ICE on "computing whether GenFuture<_> is `Copy`"

[aliemjay]

Code

use std::{
    future::Future,
    marker::PhantomData,
    pin::Pin,
    task::{Context, Poll},
};

mod object {
    use super::*;

    pub trait Object<'a> {
        type Error;
        type Future: Future<Output = Self>;
        fn create() -> Self::Future;
    }

    impl<'a> Object<'a> for u8 {
        type Error = ();
        type Future = Pin<Box<dyn Future<Output = Self>>>;
        fn create() -> Self::Future {
            unimplemented!()
        }
    }

    impl<'a, E, A: Object<'a, Error = E>> Object<'a> for (A,) {
        type Error = ();
        type Future = CustomFut<'a, E, A>;
        fn create() -> Self::Future {
            unimplemented!()
        }
    }

    pub struct CustomFut<'f, E, A: Object<'f, Error = E>> {
        ph: PhantomData<(A::Future,)>,
    }

    impl<'f, E, A: Object<'f, Error = E>> Future for CustomFut<'f, E, A> {
        type Output = (A,);
        fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
            unimplemented!()
        }
    }
}

mod async_fn {
    use super::*;

    pub trait AsyncFn {
        type Future: Future<Output = ()>;
        fn call(&self) -> Self::Future;
    }

    impl<F, Fut> AsyncFn for F
    where
        F: Fn() -> Fut,
        Fut: Future<Output = ()>,
    {
        type Future = Fut;
        fn call(&self) -> Self::Future {
            (self)()
        }
    }
}

pub async fn test() {
    use self::{async_fn::AsyncFn, object::Object};

    async fn create<T: Object<'static>>() {
        T::create().await;
    }

    async fn call_async_fn(inner: impl AsyncFn) {
        inner.call().await;
    }

    call_async_fn(create::<(u8,)>).await;
}

Compiled with the command rustc --edition 2021 --crate-type lib repro.rs. Surprisingly, It is not reproducible with --crate-type bin!

With any of the following changes, the code compiles fine:

     impl<'a, E, A: Object<'a, Error = E>> Object<'a> for (A,) {
         type Error = ();
-        type Future = CustomFut<'a, E, A>;
+        type Future = CustomFut<'a, A::Error, A>;
         fn create() -> Self::Future {
             unimplemented!()
         }

or

     pub struct CustomFut<'f, E, A: Object<'f, Error = E>> {
-        ph: PhantomData<(A::Future,)>,
+        ph: PhantomData<(E, A::Future,)>,
     }

Meta

The code compiles fine on v1.55.0. It was reproduced on v1.56.0 up to the current nightly with a similar error output.

Last good nightly: 1.56.0-nightly (b03ccace5 2021-08-24)
Regressed nightly: 1.56.0-nightly (0afc20860 2021-08-25)

rustc --version --verbose:

rustc 1.61.0-nightly (52b34550a 2022-03-15)
binary: rustc
commit-hash: 52b34550aca5f7dd7e152f773e3ab786acb86f6f
commit-date: 2022-03-15
host: x86_64-unknown-linux-gnu
release: 1.61.0-nightly
LLVM version: 14.0.0

