Async/.await and const-generics causes "Broken MIR:"

[rot256]

Code

The use of async/await and const generics can cause ICE (in the type checker?). Here is a minimal example which induces ICE:

#![feature(const_generics)]
#![allow(incomplete_features)]

const SIZE: usize = 16;

struct Bar<const H: usize> {}

struct Foo<const H: usize> {}

impl<const H: usize> Foo<H> {
    async fn biz(_: &[[u8; SIZE]]) -> Vec<()> {
        vec![]
    }

    pub async fn baz(&self) -> Bar<H> {
        Self::biz(&vec![]).await;
        Bar {}
    }
}

The behaviour is not triggered without the use of async/await.

The full library source.

Meta

I am running recent nightly compiler:

rustc --version --verbose:

rustc 1.47.0-nightly (6c8927b0c 2020-07-26)
binary: rustc
commit-hash: 6c8927b0cf80ceee19386026cf9d7fd4fd9d486f
commit-date: 2020-07-26
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0

Error output

RUST_BACKTRACE=full cargo build

   Compiling bug v0.1.0 (/home/rot256/src/bug)
warning: constant is never used: `SIZE`
 --> src/lib.rs:4:1
   |
4 | const SIZE: usize = 16;
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
  = note: `#[warn(dead_code)]` on by default

warning: struct is never constructed: `Bar`
 --> src/lib.rs:6:8
   |
6 | struct Bar<const H: usize> {}
   |        ^^^

warning: struct is never constructed: `Foo`
 --> src/lib.rs:8:8
  |
8 | struct Foo<const H: usize> {}
   |        ^^^

warning: associated function is never used: `biz`
  --> src/lib.rs:11:14
     |
11 |     async fn biz(_: &[[u8; SIZE]]) -> Vec<()> {
     |              ^^^

warning: associated function is never used: `baz`
  --> src/lib.rs:15:18
     |
15 |     pub async fn baz(&self) -> Bar<H> {
     |                  ^^^

error: internal compiler error: src/librustc_mir/transform/generator.rs:739:13: Broken MIR: generator contains type std::vec::Vec<[u8; _]> in MIR, but typeck only knows about for<'r, 's, 't0, 't1> {std::future::ResumeTy, &'r Foo<H>, std::vec::Vec<[u8; _]>, &'s [[u8; _]], &'t0 std::vec::Vec<[u8; _]>, impl std::future::Future, ()}
  --> src/lib.rs:15:39
     |
15 |       pub async fn baz(&self) -> Bar<H> {
     |  _______________________________________^
16 | |         Self::biz(&vec![]).await;
17 | |         Bar {}
18 | |     }
     | |_____^

