FullfillmentError ICE with const fn and existential type

[DutchGhost]

Again something that is highly unlikely to ever be found in real code, but nonetheless ICE's:

MCVE: #53092 (comment)

#![feature(const_fn)]
#![feature(existential_type)]
#![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<F, U>: Fn(F) -> U + Copy;

const BAZR: Foo<u8, ()> = transmute(|x: ()| { x });

fn bar<T, U: From<T>>() -> Foo<T, U> { |x| x.into() }

fn main() {
    let x = BAZR(0);
}

Playground: https://play.rust-lang.org/?gist=47ee8d54d29329821783e315423aa352&version=nightly&mode=release&edition=2015

Backtrace:

error: internal compiler error: librustc/traits/codegen/mod.rs:169: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<() as std::convert::From<u8>>)),depth=1),Unimplemented)]` resolving bounds after type-checking

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:516:9
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::begin_panic
   7: rustc_errors::Handler::span_bug
   8: rustc::session::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::session::opt_span_bug_fmt
  13: rustc::session::span_bug_fmt
  14: rustc::traits::codegen::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::drain_fulfillment_cx_or_panic
  15: rustc::ty::context::tls::with_related_context
  16: rustc::traits::codegen::codegen_fulfill_obligation
  17: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  18: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::codegen_fulfill_obligation<'tcx>>::compute
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
  20: rustc::ty::context::tls::with_related_context
  21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  23: rustc::ty::instance::Instance::resolve
  24: rustc_mir::monomorphize::collector::visit_fn_use
  25: <rustc_mir::monomorphize::collector::MirNeighborCollector<'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_terminator_kind
  26: rustc_mir::monomorphize::collector::collect_items_rec
  27: rustc_mir::monomorphize::collector::collect_items_rec
  28: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  29: rustc::util::common::time
  30: rustc_mir::monomorphize::collector::collect_crate_mono_items
  31: rustc::util::common::time
  32: rustc_codegen_llvm::base::collect_and_partition_mono_items
  33: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::collect_and_partition_mono_items<'tcx>>::compute
  34: rustc::dep_graph::graph::DepGraph::with_task_impl
  35: rustc::ty::context::tls::with_related_context
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  37: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  38: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  39: rustc::util::common::time
  40: rustc_driver::driver::phase_4_codegen
  41: rustc_driver::driver::compile_input::{{closure}}
  42: rustc::ty::context::tls::enter_context
  43: <std::thread::local::LocalKey<T>>::with
  44: rustc::ty::context::TyCtxt::create_and_enter
  45: rustc_driver::driver::compile_input
  46: rustc_driver::run_compiler_with_pool
  47: <scoped_tls::ScopedKey<T>>::set
  48: syntax::with_globals
  49: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  50: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  51: rustc_driver::run
  52: rustc_driver::main
  53: std::rt::lang_start::{{closure}}
  54: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  55: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  56: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  57: main
  58: __libc_start_main
  59: <unknown>
query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::convert::Into` fulfills its obligations
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error


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 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.

Changing

fn bar<T, U: From<T>>() -> Foo<T, U> { |x| x.into() }

to

fn bar<T, U: From<T>>() -> Foo<T, U> { |x| U::from(x) }

also ICE's but with a different message:

error: internal compiler error: librustc/traits/codegen/mod.rs:68: Encountered error `Unimplemented` selecting `Binder(<() as std::convert::From<u8>>)` during codegen`

Removing the Copy from the existential type, gives the same ICE as #53087

[Alexendoo]

Updated reproduction:

Playground

#![feature(const_fn)]
#![feature(const_fn_union)]
#![feature(type_alias_impl_trait)]
#![feature(untagged_unions)]

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

type Foo<F, U> = impl Fn(F) -> U + Copy;

const BAZR: Foo<u8, ()> = transmute(|x: ()| { x });

fn bar<T, U: From<T>>() -> Foo<T, U> { |x| x.into() }

fn main() {
    let x = BAZR(0);
}

[JohnTitor]

The ICE doesn't appear with the latest example on the latest nightly, marked as E-needstest