$ export RUST_BACKTRACE=1
$ echo | rustc +nightly --crate-type cdylib --target aarch64-apple-ios -
warning: dropping unsupported crate type `cdylib` for target `aarch64-apple-ios`

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::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: rustc_trans::base::write_metadata
  11: rustc::util::common::time
  12: rustc_trans::base::trans_crate
  13: <rustc_trans::LlvmTransCrate as rustc_trans_utils::trans_crate::TransCrate>::trans_crate
  14: rustc::util::common::time
  15: rustc_driver::driver::phase_4_translate_to_llvm
  16: rustc_driver::driver::compile_input::{{closure}}
  17: <std::thread::local::LocalKey<T>>::with
  18: <std::thread::local::LocalKey<T>>::with
  19: rustc::ty::context::TyCtxt::create_and_enter
  20: rustc_driver::driver::compile_input
  21: rustc_driver::run_compiler

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.25.0-nightly (7d6e5b9da 2018-01-27) running on x86_64-apple-darwin