thread 'rustc' panicked at 'Box<Any>', /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/libstd/macros.rs:13:23
stack backtrace:
   0:     0x7f35c2d05215 - backtrace::backtrace::libunwind::trace::h75aedf5f78e5147f
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1:     0x7f35c2d05215 - backtrace::backtrace::trace_unsynchronized::h18fb73c9ac9ae753
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2:     0x7f35c2d05215 - std::sys_common::backtrace::_print_fmt::h65f97470ff13ec84
                               at src/libstd/sys_common/backtrace.rs:78
   3:     0x7f35c2d05215 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hee061c54ddc9f024
                               at src/libstd/sys_common/backtrace.rs:59
   4:     0x7f35c2d4141c - core::fmt::write::hfbd2baad61ed21a8
                               at src/libcore/fmt/mod.rs:1117
   5:     0x7f35c2cf6f52 - std::io::Write::write_fmt::h72f9bd227f40dc62
                               at src/libstd/io/mod.rs:1508
   6:     0x7f35c2d0a0b0 - std::sys_common::backtrace::_print::h2d2cd8fe02feb5fa
                               at src/libstd/sys_common/backtrace.rs:62
   7:     0x7f35c2d0a0b0 - std::sys_common::backtrace::print::h801b12991252ba7c
                               at src/libstd/sys_common/backtrace.rs:49
   8:     0x7f35c2d0a0b0 - std::panicking::default_hook::{{closure}}::h25fc1fbf3b63b5c8
                               at src/libstd/panicking.rs:198
   9:     0x7f35c2d09dfc - std::panicking::default_hook::h62c897957a5e0f26
                               at src/libstd/panicking.rs:217
  10:     0x7f35c348e719 - rustc_driver::report_ice::h54e3b417084b44b0
  11:     0x7f35c2d0a828 - std::panicking::rust_panic_with_hook::hb8a276f163c59810
                               at src/libstd/panicking.rs:530
  12:     0x7f35c4648a43 - std::panicking::begin_panic::h56ab19f2cedb9db6
  13:     0x7f35c46cf5ac - rustc_errors::HandlerInner::span_bug::h6725b1754e0b8350
  14:     0x7f35c46cf9f0 - rustc_errors::Handler::span_bug::h690acfedaaa39918
  15:     0x7f35c46d3ec4 - rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}::h9ad20ff21dfdee15
  16:     0x7f35c46d2c2b - rustc_middle::ty::context::tls::with_opt::{{closure}}::h55dd76028893bf0b
  17:     0x7f35c46d2bd2 - rustc_middle::ty::context::tls::with_opt::ha4f5888994b68db9
  18:     0x7f35c46d3dc9 - rustc_middle::util::bug::opt_span_bug_fmt::hff1523241ff1d646
  19:     0x7f35c46d3d83 - rustc_middle::util::bug::span_bug_fmt::h6642bef07315747d
  20:     0x7f35c4677dbf - <rustc_mir::transform::generator::StateTransform as rustc_mir::transform::MirPass>::run_pass::ha0b6b0061f9616e8
  21:     0x7f35c4a076d5 - rustc_mir::transform::run_passes::h49d12d42b645883c
  22:     0x7f35c4a097ee - rustc_mir::transform::run_optimization_passes::h4808514d2a3dc8d0
  23:     0x7f35c4a09bc0 - rustc_mir::transform::inner_optimized_mir::h7bbfaea3bf718bc9
  24:     0x7f35c4a0997d - rustc_mir::transform::optimized_mir::h2c34b792ca19fc38
  25:     0x7f35c5e8e061 - rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps::h4c010a4768f0f01c
  26:     0x7f35c58f5b4f - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl::h1d87895a9b5e641e
  27:     0x7f35c5c846b2 - rustc_data_structures::stack::ensure_sufficient_stack::hf48d61a9d17fd483
  28:     0x7f35c5acc793 - rustc_query_system::query::plumbing::get_query_impl::hef59961961c2f14e
  29:     0x7f35c59bb801 - rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached::h232eb1b291401511
  30:     0x7f35c59b6cff - rustc_middle::ty::layout::layout_raw::hf9eb140d1022d561
  31:     0x7f35c59e5c21 - rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::layout_raw>::compute::h9a2c6c02944bd2dd
  32:     0x7f35c5e92075 - rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps::he44c72eff6c9f466
  33:     0x7f35c590c0a0 - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl::h7a291f01f92f5108
  34:     0x7f35c5c7762f - rustc_data_structures::stack::ensure_sufficient_stack::h353561557dc75241
  35:     0x7f35c5abd6e7 - rustc_query_system::query::plumbing::get_query_impl::hdc6080a3a10cbfb0
  36:     0x7f35c59c57f4 - <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of::hdd5ae6209bb8c384
  37:     0x7f35c5f24fbe - <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next::h3dc9bd222e471f34
  38:     0x7f35c58b312c - <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter::h02b7a95b20eda5b5
  39:     0x7f35c5f24dbd - <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next::h19c51e30891d4acc
  40:     0x7f35c58d73b0 - <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter::hecfa2cc5545dc974
  41:     0x7f35c59bc2e2 - rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached::h232eb1b291401511
  42:     0x7f35c59b6cff - rustc_middle::ty::layout::layout_raw::hf9eb140d1022d561
  43:     0x7f35c59e5c21 - rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::layout_raw>::compute::h9a2c6c02944bd2dd
  44:     0x7f35c5e92075 - rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps::he44c72eff6c9f466
  45:     0x7f35c590c0a0 - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl::h7a291f01f92f5108
  46:     0x7f35c5c7762f - rustc_data_structures::stack::ensure_sufficient_stack::h353561557dc75241
  47:     0x7f35c5abd6e7 - rustc_query_system::query::plumbing::get_query_impl::hdc6080a3a10cbfb0
  48:     0x7f35c59c6351 - <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::query::TyCtxtAt> as rustc_target::abi::LayoutOf>::layout_of::h249f6e77b4e78581
  49:     0x7f35c470fa5e - <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass::heeb6bfd954ebfdb5
  50:     0x7f35c4a076d5 - rustc_mir::transform::run_passes::h49d12d42b645883c
  51:     0x7f35c4a097ee - rustc_mir::transform::run_optimization_passes::h4808514d2a3dc8d0
  52:     0x7f35c4a09bc0 - rustc_mir::transform::inner_optimized_mir::h7bbfaea3bf718bc9
  53:     0x7f35c4a0997d - rustc_mir::transform::optimized_mir::h2c34b792ca19fc38
  54:     0x7f35c4e721e2 - rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::optimized_mir>::compute::hd8f3e45025769508
  55:     0x7f35c4f44a2a - rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps::hbbb1691ceb2a7378
  56:     0x7f35c5033abc - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl::h2a116afb948656f3
  57:     0x7f35c4fb6de3 - rustc_data_structures::stack::ensure_sufficient_stack::h50171c7f6732d5e7
  58:     0x7f35c4fdab9a - rustc_query_system::query::plumbing::get_query_impl::h23e08d9912664785
  59:     0x7f35c4e900bd - rustc_metadata::rmeta::encoder::EncodeContext::encode_optimized_mir::h21e11081f3184770
  60:     0x7f35c4e9437b - <rustc_metadata::rmeta::encoder::EncodeContext as rustc_hir::intravisit::Visitor>::visit_item::hc5fad122cac52247
  61:     0x7f35c508fc67 - rustc_hir::hir::Crate::visit_all_item_likes::h57ce95c9da9b50ad
  62:     0x7f35c4e98da5 - rustc_metadata::rmeta::encoder::encode_metadata_impl::h2e797180c72c7de0
  63:     0x7f35c4fb437c - rustc_data_structures::sync::join::h26f229ec55e7a0f5
  64:     0x7f35c4eb2630 - rustc_metadata::rmeta::decoder::cstore_impl::<impl rustc_middle::middle::cstore::CrateStore for rustc_metadata::creader::CStore>::encode_metadata::hab1eceef04d8a455
  65:     0x7f35c5cdde50 - rustc_middle::ty::context::TyCtxt::encode_metadata::h6eaf9864b984a444
  66:     0x7f35c362e8f4 - rustc_interface::passes::start_codegen::h4cb47030bcada22e
  67:     0x7f35c36488e6 - rustc_middle::ty::context::tls::enter_global::hc8731f69dcd0bbcb
  68:     0x7f35c364445e - rustc_interface::queries::Queries::ongoing_codegen::ha8e667ca2563fa09
  69:     0x7f35c34498d3 - rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter::h3b5436b320054acc
  70:     0x7f35c34e8277 - rustc_span::with_source_map::hd3215c6d07150bbc
  71:     0x7f35c344afb6 - rustc_interface::interface::create_compiler_and_run::hc502b394ead637d1
  72:     0x7f35c347432d - scoped_tls::ScopedKey<T>::set::hb09c39dbfa5a4396
  73:     0x7f35c349a4a2 - std::sys_common::backtrace::__rust_begin_short_backtrace::h4fa4ac8c571754e6
  74:     0x7f35c345726e - core::ops::function::FnOnce::call_once{{vtable.shim}}::h23cc72206c0cf83b
  75:     0x7f35c2d18d3a - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h3e41d894970cea2f
                               at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/liballoc/boxed.rs:1074
  76:     0x7f35c2d18d3a - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h9a318bb00b7f8871
                               at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/liballoc/boxed.rs:1074
  77:     0x7f35c2d18d3a - std::sys::unix::thread::Thread::new::thread_start::hf522342530b04cb6
                               at src/libstd/sys/unix/thread.rs:87
  78:     0x7f35c2c48f27 - start_thread
                               at ./nptl/pthread_create.c:479
  79:     0x7f35c2b6831f - __clone
                               at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
  80:                0x0 - <unknown>

