"internal compiler error: escaping regions in predicate Obligation" with impl Trait

[niklasf]

Condensed code example (playground):

#![feature(conservative_impl_trait)]

struct Move { }
struct Pos { }

impl Pos {
    fn legals(&self) -> impl Iterator<Item=&Move> {
    }
}

fn main() { }

I expected to see this happen: Compiler error due to missing method body.

Instead, this happened: Internal compiler error.

Meta

rustc --version --verbose:

rustc 1.20.0-nightly (3bfc18a96 2017-06-29)
binary: rustc
commit-hash: 3bfc18a9619a5151ff4f11618db9cd882996ba6f
commit-date: 2017-06-29
host: x86_64-unknown-linux-gnu
release: 1.20.0-nightly
LLVM version: 4.0

Backtrace:

error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:617: escaping regions in predicate Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { trait_ref: <_ as std::iter::Iterator>, item_def_id: DefId { krate: CrateNum(2), node: DefIndex(1612) => core/3f7a343::iter[0]::iterator[0]::Iterator[0]::Item[0] } }, &Move)),depth=0)
 --> err.rs:7:29
  |
7 |         fn legals(&self) -> impl Iterator<Item=&Move> {
  |                             ^^^^^^^^^^^^^^^^^^^^^^^^^

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 1.20.0-nightly (3bfc18a96 2017-06-29) running on x86_64-unknown-linux-gnu

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:426
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:355
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:365
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:549
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::span_bug
   7: rustc::session::opt_span_bug_fmt::{{closure}}
   8: rustc::session::span_bug_fmt
   9: rustc_typeck::check::Inherited::register_predicate
  10: <rustc::ty::fold::BottomUpFolder<'a, 'gcx, 'tcx, F> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  11: rustc_typeck::check::check_fn
  12: rustc_typeck::check::typeck_tables_of::{{closure}}
  13: rustc_typeck::check::typeck_tables_of
  14: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::try_get
  15: rustc::ty::maps::TyCtxtAt::typeck_tables_of
  16: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_tables_of
  17: rustc_typeck::check::typeck_item_bodies
  18: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::try_get
  19: rustc::ty::maps::TyCtxtAt::typeck_item_bodies
  20: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_item_bodies
  21: rustc_typeck::check_crate
  22: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  23: rustc_driver::driver::phase_3_run_analysis_passes
  24: rustc_driver::driver::compile_input
  25: rustc_driver::run_compiler

[niklasf]

Seems to be a duplicate of #42479.