ICE in presence of bang type

[bjorn3]

This test is compile-fail/never_transmute_humans.rs from miri

// This should fail even without validation
// compile-flags: -Zmir-emit-validate=0

#![feature(never_type)]
#![allow(unreachable_code)]
#![allow(unused_variables)]

struct Human;

fn main() {
    let x: ! = unsafe {
        std::mem::transmute::<Human, !>(Human) //~ ERROR entered unreachable code
    };
    f(x)
}

fn f(x: !) -> ! { x }

backtrace

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
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: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: rustc_trans::mir::block::<impl rustc_trans::mir::FunctionCx<'a, 'tcx>>::trans_terminator
  11: rustc_trans::mir::trans_mir
  12: rustc_trans::base::trans_instance
  13: rustc_trans::trans_item::MonoItemExt::define
  14: rustc_trans::base::compile_codegen_unit
  15: rustc::ty::maps::<impl rustc::ty::maps::config::QueryConfig<'tcx> for rustc::ty::maps::queries::compile_codegen_unit<'tcx>>::compute
  16: rustc::dep_graph::graph::DepGraph::with_task_impl
  17: rustc::ty::context::tls::with_related_context
  18: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  19: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  20: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::compile_codegen_unit
  21: rustc_trans::base::trans_crate
  22: <rustc_trans::LlvmTransCrate as rustc_trans_utils::trans_crate::TransCrate>::trans_crate
  23: rustc::util::common::time
  24: rustc_driver::driver::phase_4_translate_to_llvm
  25: rustc_driver::driver::compile_input::{{closure}}
  26: rustc::ty::context::tls::enter_context
  27: <std::thread::local::LocalKey<T>>::with
  28: rustc::ty::context::TyCtxt::create_and_enter
  29: rustc_driver::driver::compile_input
  30: rustc_driver::run_compiler_impl
  31: <scoped_tls::ScopedKey<T>>::set
  32: syntax::with_globals
  33: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  34: __rust_maybe_catch_panic
  35: rustc_driver::run
  36: rustc_driver::main
  37: std::rt::lang_start::{{closure}}
  38: std::panicking::try::do_call
  39: __rust_maybe_catch_panic
  40: std::rt::lang_start_internal
  41: main
query stack during panic:
#0 [compile_codegen_unit] compile_codegen_unit
end of query stack

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 1.27.0-nightly (8ff4b4206 2018-05-08) running on x86_64-apple-darwin

[varkor]

There's some discussion of mem::transmute with ! here: rust-lang/rfcs#1892. However, it doesn't look like anything was decided in terms of forbidding it since rust-lang/rfcs#1891 was closed.

[RalfJung]

This code is insta-UB if it ever gets executed, but it should still not ICE, obviously.