MIR inliner ICEs trying to inline async closures

[wesleywiser]

As @bjorn3 noted, the MIR inliner seems to fail when dealing with opaque return types.

Test case:

#![feature(async_closure)]

use std::future::Future;

fn async_closure() -> impl Future<Output = u8> {
    (async move || -> u8 {
        42
    })()
}

fn main() {
    let _fut = async_closure();
}

Invocation:

rustc -Z mir-opt-level=2 --edition 2018 test.rs

ICE:

error: internal compiler error: src/librustc/traits/codegen/mod.rs:57: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@async-closure.rs:10:5: 12:7] as std::ops::Fn<()>>), Binder(<[closure@async-closure.rs:10:5: 12:7] as std::ops::Fn<()>>), Sorts(ExpectedFound { expected: impl std::future::Future, found: std::future::GenFuture<[static generator@async-closure.rs:10:26: 12:6 {}]> }))` selecting `Binder(<[closure@async-closure.rs:10:5: 12:7] as std::ops::Fn<()>>)` during codegen

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:883:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /home/wesley/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /home/wesley/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   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:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at ./src/liballoc/boxed.rs:1029
  11: rustc_driver::report_ice
             at src/librustc_driver/lib.rs:1157
  12: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  13: std::panicking::begin_panic
             at ./src/libstd/panicking.rs:397
  14: rustc_errors::HandlerInner::bug
             at src/librustc_errors/lib.rs:883
  15: rustc_errors::Handler::bug
             at src/librustc_errors/lib.rs:658
  16: rustc::util::bug::opt_span_bug_fmt::{{closure}}
             at src/librustc/util/bug.rs:36
  17: rustc::ty::context::tls::with_opt::{{closure}}
             at src/librustc/ty/context.rs:1805
  18: rustc::ty::context::tls::with_context_opt
             at src/librustc/ty/context.rs:1757
  19: rustc::ty::context::tls::with_opt
             at src/librustc/ty/context.rs:1805
  20: rustc::util::bug::opt_span_bug_fmt
             at src/librustc/util/bug.rs:32
  21: rustc::util::bug::bug_fmt
             at src/librustc/util/bug.rs:12
  22: rustc::traits::codegen::codegen_fulfill_obligation::{{closure}}
             at src/librustc/traits/codegen/mod.rs:57
  23: rustc::infer::InferCtxtBuilder::enter::{{closure}}
             at src/librustc/infer/mod.rs:535
  24: rustc::ty::context::GlobalCtxt::enter_local::{{closure}}::{{closure}}
             at src/librustc/ty/context.rs:1529
  25: rustc::ty::context::tls::enter_context::{{closure}}
             at src/librustc/ty/context.rs:1696
  26: rustc::ty::context::tls::set_tlv
             at src/librustc/ty/context.rs:1681
  27: rustc::ty::context::tls::enter_context
             at src/librustc/ty/context.rs:1696
  28: rustc::ty::context::GlobalCtxt::enter_local::{{closure}}
             at src/librustc/ty/context.rs:1529
  29: rustc::ty::context::tls::with_related_context::{{closure}}
             at src/librustc/ty/context.rs:1784
  30: rustc::ty::context::tls::with_context::{{closure}}
             at src/librustc/ty/context.rs:1768
  31: rustc::ty::context::tls::with_context_opt
             at src/librustc/ty/context.rs:1757
  32: rustc::ty::context::tls::with_context
             at src/librustc/ty/context.rs:1768
  33: rustc::ty::context::tls::with_related_context
             at src/librustc/ty/context.rs:1781
  34: rustc::ty::context::GlobalCtxt::enter_local
             at src/librustc/ty/context.rs:1521
  35: rustc::infer::InferCtxtBuilder::enter
             at src/librustc/infer/mod.rs:534
  36: rustc::traits::codegen::codegen_fulfill_obligation
             at src/librustc/traits/codegen/mod.rs:34
  37: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute::{{closure}}
             at src/librustc/ty/query/plumbing.rs:951
  38: rustc::ty::query::__query_compute::codegen_fulfill_obligation
             at src/librustc/ty/query/plumbing.rs:902
  39: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
             at src/librustc/ty/query/plumbing.rs:943
  40: rustc::dep_graph::graph::DepGraph::with_task_impl
             at src/librustc/dep_graph/graph.rs:341
  41: rustc::dep_graph::graph::DepGraph::with_task
             at src/librustc/dep_graph/graph.rs:209
  42: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job::{{closure}}::{{closure}}
             at src/librustc/ty/query/plumbing.rs:557
  43: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at src/librustc/ty/query/plumbing.rs:278
  44: rustc::ty::context::tls::enter_context::{{closure}}
