ICE: Broken MIR in generators

[Nemo157]

#![feature(async_await)]

async fn bar() {
}

pub async fn foo() {
    std::thread::spawn(move || ());
    bar().await;
}

(playground), error:

error: internal compiler error: src/librustc_mir/transform/generator.rs:532: Broken MIR: generator contains type std::thread::JoinHandle<()> in MIR, but typeck only knows about {impl std::future::Future, ()}
 --> src/lib.rs:6:20
  |
6 |   pub async fn foo() {
  |  ____________________^
7 | |     std::thread::spawn(move || ());
8 | |     bar().await;
9 | | }
  | |_^

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:573:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:197
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   8: std::panicking::begin_panic
   9: rustc_errors::Handler::span_bug
  10: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  11: rustc::ty::context::tls::with_opt::{{closure}}
  12: rustc::ty::context::tls::with_context_opt
  13: rustc::ty::context::tls::with_opt
  14: rustc::util::bug::opt_span_bug_fmt
  15: rustc::util::bug::span_bug_fmt
  16: <rustc_mir::transform::generator::StateTransform as rustc_mir::transform::MirPass>::run_pass
  17: rustc_mir::transform::run_passes::{{closure}}
  18: rustc_mir::transform::run_passes
  19: rustc_mir::transform::optimized_mir
  20: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  21: rustc::dep_graph::graph::DepGraph::with_task_impl
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  23: rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt>::layout_raw_uncached
  24: rustc::ty::layout::layout_raw
  25: rustc::ty::query::__query_compute::layout_raw
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::layout_raw>::compute
  27: rustc::dep_graph::graph::DepGraph::with_task_impl
  28: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  29: <rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  30: <rustc_mir::transform::const_prop::ConstPropagator as rustc::mir::visit::MutVisitor>::visit_statement
  31: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  32: rustc_mir::transform::run_passes::{{closure}}
  33: rustc_mir::transform::run_passes
  34: rustc_mir::transform::optimized_mir
  35: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  36: rustc::dep_graph::graph::DepGraph::with_task_impl
  37: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  38: rustc_mir::monomorphize::collector::collect_items_rec
  39: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  40: rustc::util::common::time
  41: rustc_mir::monomorphize::collector::collect_crate_mono_items
  42: rustc::util::common::time
  43: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  44: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  45: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute
  46: rustc::dep_graph::graph::DepGraph::with_task_impl
  47: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  48: rustc_codegen_ssa::back::symbol_export::exported_symbols_provider_local
  49: rustc::ty::query::__query_compute::exported_symbols
  50: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::exported_symbols>::compute
  51: rustc::dep_graph::graph::DepGraph::with_task_impl
  52: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  53: rustc_metadata::encoder::EncodeContext::encode_crate_root
  54: rustc::dep_graph::graph::DepGraph::with_ignore
  55: rustc_metadata::encoder::encode_metadata
  56: rustc_metadata::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::cstore::CStore>::encode_metadata
  57: rustc::ty::context::TyCtxt::encode_metadata
  58: rustc_interface::passes::encode_and_write_metadata
  59: rustc::util::common::time
  60: rustc_interface::passes::start_codegen
  61: rustc::ty::context::tls::enter_global
  62: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  63: rustc_interface::passes::create_global_ctxt::{{closure}}
  64: rustc_interface::passes::BoxedGlobalCtxt::enter
  65: rustc_interface::queries::Query<T>::compute
  66: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  67: rustc_interface::interface::run_compiler_in_existing_thread_pool
  68: std::thread::local::LocalKey<T>::with
  69: scoped_tls::ScopedKey<T>::set
  70: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
#0 [optimized_mir] processing `foo::{{closure}}#0`
#1 [layout_raw] computing layout of `[static generator@src/lib.rs:6:20: 9:2 {std::future::GenFuture<[static generator@src/lib.rs:3:16: 4:2 {}]>, ()}]`
#2 [optimized_mir] processing `foo`
#3 [collect_and_partition_mono_items] collect_and_partition_mono_items
#4 [exported_symbols] exported_symbols
end of query stack
error: aborting due to previous error
note: rustc 1.37.0-nightly (7cdaffd79 2019-06-05) running on x86_64-unknown-linux-gnu

This is minimized from failing doc-tests in futures (rust-lang/futures-rs#1657), from the CI builds there it appears the ICE was introduced between rustc 1.37.0-nightly (5d8f59f4b 2019-06-04) and rustc 1.37.0-nightly (7cdaffd79 2019-06-05).

[Nemo157]

cc @Zoxc, skimming the commits between the above nightlies #61069 seems a likely candidate.

[Zoxc]

We don't generate StorageDead for the JoinHandle MIR local here, so it appears live during the await.

[DoumanAsh]

In case if it will be useful, I caught similar ICE with a different callstack https://gist.github.com/DoumanAsh/2ce9e8aa6c360f5d74bdd169188fec8d

But I imagine it is the same issue.

Code in question: https://gitlab.com/Douman/yukikaze/blob/1.0/src/client/mod.rs#L216-317

[Zoxc]

@tmandry Did you see any reason for why StorageDead is not generated when poking around in scope.rs?

[jbg]

For anyone hitting this issue looking (like me) for a workaround, assigning the value to a local variable works:

pub async fn foo() {
    let _ = std::thread::spawn(move || ());
    bar().await;
}

[nikomatsakis]

A variant that also ICEs (playground):

#![feature(async_await)]

#[derive(Default)]
struct Foo {
    
}

impl Drop for Foo {
    fn drop(&mut self) { }
}

async fn bar() {
}

pub async fn foo() {
    Foo::default();
    bar().await;
}

seems like the problem is "dropping" a value that needs drop from a call