Consts + nevertype ICE

[DutchGhost]

Probably one should never write this, and I wonder if it will be disallowed in the future.
Happens on stable, beta and nightly, in all const-forms existing (includes const generics)

pub const unsafe fn fake_type<T>() -> T {
    hint_unreachable()
}

#[allow(unconditional_recursion)]
pub const unsafe fn hint_unreachable() -> ! {
    hint_unreachable()
}

trait Const {
    const CONSTANT: i32 = unsafe { fake_type() };
}

impl <T> Const for T {}


pub fn main() -> () {
    dbg!(i32::CONSTANT);
}

Backtrace:

 Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e/src/libcore/slice/mod.rs:2686:10
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
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:381
   7: rust_begin_unwind
             at src/libstd/panicking.rs:308
   8: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   9: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:61
  10: rustc_mir::interpret::eval_context::InterpretCx<M>::layout_of_local
  11: rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_fn_call::{{closure}}
  12: rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_fn_call
  13: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::run
  14: rustc_mir::const_eval::eval_body_using_ecx
  15: rustc_mir::const_eval::const_eval_raw_provider
  16: rustc::ty::query::__query_compute::const_eval_raw
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_raw>::compute
  18: rustc::dep_graph::graph::DepGraph::with_task_impl
  19: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  20: rustc_mir::interpret::eval_context::InterpretCx<M>::const_eval_raw
  21: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_const_to_op
  22: rustc_mir::transform::const_prop::ConstPropagator::eval_constant
  23: rustc_mir::transform::const_prop::ConstPropagator::const_prop
  24: <rustc_mir::transform::const_prop::ConstPropagator as rustc::mir::visit::Visitor>::visit_statement
  25: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  26: rustc_mir::transform::run_passes::{{closure}}
  27: rustc_mir::transform::run_passes
  28: rustc_mir::transform::optimized_mir
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc_mir::monomorphize::collector::collect_items_rec
  33: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  34: rustc::util::common::time
  35: rustc_mir::monomorphize::collector::collect_crate_mono_items
  36: rustc::util::common::time
  37: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  38: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  39: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute
  40: rustc::dep_graph::graph::DepGraph::with_task_impl
  41: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  42: rustc_codegen_ssa::base::codegen_crate
  43: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  44: rustc::util::common::time
  45: rustc_interface::passes::start_codegen
  46: rustc::ty::context::tls::enter_global
  47: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  48: rustc_interface::passes::create_global_ctxt::{{closure}}
  49: rustc_interface::passes::BoxedGlobalCtxt::enter
  50: rustc_interface::queries::Query<T>::compute
  51: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  52: rustc_interface::interface::run_compiler_in_existing_thread_pool
  53: std::thread::local::LocalKey<T>::with
  54: scoped_tls::ScopedKey<T>::set
  55: syntax::with_globals
query stack during panic:
#0 [const_eval_raw] const-evaluating `Const::CONSTANT`
  --> src/main.rs:18:10
   |
18 |     dbg!(i32::CONSTANT);
   |          ^^^^^^^^^^^^^
#1 [optimized_mir] processing `main`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
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 1.35.0 (3c235d560 2019-05-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -C opt-level=3 -C codegen-units=1 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[DutchGhost]

Note:
You could also write it the following way, in order to not need the #[allow(unconditional_recursion)] attribute, as the previous code complains about recursion without that attribute.

pub const unsafe fn fake_type<T>() -> T {
    hint_unreachable()
}

pub const unsafe fn hint_unreachable() -> ! {
    fake_type()
}

[hellow554]

Regression introduced in f40aaa6

[varkor]

cc @oli-obk: due to #58000.

Edit: I think I have a fix for this.