ICE for impl Trait with missing lifetime

[wagnerf42]

Hi, the compiler (nightly) panics on this code:

#![feature(conservative_impl_trait)]
use std::iter::once;

struct Foo {
    x: i32,
}

impl Foo {
    fn inside(&self) -> impl Iterator<Item = &i32> {
        once(&self.x)
    }
}

fn main() {
    println!("hi");
}

I know this code is not ok. in fact once the lifetime is added, the compiler does not panic anymore.

here is the log:

/tmp ❯❯❯ rustc --version
rustc 1.19.0-nightly (e0cc22b4b 2017-05-31)
/tmp ❯❯❯ rustc test.rs
error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:582: escaping regions in predicate Obligatio
n(predicate=Binder(ProjectionPredicate(ProjectionTy { trait_ref: <_ as std::iter::Iterator>, item_name: Item(92) }, &i3
2)),depth=0)
 --> test.rs:9:25
  |
9 |     fn inside(&self) -> impl Iterator<Item = &i32> {
  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:421
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

[tirr-c]

There are some duplicate open issues I can find: #39553, #39929, #41182, #42399. It seems that the ICE occurs whenever there is elided lifetime parameter in impl Trait.

[fhahn]

I'd like to look into this.

[kennytm]

I think this should be closed in favor of #43396, which is tracked by #42183.