             at src/librustc/ty/context.rs:1696
  45: rustc::ty::context::tls::set_tlv
             at src/librustc/ty/context.rs:1681
  46: rustc::ty::context::tls::enter_context
             at src/librustc/ty/context.rs:1696
  47: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query::{{closure}}
             at src/librustc/ty/query/plumbing.rs:278
  48: rustc::ty::context::tls::with_related_context::{{closure}}
             at src/librustc/ty/context.rs:1784
  49: rustc::ty::context::tls::with_context::{{closure}}
             at src/librustc/ty/context.rs:1768
  50: rustc::ty::context::tls::with_context_opt
             at src/librustc/ty/context.rs:1757
  51: rustc::ty::context::tls::with_context
             at src/librustc/ty/context.rs:1768
  52: rustc::ty::context::tls::with_related_context
             at src/librustc/ty/context.rs:1781
  53: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query
             at src/librustc/ty/query/plumbing.rs:267
  54: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job::{{closure}}
             at src/librustc/ty/query/plumbing.rs:547
  55: rustc::ty::query::plumbing::with_diagnostics
             at src/librustc/ty/query/plumbing.rs:212
  56: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job
             at src/librustc/ty/query/plumbing.rs:546
  57: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
             at src/librustc/ty/query/plumbing.rs:380
  58: rustc::ty::query::TyCtxtAt::codegen_fulfill_obligation
             at src/librustc/ty/query/plumbing.rs:1057
  59: rustc::ty::query::<impl rustc::ty::context::TyCtxt>::codegen_fulfill_obligation
             at src/librustc/ty/query/plumbing.rs:1020
  60: rustc::ty::instance::resolve_associated_item
             at src/librustc/ty/instance.rs:417
  61: rustc::ty::instance::Instance::resolve
             at src/librustc/ty/instance.rs:270
  62: rustc_mir::transform::inline::Inliner::get_valid_function_call
             at src/librustc_mir/transform/inline.rs:189
  63: rustc_mir::transform::inline::Inliner::run_pass
             at src/librustc_mir/transform/inline.rs:83
  64: <rustc_mir::transform::inline::Inline as rustc_mir::transform::MirPass>::run_pass
             at src/librustc_mir/transform/inline.rs:43
  65: rustc_mir::transform::run_passes::{{closure}}
             at src/librustc_mir/transform/mod.rs:167
  66: rustc_mir::transform::run_passes
             at src/librustc_mir/transform/mod.rs:174
  67: rustc_mir::transform::run_optimization_passes
             at src/librustc_mir/transform/mod.rs:272
  68: rustc_mir::transform::optimized_mir
             at src/librustc_mir/transform/mod.rs:343
  69: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
             at src/librustc/ty/query/plumbing.rs:943
  70: rustc::dep_graph::graph::DepGraph::with_task_impl
             at src/librustc/dep_graph/graph.rs:341
  71: rustc::dep_graph::graph::DepGraph::with_task
             at src/librustc/dep_graph/graph.rs:209
  72: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job::{{closure}}::{{closure}}
             at src/librustc/ty/query/plumbing.rs:557
  73: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at src/librustc/ty/query/plumbing.rs:278
  74: rustc::ty::context::tls::enter_context::{{closure}}
             at src/librustc/ty/context.rs:1696
  75: rustc::ty::context::tls::set_tlv
             at src/librustc/ty/context.rs:1681
  76: rustc::ty::context::tls::enter_context
             at src/librustc/ty/context.rs:1696
  77: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query::{{closure}}
             at src/librustc/ty/query/plumbing.rs:278
  78: rustc::ty::context::tls::with_related_context::{{closure}}
             at src/librustc/ty/context.rs:1784
  79: rustc::ty::context::tls::with_context::{{closure}}
             at src/librustc/ty/context.rs:1768
  80: rustc::ty::context::tls::with_context_opt
             at src/librustc/ty/context.rs:1757
  81: rustc::ty::context::tls::with_context
             at src/librustc/ty/context.rs:1768
  82: rustc::ty::context::tls::with_related_context
             at src/librustc/ty/context.rs:1781
  83: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query
             at src/librustc/ty/query/plumbing.rs:267
  84: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job::{{closure}}
             at src/librustc/ty/query/plumbing.rs:547
  85: rustc::ty::query::plumbing::with_diagnostics
             at src/librustc/ty/query/plumbing.rs:212
  86: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job
             at src/librustc/ty/query/plumbing.rs:546
  87: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
