ICE with const transmute and existential type

[DutchGhost]

Not sure if it's really something that is ever gonna happen in real code, but the following ICE's:

#![feature(existential_type)]
#![feature(const_fn)]
#![feature(untagged_unions)]

const fn transmute<T, U>(t: T) -> U {
    union Transform<TT, UU> {
        t: TT,
        u: UU,
    }
    
    unsafe { Transform { t }.u }
}


existential type Foo: Fn();
const foo: Foo = transmute(|| {});

fn bar() -> Foo { || {} }

fn main() {
    foo();
}

Backtrace:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Foo`,
 right: `[closure@src/main.rs:18:19: 18:24]`', librustc_mir/transform/const_prop.rs:338:17
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:479
   6: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   7: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:345
   8: <rustc_mir::transform::const_prop::ConstPropagator<'b, 'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_statement
   9: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  10: rustc_mir::transform::optimized_mir::{{closure}}
  11: rustc_mir::transform::optimized_mir
  12: rustc::ty::query::__query_compute::optimized_mir
  13: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::optimized_mir<'tcx>>::compute
  14: rustc::dep_graph::graph::DepGraph::with_task_impl
  15: rustc::ty::context::tls::with_related_context
  16: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_query
  18: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::instance_mir
  19: rustc_mir::monomorphize::collector::collect_items_rec
  20: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  21: rustc::util::common::time
  22: rustc_mir::monomorphize::collector::collect_crate_mono_items
  23: rustc::util::common::time
  24: rustc_codegen_llvm::base::collect_and_partition_mono_items
  25: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::collect_and_partition_mono_items<'tcx>>::compute
  26: rustc::dep_graph::graph::DepGraph::with_task_impl
  27: rustc::ty::context::tls::with_related_context
  28: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  30: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  31: rustc::util::common::time
  32: rustc_driver::driver::phase_4_codegen
  33: rustc_driver::driver::compile_input::{{closure}}
  34: rustc::ty::context::tls::enter_context
  35: <std::thread::local::LocalKey<T>>::with
  36: rustc::ty::context::TyCtxt::create_and_enter
  37: rustc_driver::driver::compile_input
  38: rustc_driver::run_compiler_with_pool
  39: <scoped_tls::ScopedKey<T>>::set
  40: syntax::with_globals
  41: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  42: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  43: rustc_driver::run
  44: rustc_driver::main
  45: std::rt::lang_start::{{closure}}
  46: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  47: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  48: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  49: main
  50: __libc_start_main
  51: <unknown>
query stack during panic:
#0 [optimized_mir] processing `main`
#1 [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 1.30.0-nightly (3edb355b7 2018-08-03) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --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]

Just figured out that declaring the existential type like this:
existential type Foo: Fn() + Copy won't panic. I guess because its being copied?
But I think that it should not be allowed,

#![feature(existential_type)]
#![feature(const_fn)]
#![feature(untagged_unions)]

const fn transmute<T, U>(t: T) -> U {
    union Transform<TT, UU> {
        t: TT,
        u: UU,
    }
    
    unsafe { Transform { t }.u }
}


existential type Foo: Fn() + Copy;
const BAZR: Foo = transmute(|| {});

fn bar() -> Foo { || {} }

fn main() {
    let x = BAZR();
    
    println!("{:?}", x);
}

[DutchGhost]

A version without transmutes and unsafe {}, but with pointer casts instead:

#![feature(existential_type)]

existential type Foo: Copy;

const BAZ: *const Foo = &0u8 as *const _ as *const _;

fn bar() -> Foo { 0u32 }

fn main() {
    let _ = BAZ;
}