic, error 5
Abort trap: 6