assertion failed: current_depth > 1 when using generators and conservative impl trait with an impl lifetime

[LunaBorowska]

The following code causes an ICE.

Code

I'm pretty it can be simplified to not depend on futures crate, but that's the best I was able to get.

#![feature(generators)]

extern crate futures;

use futures::{future, Future, FutureExt};

fn main() {
    || {
        let _future = Struct::method();
        yield ();
    };
}

struct Struct;

impl<'a> Struct {
    fn method() -> impl Future<Item = (), Error = ()> + 'a {
        future::loop_fn(future::ok(()), |builder| {
            builder.map(|_| unimplemented!())
        })
    }
}

Meta

rustc 1.26.0-nightly (2789b067d 2018-03-06)
binary: rustc
commit-hash: 2789b067da2ac921b86199bde21dd231ace1da39
commit-date: 2018-03-06
host: x86_64-unknown-linux-gnu
release: 1.26.0-nightly
LLVM version: 6.0

Error message

thread 'rustc' panicked at 'assertion failed: current_depth > 1', librustc/infer/higher_ranked/mod.rs:735:21
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:59
             at libstd/panicking.rs:207
   3: std::panicking::default_hook
             at libstd/panicking.rs:223
   4: core::ops::function::Fn::call
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:403
   6: std::panicking::begin_panic
   7: rustc::infer::higher_ranked::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::plug_leaks::{{closure}}
   8: rustc::ty::subst::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::Slice<rustc::ty::subst::Kind<'tcx>>>::super_fold_with
   9: rustc::ty::fold::TypeFoldable::fold_with
  10: rustc::ty::fold::TypeFoldable::fold_with
  11: rustc::traits::project::poly_project_and_unify_type::{{closure}}
  12: rustc::traits::project::poly_project_and_unify_type
  13: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  14: rustc::traits::select::SelectionContext::evaluate_candidate::{{closure}}
  15: rustc::traits::select::SelectionContext::evaluate_candidate
  16: rustc::traits::select::SelectionContext::evaluate_stack
  17: rustc::traits::select::SelectionContext::in_task
  18: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  19: rustc::traits::select::SelectionContext::evaluate_candidate::{{closure}}
  20: rustc::traits::select::SelectionContext::evaluate_candidate
  21: rustc::traits::select::SelectionContext::evaluate_stack
  22: rustc::traits::select::SelectionContext::in_task
  23: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  24: rustc::traits::select::SelectionContext::evaluate_candidate::{{closure}}
  25: rustc::traits::select::SelectionContext::evaluate_candidate
  26: rustc::traits::select::SelectionContext::evaluate_stack
  27: rustc::traits::select::SelectionContext::in_task
  28: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  29: rustc::traits::select::SelectionContext::evaluate_candidate::{{closure}}
  30: rustc::traits::select::SelectionContext::evaluate_candidate
  31: rustc::traits::select::SelectionContext::evaluate_stack
  32: rustc::traits::select::SelectionContext::in_task
  33: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  34: rustc::traits::select::SelectionContext::evaluate_candidate::{{closure}}
  35: rustc::traits::select::SelectionContext::evaluate_candidate
  36: rustc::traits::select::SelectionContext::evaluate_stack
  37: rustc::traits::select::SelectionContext::in_task
  38: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  39: rustc::traits::select::SelectionContext::evaluate_obligation_conservatively
  40: rustc::traits::type_known_to_meet_bound
  41: rustc::ty::util::is_freeze_raw
  42: rustc::ty::maps::<impl rustc::ty::maps::queries::is_freeze_raw<'tcx>>::compute_result
  43: rustc::dep_graph::graph::DepGraph::with_task_impl
  44: rustc::ty::maps::<impl rustc::ty::maps::queries::is_freeze_raw<'tcx>>::force
  45: rustc::ty::maps::<impl rustc::ty::maps::queries::is_freeze_raw<'tcx>>::try_get
  46: rustc::ty::maps::TyCtxtAt::is_freeze_raw
  47: rustc::ty::util::<impl rustc::ty::TyS<'tcx>>::is_freeze
  48: rustc_mir::transform::qualify_consts::Qualifier::new
  49: <rustc_mir::transform::qualify_consts::QualifyAndPromoteConstants as rustc_mir::transform::MirPass>::run_pass
  50: rustc_mir::transform::mir_validated::{{closure}}
  51: rustc_mir::transform::mir_validated
  52: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_validated<'tcx>>::compute_result
  53: rustc::dep_graph::graph::DepGraph::with_task_impl
  54: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_validated<'tcx>>::force
  55: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_validated<'tcx>>::try_get
  56: rustc::ty::maps::TyCtxtAt::mir_validated
  57: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_validated
  58: rustc_borrowck::borrowck::borrowck
  59: rustc::dep_graph::graph::DepGraph::with_task_impl
  60: rustc::ty::maps::<impl rustc::ty::maps::queries::borrowck<'tcx>>::force
  61: rustc::ty::maps::<impl rustc::ty::maps::queries::borrowck<'tcx>>::try_get
  62: rustc::ty::maps::TyCtxtAt::borrowck
  63: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::borrowck
  64: <rustc_borrowck::borrowck::unused::UsedMutFinder<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_nested_body
  65: rustc::hir::intravisit::walk_expr
  66: rustc_borrowck::borrowck::borrowck
  67: rustc::dep_graph::graph::DepGraph::with_task_impl
  68: rustc::ty::maps::<impl rustc::ty::maps::queries::borrowck<'tcx>>::force
  69: rustc::ty::maps::<impl rustc::ty::maps::queries::borrowck<'tcx>>::try_get
  70: rustc::ty::maps::TyCtxtAt::borrowck
  71: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::borrowck
  72: rustc_borrowck::borrowck::check_crate
  73: rustc::ty::context::TyCtxt::create_and_enter
  74: rustc_driver::driver::compile_input
  75: rustc_driver::run_compiler

error: internal compiler error: unexpected panic

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 1.26.0-nightly (2789b067d 2018-03-06) 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

[eddyb]

Shorter testcase at #51353 (comment).

[LunaBorowska]

I think the issue fixed itself.