ICE with trait_alias in Nightly: unwrap None in assemble_extension_candidates_for_trait

[atorstling]

This is the stacktrace I get:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:355:21
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::_print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::continue_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: rustc_metadata::cstore_impl::provide_extern::generics_of
  11: rustc::ty::query::__query_compute::generics_of
  12: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::generics_of<'tcx>>::compute
  13: rustc::ty::context::tls::with_context
  14: rustc::dep_graph::graph::DepGraph::with_task_impl
  15: rustc::ty::context::tls::with_related_context
  16: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  18: rustc::ty::subst::<impl rustc::ty::List<rustc::ty::subst::Kind<'tcx>>>::for_item
  19: rustc_typeck::check::method::probe::ProbeContext::assemble_extension_candidates_for_trait
  20: rustc_typeck::check::method::probe::ProbeContext::assemble_extension_candidates_for_all_traits
  21: rustc::infer::InferCtxt::probe
  22: rustc_typeck::check::FnCtxt::suggest_ref_or_into
  23: rustc_typeck::check::demand::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::demand_coerce
  24: rustc_typeck::check::FnCtxt::check_decl_local
  25: rustc_typeck::check::FnCtxt::check_block_with_expected
  26: rustc_typeck::check::FnCtxt::check_expr_kind
  27: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  28: rustc_typeck::check::FnCtxt::check_return_expr
  29: rustc_typeck::check::check_fn
  30: rustc::ty::context::tls::with_related_context
  31: rustc::infer::InferCtxtBuilder::enter
  32: rustc_typeck::check::typeck_tables_of
  33: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  34: rustc::ty::context::tls::with_context
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::context::tls::with_related_context
  37: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  38: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  39: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
  40: rustc::session::Session::track_errors
  41: rustc_typeck::check::typeck_item_bodies
  42: rustc::ty::query::__query_compute::typeck_item_bodies
  43: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_item_bodies<'tcx>>::compute
  44: rustc::ty::context::tls::with_context
  45: rustc::dep_graph::graph::DepGraph::with_task_impl
  46: rustc::ty::context::tls::with_related_context
  47: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  48: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  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: rustc_driver::run_compiler
  57: <scoped_tls::ScopedKey<T>>::set
  58: syntax::with_globals
  59: __rust_maybe_catch_panic
  60: <F as alloc::boxed::FnBox<A>>::call_box
  61: std::sys::unix::thread::Thread::new::thread_start
  62: _pthread_body
  63: _pthread_start
query stack during panic:
#0 [generics_of] processing `repro::TestAlias`
#1 [typeck_tables_of] processing `main`
#2 [typeck_item_bodies] type-checking all item bodies
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 1.33.0-nightly (2d3e909e4 2018-12-22) running on x86_64-apple-darwin

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

I can only repro this when using an external crate. The condition seems to be that you have to

1. Have a compile error which triggers FnCtxt::suggest_ref_or_into, combined with
2. Having a trait alias defined in an external crate.

I'm attaching a small cargo project which reproduces the problem.
The structure is really simple, these are the main files:

src/main.rs:

use repro;
use std::path::Path;

fn main() {
    //compile error which triggers suggest_ref_or_into
    let d: Path = Path::new(".");
}

src/lib.rs:

#![feature(trait_alias)]
trait TestAlias = core::fmt::Debug;

To repro, unzip the cargo project and run RUST_BACKTRACE=1 cargo build.

Thank you.
repro.zip

[davidtwco]

Submitted #57786 with a fix for this.

[davidtwco]

Looks like this was already fixed by #57502.

[atorstling]

Confirmed fixed in rustc 1.33.0-nightly (7164a9f15 2019-01-21). Thank you. Closing.