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 1.47.0-nightly (6c8927b0c 2020-07-26) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [optimized_mir] optimizing MIR for `Foo::<H>::baz::{{closure}}#0`
#1 [layout_raw] computing layout of `[static generator@src/lib.rs:15:39: 18:6 self:&Foo<H> for<'r, 's, 't0, 't1> {std::future::ResumeTy, &'r Foo<H>, std::vec::Vec<[u8; 16]>, &'s [[u8; 16]], &'t0 std::vec::Vec<[u8; 16]>, impl std::future::Future, ()}]`
#2 [layout_raw] computing layout of `std::future::from_generator::GenFuture<[static generator@src/lib.rs:15:39: 18:6 self:&Foo<H> for<'r, 's, 't0, 't1> {std::future::ResumeTy, &'r Foo<H>, std::vec::Vec<[u8; 16]>, &'s [[u8; 16]], &'t0 std::vec::Vec<[u8; 16]>, impl std::future::Future, ()}]>`
#3 [optimized_mir] optimizing MIR for `Foo::<H>::baz`
end of query stack
error: aborting due to previous error; 5 warnings emitted

error: could not compile `bug`.

To learn more, run the command again with --verbose.

[jonas-schievink]

Guessing: It might be comparing an Unevaluated const to another Unevaluated const and expects them to compare equal.

Slightly minified:

#![feature(const_generics)]
#![allow(incomplete_features)]

async fn baz<const H: usize>() {
    biz(&Vec::new()).await;
}

const SIZE: usize = 16;

async fn biz(_: &[[u8; SIZE]]) {}

[tmandry]

Related to #72651. Not going to mark as a duplicate yet, since the MVCE is quite different.

[varkor]

Note that this ICEs with const_generics but not min_const_generics, where it compiles successfully. We should add a test.

[JohnTitor]

Triage: It's now compiled successfully with const_genetics and the latest nightly.

[tmandry]

Great! Leaving open to track adding a regression test.

[hameerabbasi]

I'd like to add the test.
@rustbot claim