ICE with lifetimes and associated types

[mneumann]

The program in question:

// t.rs
#![feature(associated_types)]

struct Foo<'a> {
    buf: &'a[u8]
}

impl<'a> Iterator for Foo<'a> {
    type Item = &'a[u8];

    fn next(&mut self) -> Option<<Self as Iterator>::Item> {
    //THIS WORKS: fn next(&mut self) -> Option<&'a[u8]> {
        Some(self.buf)
    }
}

fn main() {
}

Fails with:

# rustc --version
rustc 0.13.0-nightly (c6c786671 2015-01-04 00:50:59 +0000)

# RUST_BACKTRACE=1 rustc t.rs
t.rs:3:1: 5:2 warning: struct is never used: `Foo`, #[warn(dead_code)] on by default
t.rs:3 struct Foo<'a> {
t.rs:4     buf: &'a[u8]
t.rs:5 }
t.rs:1:1: 1:1 error: internal compiler error: cannot relate bound region: '_#0r <= ReEarlyBound(38, TypeSpace, 0, 'a)
t.rs:1 #![feature(associated_types)]
       ^
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:123

stack backtrace:
   1:     0x7fdf7acc52d0 - sys::backtrace::write::h8532e701ef86014f4it
   2:     0x7fdf7aceab00 - failure::on_fail::h7532e1f79d134d5dzvz
   3:     0x7fdf7ac501c0 - rt::unwind::begin_unwind_inner::h97b151606151d62deaz
   4:     0x7fdf75b24c60 - rt::unwind::begin_unwind::h15809447133099964284
   5:     0x7fdf75b24bf0 - diagnostic::SpanHandler::span_bug::he8142ababcc30c39DFF
   6:     0x7fdf78f883c0 - middle::infer::region_inference::RegionVarBindings<'a, 'tcx>::make_subregion::h4894010abd5842ddRew
   7:     0x7fdf78f24620 - middle::infer::region_inference::RegionVarBindings<'a, 'tcx>::make_eqregion::haf414bd9d86c8f74few
   8:     0x7fdf78f24350 - middle::infer::equate::Equate<'f, 'tcx>.Combine<'tcx>::regions::hfbdb19d5c7340e32vbr
   9:     0x7fdf78f291a0 - middle::infer::combine::Combine::substs_variances::h10247305425180389096
  10:     0x7fdf78f27ca0 - middle::infer::combine::Combine::substs::h14741200973438308956
  11:     0x7fdf78f1d910 - middle::infer::equate::Equate<'f, 'tcx>.Combine<'tcx>::tys::hc9258a93ddaf1f1d9hr
  12:     0x7fdf78f354b0 - middle::infer::combine::Combine::tps::unboxed_closure.74647
  13:     0x7fdf78f33850 - middle::infer::combine::Combine::substs_variances::h15023683884087328428
  14:     0x7fdf78f33790 - middle::infer::combine::Combine::substs::h6409605246681804273
  15:     0x7fdf78f334f0 - middle::infer::combine::Combine::trait_refs::h10655637296263412357
  16:     0x7fdf78fab7d0 - middle::infer::InferCtxt<'a, 'tcx>::sub_trait_refs::h47d2b8aae7b2596472A
  17:     0x7fdf79044d10 - middle::traits::select::SelectionContext<'cx, 'tcx>::match_impl::hb52ab063e7e4913dPzT
  18:     0x7fdf7904eaa0 - middle::traits::select::SelectionContext<'cx, 'tcx>::assemble_candidates_from_impls::h3156cc6e5773eeeafmS
  19:     0x7fdf790490e0 - middle::traits::select::SelectionContext<'cx, 'tcx>::candidate_from_obligation_no_cache::hee5fb56560ec1364MER
  20:     0x7fdf79039e40 - middle::traits::select::SelectionContext<'cx, 'tcx>::candidate_from_obligation::hb1931a349a9a6aa5exR
  21:     0x7fdf79016a20 - middle::traits::select::SelectionContext<'cx, 'tcx>::select::h645cb0d6a6e6eb40dcR
  22:     0x7fdf79023e70 - middle::traits::project::project_type::h947eece142ef049d52P
  23:     0x7fdf79020fd0 - middle::traits::project::opt_normalize_projection_type::hb3defb9cc9365d1e8UP
  24:     0x7fdf7900eba0 - middle::traits::project::normalize_projection_type::hdc293893275ee559JTP
  25:     0x7fdf79022470 - middle::traits::project::AssociatedTypeNormalizer<'a, 'b, 'tcx>.TypeFolder<'tcx>::fold_ty::h801cbd2cdff2eff1kSP
  26:     0x7fdf790234c0 - vec::Vec<T>.FromIterator<T>::from_iter::h16193845141216249604
  27:     0x7fdf79023380 - middle::subst::VecPerParamSpace<T>::map_enumerated::h14775177066643949913
  28:     0x7fdf79022fc0 - middle::ty_fold::super_fold_substs::h1314162558240838980
  29:     0x7fdf79022470 - middle::traits::project::AssociatedTypeNormalizer<'a, 'b, 'tcx>.TypeFolder<'tcx>::fold_ty::h801cbd2cdff2eff1kSP
  30:     0x7fdf79023bc0 - middle::ty_fold::super_fold_fn_sig::h17017241928507571730
  31:     0x7fdf79022470 - middle::traits::project::AssociatedTypeNormalizer<'a, 'b, 'tcx>.TypeFolder<'tcx>::fold_ty::h801cbd2cdff2eff1kSP
  32:     0x7fdf79bced70 - trans::base::decl_rust_fn::h076b87abca711100IYr
  33:     0x7fdf79be41e0 - trans::base::register_fn::he826b21665129f50jxu
  34:     0x7fdf79be61d0 - trans::base::register_method::ha6bb45ad72d0525eDbv
  35:     0x7fdf79afb580 - trans::base::get_item_val::he68ea40c5ecdc62fjUu
  36:     0x7fdf79af9300 - trans::base::trans_item::ha0e598b4af14d13fQpu
  37:     0x7fdf79be6a60 - trans::base::trans_crate::hf12b6e99a3af9ab9blv
  38:     0x7fdf7b225250 - driver::phase_4_translate_to_llvm::h40ce54ee4a878885vFa
  39:     0x7fdf7b204ae0 - driver::compile_input::h68b8602933aad8d7wba
  40:     0x7fdf7b2cfeb0 - thunk::F.Invoke<A, R>::invoke::h18029802347644288836
  41:     0x7fdf7b2cec60 - rt::unwind::try::try_fn::h6518866316425934196
  42:     0x7fdf7ad51400 - rust_try_inner
  43:     0x7fdf7ad513f0 - rust_try
  44:     0x7fdf7b2cefb0 - thunk::F.Invoke<A, R>::invoke::h15513809553472565307
  45:     0x7fdf7acd6e40 - sys::thread::thread_start::h5ea7ba97235331d5a9v
  46:     0x7fdf75348fa0 - <unknown>
  47:     0x7fdf7a907479 - clone
  48:                0x0 - <unknown>