thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:335:20
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::_print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: <core::option::Option<T>>::unwrap    <-- nothing about "bar"
  11: foo::main
  12: __rust_maybe_catch_panic
  13: std::rt::lang_start
  14: main