ICE with HRTB and associated type: "leak-check would have us replace ReErased with BrNamed"

[alecmocatta]

Affects stable and nightly, with and without #![feature(nll)].

use std::marker::PhantomData;

trait DistributedIterator {
    fn reduce(self)
    where
        Self: Sized,
    {
        unreachable!()
    }
}

trait DistributedIteratorMulti<Source> {
    type Item;
}

struct Connect<I>(PhantomData<fn(I)>);
impl<I: for<'a> DistributedIteratorMulti<&'a ()>> DistributedIterator for Connect<I> where {}

struct Cloned<Source>(PhantomData<fn(Source)>);
impl<'a, Source> DistributedIteratorMulti<&'a Source> for Cloned<&'a Source> {
    type Item = ();
}

struct Map<I, F> {
    i: I,
    f: F,
}
impl<I: DistributedIteratorMulti<Source>, F, Source> DistributedIteratorMulti<Source> for Map<I, F>
where
    F: A<<I as DistributedIteratorMulti<Source>>::Item>,
{
    type Item = ();
}

trait A<B> {}

struct X;
impl A<()> for X {}

fn multi<I>(_reducer: I)
where
    I: for<'a> DistributedIteratorMulti<&'a ()>,
{
    DistributedIterator::reduce(Connect::<I>(PhantomData))
}

fn main() {
    multi(Map {
        i: Cloned(PhantomData),
        f: X,
    });
}

(Playground)

thread 'main' panicked at 'leak-check would have us replace ReErased with BrNamed(crate0:DefIndex(1:14), 'a)', librustc/infer/higher_ranked/mod.rs:507:21
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:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   7: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:345
   8: rustc::infer::higher_ranked::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::plug_leaks::{{closure}}
   9: rustc::ty::fold::TypeFolder::fold_ty
  10: <smallvec::SmallVec<A> as core::iter::traits::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  11: rustc::ty::fold::TypeFoldable::fold_with
  12: rustc::ty::fold::TypeFolder::fold_ty
  13: <smallvec::SmallVec<A> as core::iter::traits::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  14: rustc::ty::fold::TypeFoldable::fold_with
  15: rustc::ty::fold::TypeFoldable::fold_with
  16: <core::iter::Map<I, F> as core::iter::iterator::Iterator>::fold
  17: rustc::ty::fold::TypeFoldable::fold_with
  18: rustc::infer::higher_ranked::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::plug_leaks
  19: rustc::traits::select::SelectionContext::impl_or_trait_obligations
  20: rustc::traits::select::SelectionContext::vtable_impl
  21: rustc::infer::InferCtxt::in_snapshot
  22: rustc::traits::select::SelectionContext::confirm_candidate
  23: rustc::infer::InferCtxt::probe
  24: rustc::traits::select::SelectionContext::evaluate_stack
  25: rustc::dep_graph::graph::DepGraph::with_anon_task
  26: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  27: rustc::traits::select::SelectionContext::evaluate_obligation_recursively
  28: rustc::ty::context::tls::with_related_context
  29: rustc::infer::InferCtxtBuilder::enter_with_canonical
  30: rustc_traits::evaluate_obligation::evaluate_obligation
  31: rustc::ty::query::__query_compute::evaluate_obligation
  32: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::evaluate_obligation<'tcx>>::compute
  33: rustc::dep_graph::graph::DepGraph::with_task_impl
  34: rustc::ty::context::tls::with_related_context
  35: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  37: rustc::traits::query::evaluate_obligation::<impl rustc::infer::InferCtxt<'cx, 'gcx, 'tcx>>::evaluate_obligation
  38: rustc::traits::query::evaluate_obligation::<impl rustc::infer::InferCtxt<'cx, 'gcx, 'tcx>>::evaluate_obligation_no_overflow
  39: <rustc_data_structures::obligation_forest::ObligationForest<O>>::process_obligations
  40: <rustc::traits::fulfill::FulfillmentContext<'tcx> as rustc::traits::engine::TraitEngine<'tcx>>::select_where_possible
  41: <rustc::traits::fulfill::FulfillmentContext<'tcx> as rustc::traits::engine::TraitEngine<'tcx>>::select_all_or_error
  42: rustc::traits::codegen::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::drain_fulfillment_cx_or_panic
  43: rustc::ty::context::tls::with_related_context
  44: rustc::infer::InferCtxtBuilder::enter
  45: rustc::traits::codegen::codegen_fulfill_obligation
  46: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  47: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::codegen_fulfill_obligation<'tcx>>::compute
  48: rustc::dep_graph::graph::DepGraph::with_task_impl
  49: rustc::ty::context::tls::with_related_context
  50: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  51: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  52: rustc::ty::instance::Instance::resolve
  53: rustc_mir::monomorphize::collector::visit_fn_use
  54: <rustc_mir::monomorphize::collector::MirNeighborCollector<'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_terminator_kind
  55: rustc_mir::monomorphize::collector::collect_items_rec
  56: rustc_mir::monomorphize::collector::collect_items_rec
  57: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  58: rustc::util::common::time
  59: rustc_mir::monomorphize::collector::collect_crate_mono_items
  60: rustc::util::common::time
  61: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  62: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::collect_and_partition_mono_items<'tcx>>::compute
  63: rustc::dep_graph::graph::DepGraph::with_task_impl
  64: rustc::ty::context::tls::with_related_context
  65: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  66: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  67: rustc_codegen_llvm::base::codegen_crate
  68: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  69: rustc::util::common::time
  70: rustc_driver::driver::phase_4_codegen
  71: rustc_driver::driver::compile_input::{{closure}}
  72: rustc::ty::context::tls::enter_context
  73: <std::thread::local::LocalKey<T>>::with
  74: rustc::ty::context::TyCtxt::create_and_enter
  75: rustc_driver::driver::compile_input
  76: rustc_driver::run_compiler_with_pool
  77: rustc_driver::driver::spawn_thread_pool
  78: rustc_driver::run_compiler
  79: <scoped_tls::ScopedKey<T>>::set
  80: syntax::with_globals
  81: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  82: rustc_driver::run
  83: rustc_driver::main
  84: std::rt::lang_start::{{closure}}
  85: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  86: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  87: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:398
             at libstd/rt.rs:58
  88: main
  89: __libc_start_main
  90: <unknown>
query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `for<'a> Map<Cloned<&()>, X>: DistributedIteratorMulti<&'a ()>`
#1 [codegen_fulfill_obligation] checking if `DistributedIterator` fulfills its obligations
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

error: internal compiler error: unexpected panic

note: rustc 1.32.0-nightly (13dab66a6 2018-11-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

[estebank]

Fixed in beta:

error[E0308]: mismatched types
  --> src/main.rs:49:5
   |
49 |     multi(Map {
   |     ^^^^^ one type is more general than the other
   |
   = note: expected type `DistributedIteratorMulti<&'a ()>`
              found type `DistributedIteratorMulti<&()>`