ICE with generic associated type

[antonok-edm]

The following minimal example causes an internal compiler error.

struct Foo;

impl Iterator for Foo {
    type Item<'b> = &'b Foo;

    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}

fn main() {}

Error in question:

> RUST_BACKTRACE=1 cargo +stable build
   Compiling icetest v0.1.0 (/tmp/icetest)
error[E0658]: generic associated types are unstable (see issue #44265)
 --> src/main.rs:4:5
  |
4 |     type Item<'b> = &'b Foo;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: src/librustc/ty/subst.rs:426: Region parameter out of range when substituting in region 'b (root type=Some(&'b Foo)) (index=0)

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:558:9
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:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:482
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::span_bug
   8: rustc::util::bug::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::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::span_bug_fmt
  14: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_region
  15: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
  16: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  17: rustc::traits::project::opt_normalize_projection_type
  18: rustc::traits::project::normalize_projection_type
  19: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  20: <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  21: rustc::ty::fold::TypeFoldable::fold_with
  22: rustc::ty::fold::TypeFoldable::fold_with
  23: rustc::ty::fold::TypeFoldable::fold_with
  24: rustc::traits::project::normalize
  25: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
  26: <core::iter::adapters::flatten::FlatMap<I, U, F> as core::iter::traits::iterator::Iterator>::next
  27: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  28: rustc::ty::wf::trait_obligations
  29: rustc::ty::context::GlobalCtxt::enter_local
  30: rustc_typeck::check::wfcheck::check_item_well_formed
  31: rustc::ty::query::__query_compute::check_item_well_formed
  32: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::check_item_well_formed<'tcx>>::compute
  33: rustc::dep_graph::graph::DepGraph::with_task_impl
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  35: rustc::hir::Crate::visit_all_item_likes
  36: rustc::util::common::time
  37: rustc_typeck::check_crate
  38: <std::thread::local::LocalKey<T>>::with
  39: rustc::ty::context::TyCtxt::create_and_enter
  40: rustc_driver::driver::compile_input
  41: rustc_driver::run_compiler_with_pool
  42: <scoped_tls::ScopedKey<T>>::set
  43: rustc_driver::run_compiler
  44: syntax::with_globals
  45: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:87
  46: <F as alloc::boxed::FnBox<A>>::call_box
  47: std::sys::unix::thread::Thread::new::thread_start
             at /rustc/fc50f328b0353b285421b8ff5d4100966387a997/src/liballoc/boxed.rs:759
             at src/libstd/sys_common/thread.rs:14
             at src/libstd/sys/unix/thread.rs:81
  48: start_thread
  49: __clone
query stack during panic:
#0 [check_item_well_formed] processing `<Foo as std::iter::Iterator>`
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.

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 1.34.1 (fc50f328b 2019-04-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

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

error: Could not compile `icetest`.

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

rustc version:

> rustc +stable --version --verbose
rustc 1.34.1 (fc50f328b 2019-04-24)
binary: rustc
commit-hash: fc50f328b0353b285421b8ff5d4100966387a997
commit-date: 2019-04-24
host: x86_64-unknown-linux-gnu
release: 1.34.1
LLVM version: 8.0

This error was encountered on stable, but I can confirm that it also occurs on the latest nightly (1.36.0-nightly (cfdc84a00 2019-05-07)).

[hellow554]

it also occurs on the latest nightly

Hasn't the feature gate issue been fixed? :|

[nikomatsakis]

I've removed the nomination and added this to the tracking issue for GATs #44265. The current impl is highly incomplete and ICEs are, sadly, expected. Also, I verified that a feature gate is indeed required.

[nikomatsakis]

triage: P-medium

[hellow554]

Also, I verified that a feature gate is indeed required.

I don't know if I get you wrong, but the example above ICEs on stable, beta and nightly without feature gate

[antonok-edm]

@nikomatsakis @hellow554 oops, looks like I accidentally left the feature gate in there when I tried it out, but it produces the same ICE when it is omitted as well.

[hellow554]

looks like I accidentally left the feature gate in there

No, niko edited your question.
ping @nikomatsakis I think you got something wrong there. Can you please clarify that?

[nikomatsakis]

Oh, I guess I was indeed incorreect. I tried it on play but didn't notice that, after the "correct" error, it goes on to print an ICE.

[hellow554]

What about adding the I-nominated tag again?

[pnkfelix]

I'm not sure why we would nominate this for discussion. Yes, it is an ICE (and an ICE that occurs without the user adding a feature gate), but the message pretty clearly states that you're using an unstable feature.

It would be great to fix it so that it issues the error diagnostic without ICEing, but I wouldn't re-prioritize that work above P-high P-medium.

[pnkfelix]

assigning to self since its a clear annoyance

[pnkfelix]

triage: P-medium. (dupe of #61064)

[pnkfelix]

(the remaining bug, when the feature gate is enabled, is tracked in issue #62521)