ICE: compiler/rustc_typeck/src/check/closure.rs:608:18: async fn generator return type not an inference variable

[chengniansun]

Code

impl<const H: feature> Foo {
    async fn biz() {}
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (4f7612ac1 2020-10-31)
binary: rustc
commit-hash: 4f7612ac1499258025077f1fd05d2f429f9accfb
commit-date: 2020-10-31
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly

Error output

error[E0670]: `async fn` is not permitted in the 2015 edition
 --> reduced_mutant.rs:2:5
  |
2 |     async fn biz() {}
  |     ^^^^^ to use `async fn`, switch to Rust 2018
  |
  = help: set `edition = "2018"` in `Cargo.toml`
  = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error[E0412]: cannot find type `Foo` in this scope
 --> reduced_mutant.rs:1:24
  |
1 | impl<const H: feature> Foo {
  |                        ^^^ not found in this scope

error[E0573]: expected type, found built-in attribute `feature`
 --> reduced_mutant.rs:1:15
  |
1 | impl<const H: feature> Foo {
  |               ^^^^^^^ not a type

error[E0658]: const generics are unstable
 --> reduced_mutant.rs:1:12
  |
1 | impl<const H: feature> Foo {
  |            ^
  |
  = note: see issue #74878 <https://github.com/rust-lang/rust/issues/74878> for more information
  = help: add `#![feature(min_const_generics)]` to the crate attributes to enable

error[E0601]: `main` function not found in crate `reduced_mutant`
 --> reduced_mutant.rs:1:1
  |
1 | / impl<const H: feature> Foo {
2 | |     async fn biz() {}
3 | | }
  | |_^ consider adding a `main` function to `reduced_mutant.rs`

error: internal compiler error: compiler/rustc_typeck/src/check/closure.rs:608:18: async fn generator return type not an inference variable
 --> reduced_mutant.rs:2:20
  |
2 |     async fn biz() {}
  |                    ^^

thread 'rustc' panicked at 'Box<Any>', /rustc/4f7612ac1499258025077f1fd05d2f429f9accfb/compiler/rustc_errors/src/lib.rs:888:9
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (4f7612ac1 2020-10-31) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `<impl [type error]>::biz`
#1 [mir_built] building MIR for `<impl [type error]>::biz`
end of query stack
error: aborting due to 6 previous errors

Some errors have detailed explanations: E0412, E0573, E0601, E0658, E0670.
For more information about an error, try `rustc --explain E0412`.

Backtrace

thread 'rustc' panicked at 'Box<Any>', /rustc/4f7612ac1499258025077f1fd05d2f429f9accfb/compiler/rustc_errors/src/lib.rs:888:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::span_bug
   2: rustc_errors::Handler::span_bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::span_bug_fmt
   8: rustc_typeck::check::closure::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::supplied_sig_of_closure
   9: rustc_typeck::check::closure::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_closure
  10: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  11: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  12: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_argument_types
  13: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::confirm_builtin_call
  14: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_call
  15: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  16: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  17: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  18: rustc_typeck::check::check::check_fn
  19: rustc_infer::infer::InferCtxtBuilder::enter
  20: rustc_typeck::check::typeck
  21: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  22: rustc_data_structures::stack::ensure_sufficient_stack
  23: rustc_query_system::query::plumbing::get_query_impl
  24: rustc_middle::ty::context::TyCtxt::typeck_opt_const_arg
  25: rustc_mir_build::thir::cx::Cx::new
  26: rustc_infer::infer::InferCtxtBuilder::enter
  27: rustc_mir_build::build::mir_built
  28: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_built>::compute
  29: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  30: rustc_data_structures::stack::ensure_sufficient_stack
  31: rustc_query_system::query::plumbing::get_query_impl
  32: rustc_mir::transform::check_unsafety::unsafety_check_result
  33: core::ops::function::FnOnce::call_once
  34: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::unsafety_check_result>::compute
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_query_system::query::plumbing::get_query_impl
  38: rustc_query_system::query::plumbing::ensure_query_impl
  39: rustc_mir::transform::mir_const
  40: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_const>::compute
  41: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  42: rustc_data_structures::stack::ensure_sufficient_stack
  43: rustc_query_system::query::plumbing::get_query_impl
  44: rustc_mir::transform::mir_promoted
  45: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_promoted>::compute
  46: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  47: rustc_data_structures::stack::ensure_sufficient_stack
  48: rustc_query_system::query::plumbing::get_query_impl
  49: rustc_mir::borrow_check::mir_borrowck
  50: core::ops::function::FnOnce::call_once
  51: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  52: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  53: rustc_data_structures::stack::ensure_sufficient_stack
  54: rustc_query_system::query::plumbing::get_query_impl
  55: rustc_typeck::collect::type_of::type_of
  56: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::type_of>::compute
  57: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  58: rustc_data_structures::stack::ensure_sufficient_stack
  59: rustc_query_system::query::plumbing::get_query_impl
  60: rustc_typeck::check::check::check_item_type
  61: rustc_middle::hir::map::Map::visit_item_likes_in_module
  62: rustc_typeck::check::check::check_mod_item_types
  63: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_mod_item_types>::compute
  64: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  65: rustc_data_structures::stack::ensure_sufficient_stack
  66: rustc_query_system::query::plumbing::get_query_impl
  67: rustc_query_system::query::plumbing::ensure_query_impl
  68: rustc_session::utils::<impl rustc_session::session::Session>::time
  69: rustc_typeck::check_crate
  70: rustc_interface::passes::analysis
  71: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  72: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  73: rustc_data_structures::stack::ensure_sufficient_stack
  74: rustc_query_system::query::plumbing::get_query_impl
  75: rustc_interface::passes::QueryContext::enter
  76: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  77: rustc_span::with_source_map
  78: scoped_tls::ScopedKey<T>::set

NOTE: The bug is found by our work-in-progress compiler testing tool Kira, and the test program is reduced/minimized by Perses

[JohnTitor]

Related to #64999.

[nikomatsakis]

My hunch is that this code:

rust/compiler/rustc_typeck/src/check/closure.rs

Lines 606 to 612 in 0cd1516

	let ret_vid = match *ret_ty.kind() {
	ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
	_ => span_bug!(
	self.tcx.def_span(expr_def_id),
	"async fn generator return type not an inference variable"
	),
	};

needs to be generalized to also accept a Error type. The function could just return Some(err) (or maybe None) in that case and I expect the ICE would go away.

[nikomatsakis]

I'm tagging this as E-mentor because of the above instructions ☝️ (feel free to also ping me on Zulip)

[hameerabbasi]

@rustbot claim