ICE with type error and impl Fn

[pthariensflame]

Using the conservative_impl_trait feature with the Fn traits seems to create an ICE when there’s a type error.

The following code (also available on the Playground here):

#![crate_type = "lib"]
#![feature(conservative_impl_trait)]

pub fn partial_apply<A, B, C, F>(f: F, x: A) -> impl FnOnce(B) -> C
    where F: FnOnce(A, B) -> C
{
    &|y| f(x, y)
}

causes the following compiler output:

thread 'rustc' panicked at 'Box<Any>', ../src/librustc_errors/lib.rs:590
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
 --> test.rs:7:6
  |
7 |     &|y| f(x, y)
  |      ^^^^^^^^^^^
  |
note: the requirement to implement `Fn` derives from here
 --> test.rs:4:49
  |
4 | pub fn partial_apply<A, B, C, F>(f: F, x: A) -> impl FnOnce(B) -> C
  |                                                 ^^^^^^^^^^^^^^^^^^^

error: internal compiler error: ../src/librustc_typeck/check/writeback.rs:347: invalid region in impl Trait: ReEmpty
 --> test.rs:4:49
  |
4 | pub fn partial_apply<A, B, C, F>(f: F, x: A) -> impl FnOnce(B) -> C
  |                                                 ^^^^^^^^^^^^^^^^^^^

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

Obviously, the error message is correct, if somewhat hard to decipher. The ICE is worrying, however.

Backtrace:

thread 'rustc' panicked at 'Box<Any>', ../src/librustc_errors/lib.rs:590
stack backtrace:
   1:        0x10897757a - std::sys::backtrace::tracing::imp::write::h46f28e67d38b4637
   2:        0x108985c7f - std::panicking::default_hook::{{closure}}::h1d3243f546573ff4
   3:        0x10898431d - std::panicking::default_hook::h96c288d728df3ebf
   4:        0x1089849b6 - std::panicking::rust_panic_with_hook::hb1322e5f2588b4db
   5:        0x10441551b - std::panicking::begin_panic::hb7c71bbc491f561c
   6:        0x104547813 - rustc::session::opt_span_bug_fmt::{{closure}}::h0a2486f2fff54208
   7:        0x10445e1a9 - rustc::session::span_bug_fmt::heb3fdb6acac8d0c7
   8:        0x104546455 - rustc_typeck::check::writeback::WritebackCx::visit_anon_types::{{closure}}::hfd3c46aef9bd9a7d
   9:        0x10542670c - <rustc::ty::fold::RegionFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_region::h331824092188ef10
  10:        0x1044326ce - rustc::ty::fold::TypeFolder::fold_ty::hb7dbb94e06be626a
  11:        0x10448ccb5 - rustc_typeck::check::writeback::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::resolve_type_vars_in_fn::hceb857dfd8fca940
  12:        0x1044e0d3d - rustc_typeck::check::check_bare_fn::haa7199476c92e826
  13:        0x1044e3f55 - rustc_typeck::check::check_item_body::h4df24fa6569eb6cd
  14:        0x1044de6e7 - rustc_typeck::check::check_item_bodies::h2e489dd76c967c43
  15:        0x104544457 - rustc_typeck::check_crate::h279dc52e9bc1b4c2
  16:        0x10431fdfc - rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::h2b062b0c671cd504
  17:        0x1042f6c14 - rustc_driver::driver::phase_3_run_analysis_passes::h2f1bd27ff2e4d8a0
  18:        0x1042e9836 - rustc_driver::driver::compile_input::h7dacd98cd2fd7d2b
  19:        0x104312778 - rustc_driver::run_compiler::h37c4294ab73436f7
  20:        0x104257170 - std::panicking::try::do_call::h4d040997e2efdaf3
  21:        0x10898c85a - __rust_maybe_catch_panic
  22:        0x104275164 - <F as alloc::boxed::FnBox<A>>::call_box::hefd1c85fab3e6c31
  23:        0x108983194 - std::sys::thread::Thread::new::thread_start::h5b631f48cd23f128
  24:     0x7fffb0456aba - _pthread_body
  25:     0x7fffb0456a06 - _pthread_start

rustc version:

rustc 1.13.0-nightly (91f057de3 2016-09-04)
binary: rustc
commit-hash: 91f057de35066d0a34102bd0673b56684509b93d
commit-date: 2016-09-04
host: x86_64-apple-darwin
release: 1.13.0-nightly

[eddyb]

Duplicate of #35668. Easy to ignore if you someone else wants to poke it.