"Unknown error occurred" when running cargo test with RUST_BACKTRACE=1 on the nightly

[cohen990]

$ rustup override set stable        
info: using existing install for 'stable-x86_64-apple-darwin'
info: override toolchain for '/srcroot/' set to 'stable-x86_64-apple-darwin'

  stable-x86_64-apple-darwin unchanged - rustc 1.21.0 (3b72af97e 2017-10-09)


$ RUST_BACKTRACE=1 cargo test       
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/deps/tetrus-37eb396d2309593e

running 4 tests
test well::empty_well_should::be_empty ... ok
test game::constructor_should::set_score_to_zero ... ok
test game::start_should::put_next_into_play ... ok
test tetromino_generator::next_should::return_a_tetromino ... FAILED

failures:

---- tetromino_generator::next_should::return_a_tetromino stdout ----
	thread 'tetromino_generator::next_should::return_a_tetromino' panicked at 'assertion failed: `(left != right)`
  left: `Invalid`,
 right: `Invalid`', src/tetromino_generator.rs:47:8
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::panicking::default_hook::{{closure}}
   2: std::panicking::default_hook
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic
   5: std::panicking::begin_panic_fmt
   6: tetrus::tetromino_generator::next_should::return_a_tetromino
   7: <F as test::FnBox<T>>::call_box
   8: __rust_maybe_catch_panic


failures:
    tetromino_generator::next_should::return_a_tetromino

test result: FAILED. 3 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--lib'




$ rustup override set nightly       
info: using existing install for 'nightly-x86_64-apple-darwin'
info: override toolchain for '/srcroot/' set to 'nightly-x86_64-apple-darwin'

  nightly-x86_64-apple-darwin unchanged - rustc 1.23.0-nightly (bd0e45a32 2017-11-06)

------------------------------------------------------------
$ RUST_BACKTRACE=1 cargo test       
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/deps/tetrus-0a186c44f7f3bff7

running 4 tests
test well::empty_well_should::be_empty ... ok
test game::constructor_should::set_score_to_zero ... ok
test game::start_should::put_next_into_play ... ok
error: An unknown error occurred

Let me know if you'd need more details.
Thanks a lot !

[alexcrichton]

I thinkt this is probably rust-lang/rust#45731 which was fixed very recently and I don't think has made it into the nightlies yet, but thanks for the report!