ICE when adding GAT feature in combination with async/await

[samuelpilz]

Code

• crate b (only main.rs) depends on crate a (only lib.rs).
• Feature GAT is enabled (but not used directly)
• without feature GAT, the code does not pass cargo check
• with feature GAT, the code passes cargo check and fails at cargo build, but only when code ill-formed FooImpl is generated.
• within single crate, the ICE does not happen
• I cannot reproduce this with any other traits than Future together with async/await

crate a, lib.rs

#![feature(generic_associated_types)]

use std::{future::Future, pin::Pin};
pub trait Foo {
    type Bar: AsRef<()>;
    fn foo(&self) -> Pin<Box<dyn Future<Output = Self::Bar> + '_>>;
}

crate b, main.rs

use futures::executor::block_on;

use a::Foo;
use std::{future::Future, pin::Pin};

fn main() {
    block_on(foo());
}

pub struct FooImpl;

impl Foo for FooImpl {
    type Bar = ();
    fn foo(&self) -> Pin<Box<dyn Future<Output = Self::Bar> + '_>> {
        panic!()
    }
}

async fn foo() {
    bar(&FooImpl).await;
}

async fn bar<F: Foo>(foo: &F) {
    foo.foo().await.as_ref();
}

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (7750c3d46 2020-06-26)
binary: rustc
commit-hash: 7750c3d46bc19784adb1ee6e37a5ec7e4cd7e772
commit-date: 2020-06-26
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

Error output

error: internal compiler error: src/librustc_trait_selection/traits/codegen/mod.rs:62:17: Encountered error `Unimplemented` selecting `Binder(<() as std::convert::AsRef<()>>)` during codegen

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:916:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.46.0-nightly (7750c3d46 2020-06-26) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

error: aborting due to previous error

Backtrace

error: internal compiler error: src/librustc_trait_selection/traits/codegen/mod.rs:62:17: Encountered error `Unimplemented` selecting `Binder(<() as std::convert::AsRef<()>>)` during codegen

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:916:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:524
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_middle::ty::context::GlobalCtxt::enter_local
  21: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::codegen_fulfill_obligation>::compute
  23: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  24: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  25: rustc_data_structures::stack::ensure_sufficient_stack
  26: rustc_query_system::query::plumbing::get_query_impl
  27: rustc_ty::instance::resolve_instance
  28: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::resolve_instance>::compute
  29: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  30: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  31: rustc_data_structures::stack::ensure_sufficient_stack
  32: rustc_query_system::query::plumbing::get_query_impl
  33: rustc_middle::ty::instance::Instance::resolve
  34: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
  35: rustc_mir::monomorphize::collector::collect_neighbours
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_mir::monomorphize::collector::collect_items_rec
  38: rustc_mir::monomorphize::collector::collect_items_rec
  39: rustc_mir::monomorphize::collector::collect_items_rec
  40: rustc_mir::monomorphize::collector::collect_items_rec
  41: rustc_mir::monomorphize::collector::collect_items_rec
  42: rustc_mir::monomorphize::collector::collect_items_rec
  43: rustc_mir::monomorphize::collector::collect_items_rec
  44: rustc_mir::monomorphize::collector::collect_items_rec
  45: rustc_mir::monomorphize::collector::collect_items_rec
  46: rustc_mir::monomorphize::collector::collect_items_rec
  47: rustc_mir::monomorphize::collector::collect_items_rec
  48: rustc_mir::monomorphize::collector::collect_crate_mono_items
  49: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  50: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::collect_and_partition_mono_items>::compute
  51: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  52: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  53: rustc_query_system::query::plumbing::get_query_impl
  54: rustc_codegen_ssa::base::codegen_crate
  55: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  56: rustc_session::utils::<impl rustc_session::session::Session>::time
  57: rustc_interface::passes::start_codegen
  58: rustc_middle::ty::context::tls::enter_global
  59: rustc_interface::queries::Queries::ongoing_codegen
  60: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  61: rustc_span::with_source_map
  62: rustc_interface::interface::run_compiler_in_existing_thread_pool
  63: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.46.0-nightly (7750c3d46 2020-06-26) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::convert::AsRef` fulfills its obligations
#1 [resolve_instance] resolving instance `<() as std::convert::AsRef<()>>::as_ref`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error