             at src/librustc/ty/query/plumbing.rs:380
  88: rustc::ty::query::TyCtxtAt::optimized_mir
             at src/librustc/ty/query/plumbing.rs:1057
  89: rustc::ty::query::<impl rustc::ty::context::TyCtxt>::optimized_mir
             at src/librustc/ty/query/plumbing.rs:1020
  90: rustc::ty::<impl rustc::ty::context::TyCtxt>::instance_mir
             at src/librustc/ty/mod.rs:2883
  91: rustc_mir::monomorphize::collector::collect_neighbours
             at src/librustc_mir/monomorphize/collector.rs:1162
  92: rustc_mir::monomorphize::collector::collect_items_rec
             at src/librustc_mir/monomorphize/collector.rs:371
  93: rustc_mir::monomorphize::collector::collect_items_rec
             at src/librustc_mir/monomorphize/collector.rs:381
  94: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}::{{closure}}
             at src/librustc_mir/monomorphize/collector.rs:296
  95: core::iter::traits::iterator::Iterator::for_each::call::{{closure}}
             at ./src/libcore/iter/traits/iterator.rs:656
  96: core::iter::traits::iterator::Iterator::fold::ok::{{closure}}
             at ./src/libcore/iter/traits/iterator.rs:2000
  97: core::iter::traits::iterator::Iterator::try_fold
             at ./src/libcore/iter/traits/iterator.rs:1876
  98: core::iter::traits::iterator::Iterator::fold
             at ./src/libcore/iter/traits/iterator.rs:2003
  99: core::iter::traits::iterator::Iterator::for_each
             at ./src/libcore/iter/traits/iterator.rs:659
 100: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
             at src/librustc_mir/monomorphize/collector.rs:294
 101: rustc_data_structures::profiling::VerboseTimingGuard::run
             at ./src/librustc_data_structures/profiling.rs:518
 102: rustc_session::utils::<impl rustc_session::session::Session>::time
             at ./src/librustc_session/utils.rs:9
 103: rustc_mir::monomorphize::collector::collect_crate_mono_items
             at src/librustc_mir/monomorphize/collector.rs:293
 104: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
             at src/librustc_mir/monomorphize/partitioning.rs:868
 105: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute::{{closure}}
             at ./src/librustc/ty/query/plumbing.rs:951
 106: rustc::ty::query::__query_compute::collect_and_partition_mono_items
             at ./src/librustc/ty/query/plumbing.rs:902
 107: rustc::dep_graph::graph::DepGraph::with_task_impl
             at ./src/librustc/dep_graph/graph.rs:341
 108: rustc::dep_graph::graph::DepGraph::with_eval_always_task
             at ./src/librustc/dep_graph/graph.rs:388
 109: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job::{{closure}}::{{closure}}
             at ./src/librustc/ty/query/plumbing.rs:549
 110: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at ./src/librustc/ty/query/plumbing.rs:278
 111: rustc::ty::context::tls::enter_context::{{closure}}
             at ./src/librustc/ty/context.rs:1696
 112: rustc::ty::context::tls::set_tlv
             at ./src/librustc/ty/context.rs:1681
 113: rustc::ty::context::tls::enter_context
             at ./src/librustc/ty/context.rs:1696
 114: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query::{{closure}}
             at ./src/librustc/ty/query/plumbing.rs:278
 115: rustc::ty::context::tls::with_related_context::{{closure}}
             at ./src/librustc/ty/context.rs:1784
 116: rustc::ty::context::tls::with_context::{{closure}}
             at ./src/librustc/ty/context.rs:1768
 117: rustc::ty::context::tls::with_context_opt
             at ./src/librustc/ty/context.rs:1757
 118: rustc::ty::context::tls::with_context
             at ./src/librustc/ty/context.rs:1768
 119: rustc::ty::context::tls::with_related_context
             at ./src/librustc/ty/context.rs:1781
 120: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::start_query
             at ./src/librustc/ty/query/plumbing.rs:267
 121: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job::{{closure}}
             at ./src/librustc/ty/query/plumbing.rs:547
 122: rustc::ty::query::plumbing::with_diagnostics
             at ./src/librustc/ty/query/plumbing.rs:212
 123: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::force_query_with_job
             at ./src/librustc/ty/query/plumbing.rs:546
 124: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
             at ./src/librustc/ty/query/plumbing.rs:380
 125: rustc::ty::query::TyCtxtAt::collect_and_partition_mono_items
             at ./src/librustc/ty/query/plumbing.rs:1057
 126: rustc::ty::query::<impl rustc::ty::context::TyCtxt>::collect_and_partition_mono_items
