ICE typechecking `async fn` with explicit lifetime argument

[cramertj]

Playground link

ICE disappears if the dummy argument explicit lifetime is removed.

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `DefId(0/1:15 ~ foo[b6a6]::foo[0]::{{closure}}[0])`,
 right: `DefId(0/0:14 ~ foo[b6a6]::foo[0])`', librustc/middle/region.rs:761:9
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
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             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:515
   6: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:426
   7: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:413
   8: rustc::middle::region::ScopeTree::free_scope
   9: rustc::infer::lexical_region_resolve::LexicalResolver::expand_node
  10: rustc::infer::lexical_region_resolve::resolve
  11: rustc::infer::InferCtxt::resolve_regions_and_report_errors_inner
  12: rustc_typeck::check::regionck::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::regionck_fn
  13: rustc::ty::context::tls::with_related_context
  14: rustc::infer::InferCtxtBuilder::enter
  15: rustc_typeck::check::typeck_tables_of
  16: rustc::ty::query::__query_compute::typeck_tables_of
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  18: rustc::ty::context::tls::with_context
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
  20: rustc::ty::context::tls::with_related_context
  21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  23: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_tables_of
  24: rustc_typeck::check::typeck_tables_of
  25: rustc::ty::query::__query_compute::typeck_tables_of
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  27: rustc::ty::context::tls::with_context
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
  29: rustc::ty::context::tls::with_related_context
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  32: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_tables_of
  33: rustc_typeck::collect::type_of
  34: rustc::ty::query::__query_compute::type_of
  35: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::type_of<'tcx>>::compute
  36: rustc::ty::context::tls::with_context
  37: rustc::dep_graph::graph::DepGraph::with_task_impl
  38: rustc::ty::context::tls::with_related_context
  39: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  41: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::type_of
  42: <rustc_typeck::collect::CollectItemTypesVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  43: rustc::hir::intravisit::walk_expr
  44: rustc::hir::intravisit::Visitor::visit_fn
  45: rustc::hir::intravisit::walk_item
  46: <rustc_typeck::collect::CollectItemTypesVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  47: rustc::hir::Crate::visit_all_item_likes
  48: rustc::session::Session::track_errors
  49: rustc_typeck::check_crate
  50: rustc::ty::context::tls::enter_context
  51: <std::thread::local::LocalKey<T>>::with
  52: rustc::ty::context::TyCtxt::create_and_enter
  53: rustc_driver::driver::compile_input
  54: rustc_driver::run_compiler_with_pool
  55: <scoped_tls::ScopedKey<T>>::set
  56: syntax::with_globals
  57: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  58: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  59: rustc_driver::run
  60: rustc_driver::main
  61: std::rt::lang_start::{{closure}}
  62: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  63: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  64: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  65: main
  66: __libc_start_main
  67: <unknown>
query stack during panic:
#0 [typeck_tables_of] processing `foo`
#1 [typeck_tables_of] processing `foo::{{closure}}`
#2 [type_of] processing `foo::{{closure}}`
end of query stack

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 1.28.0-nightly (84804c387 2018-06-26) 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

[jkozlowski]

See my issue above, I think it might be references with lifetimes in general that ICE it?

[Arnavion]

The ICE in the OP code is fixed by #54000, though the code still doesn't compile for lifetime reasons.