             at ./src/librustc/ty/query/plumbing.rs:1020
 127: rustc_codegen_ssa::base::codegen_crate
             at ./src/librustc_codegen_ssa/base.rs:534
 128: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
             at src/librustc_codegen_llvm/lib.rs:265
 129: rustc_interface::passes::start_codegen::{{closure}}
             at src/librustc_interface/passes.rs:968
 130: rustc_data_structures::profiling::VerboseTimingGuard::run
             at ./src/librustc_data_structures/profiling.rs:518
 131: rustc_session::utils::<impl rustc_session::session::Session>::time
             at ./src/librustc_session/utils.rs:9
 132: rustc_interface::passes::start_codegen
             at src/librustc_interface/passes.rs:967
 133: rustc_interface::queries::Queries::ongoing_codegen::{{closure}}::{{closure}}
             at src/librustc_interface/queries.rs:280
 134: rustc_interface::passes::QueryContext::enter::{{closure}}
             at src/librustc_interface/passes.rs:701
 135: rustc::ty::context::tls::enter_global::{{closure}}
             at ./src/librustc/ty/context.rs:1719
 136: rustc::ty::context::tls::enter_context::{{closure}}
             at ./src/librustc/ty/context.rs:1696
 137: rustc::ty::context::tls::set_tlv
             at ./src/librustc/ty/context.rs:1681
 138: rustc::ty::context::tls::enter_context
             at ./src/librustc/ty/context.rs:1696
 139: rustc::ty::context::tls::enter_global
             at ./src/librustc/ty/context.rs:1719
 140: rustc_interface::passes::QueryContext::enter
             at src/librustc_interface/passes.rs:701
 141: rustc_interface::queries::Queries::ongoing_codegen::{{closure}}
             at src/librustc_interface/queries.rs:274
 142: rustc_interface::queries::Query<T>::compute
             at src/librustc_interface/queries.rs:33
 143: rustc_interface::queries::Queries::ongoing_codegen
             at src/librustc_interface/queries.rs:272
 144: rustc_driver::run_compiler::{{closure}}::{{closure}}
             at src/librustc_driver/lib.rs:405
 145: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
             at ./src/librustc_interface/queries.rs:336
 146: rustc_driver::run_compiler::{{closure}}
             at src/librustc_driver/lib.rs:295
 147: rustc_interface::interface::run_compiler_in_existing_thread_pool
             at ./src/librustc_interface/interface.rs:186
 148: rustc_interface::interface::run_compiler::{{closure}}
             at ./src/librustc_interface/interface.rs:200
 149: rustc_interface::util::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}
             at ./src/librustc_interface/util.rs:155
 150: scoped_tls::ScopedKey<T>::set
             at /home/wesley/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
 151: rustc_interface::util::spawn_thread_pool::{{closure}}::{{closure}}
             at ./src/librustc_interface/util.rs:151
 152: scoped_tls::ScopedKey<T>::set
             at /home/wesley/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
 153: syntax::attr::with_globals::{{closure}}
             at ./src/libsyntax/attr/mod.rs:44
 154: scoped_tls::ScopedKey<T>::set
             at /home/wesley/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
 155: syntax::attr::with_globals
             at ./src/libsyntax/attr/mod.rs:44
 156: rustc_interface::util::spawn_thread_pool::{{closure}}
             at ./src/librustc_interface/util.rs:150
 157: rustc_interface::util::scoped_thread::{{closure}}
             at ./src/librustc_interface/util.rs:125
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 1.42.0-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -Z mir-opt-level=2 -Z dump-mir=Inline

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::ops::Fn` fulfills its obligations
#1 [optimized_mir] processing `async_closure`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

[wesleywiser]

@pnkfelix I'm not sure how async closures desugar, so this could be way off base, but this looks very similar to the other ICEs tracked in #62529. Does this look like another instance of that issue to you?

[Aaron1011]

The immediate cause of this is the fact that we call with_reveal_all during inlining:

rust/src/librustc_mir/transform/inline.rs

Line 76 in 54b7d21

param_env = param_env.with_reveal_all();

If we do this, we should also be normalizing the substs we pass to Instance::resolve, so that we reveal any opaque types. However, it looks like the opaque type occurs within a binder (due the opaque type appearing in the return type of an fn pointer), so I think lazy normalization may be required to fix this in the general case.

[JohnTitor]

Triage: It's now compiled fine with the latest nightly, I think it's fixed by #77306.