Error output

Output with backtrace

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /cargo/registry/src/github.com-1ecc6299db9ec823/ena-0.14.0/src/snapshot_vec.rs:199:10
stack backtrace:
   0: rust_begin_unwind
             at /rustc/52b34550aca5f7dd7e152f773e3ab786acb86f6f/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/52b34550aca5f7dd7e152f773e3ab786acb86f6f/library/core/src/panicking.rs:143:14
   2: core::panicking::panic_bounds_check
             at /rustc/52b34550aca5f7dd7e152f773e3ab786acb86f6f/library/core/src/panicking.rs:84:5
   3: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
   4: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
   5: <&rustc_middle::ty::list::List<rustc_middle::ty::Ty> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
   6: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::FallibleTypeFolder>::try_fold_binder::<&rustc_middle::ty::list::List<rustc_middle::ty::Ty>>
   7: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
   8: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
   9: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  10: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  11: <&rustc_middle::ty::list::List<rustc_middle::ty::Ty> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
  12: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::FallibleTypeFolder>::try_fold_binder::<&rustc_middle::ty::list::List<rustc_middle::ty::Ty>>
  13: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  14: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_infer::infer::canonical::canonicalizer::Canonicalizer>
  15: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  16: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  17: <rustc_infer::infer::canonical::canonicalizer::Canonicalizer as rustc_middle::ty::fold::FallibleTypeFolder>::try_fold_predicate
  18: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::predicate_must_hold_modulo_regions
  19: rustc_trait_selection::traits::type_known_to_meet_bound_modulo_regions
  20: <rustc_infer::infer::InferCtxtBuilder>::enter::<bool, rustc_ty_utils::common_traits::is_item_raw::{closure#0}>
  21: rustc_ty_utils::common_traits::is_copy_raw
  22: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<rustc_middle::ty::Ty>, bool>>
  23: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::is_copy_raw
  24: <rustc_middle::ty::Ty>::is_copy_modulo_regions
  25: rustc_ty_utils::needs_drop::needs_drop_raw
  26: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<rustc_middle::ty::Ty>, bool>>
  27: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::needs_drop_raw
  28: rustc_mir_dataflow::drop_flag_effects::on_all_children_bits::on_all_children_bits::<rustc_mir_dataflow::drop_flag_effects::on_all_drop_children_bits<rustc_mir_transform::elaborate_drops::find_dead_unwinds::{closure#0}>::{closure#0}>
  29: <rustc_mir_transform::elaborate_drops::ElaborateDrops as rustc_middle::mir::MirPass>::run_pass
  30: rustc_mir_transform::pass_manager::run_passes
  31: rustc_mir_transform::mir_drops_elaborated_and_const_checked
  32: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::WithOptConstParam<rustc_span::def_id::LocalDefId>, &rustc_data_structures::steal::Steal<rustc_middle::mir::Body>>>
  33: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_drops_elaborated_and_const_checked
  34: rustc_mir_transform::optimized_mir
  35: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, &rustc_middle::mir::Body>>
  36: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::optimized_mir
  37: <rustc_metadata::rmeta::encoder::EncodeContext>::encode_crate_root
  38: rustc_metadata::rmeta::encoder::encode_metadata_impl
  39: rustc_data_structures::sync::join::<rustc_metadata::rmeta::encoder::encode_metadata::{closure#0}, rustc_metadata::rmeta::encoder::encode_metadata::{closure#1}, rustc_metadata::rmeta::encoder::EncodedMetadata, ()>
  40: rustc_metadata::rmeta::encoder::encode_metadata
  41: <rustc_interface::passes::QueryContext>::enter::<<rustc_interface::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core::result::Result<alloc::boxed::Box<dyn core::any::Any>, rustc_errors::ErrorGuaranteed>>
  42: <rustc_interface::queries::Queries>::ongoing_codegen
  43: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorGuaranteed>>
  44: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
  45: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>
  46: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

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 1.61.0-nightly (52b34550a 2022-03-15) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
#0 [is_copy_raw] computing whether `core::future::from_generator::GenFuture<[static generator@src/lib.rs:72:49: 74:6]>` is `Copy`
#1 [needs_drop_raw] computing whether `core::future::from_generator::GenFuture<[static generator@src/lib.rs:72:49: 74:6]>` needs drop
#2 [mir_drops_elaborated_and_const_checked] elaborating drops for `test::{closure#0}`
#3 [optimized_mir] optimizing MIR for `test::{closure#0}`
end of query stack

@rustbot label A-async-await A-generators regression-from-stable-to-stable

[compiler-errors]

@rustbot claim

[compiler-errors]

Wow this ICE is super weird. I believe it has to do something with the Generalizer in rustc_infer, the fact that generator interiors are represented with higher-kinded types, the fact that the E parameter in the example above is bivariant, and the way that universes work.

Gonna keep working on it for a couple of hours, but I think it might be impossible to fix currently. Or at least it requires someone a lot smarter than me.

[aliemjay]

the fact that the E parameter in the example above is bivariant

Is this a thing in current rust? 🤯

Honestly, I was surprised that rustc accepts CustomFut and not complains about unused type params.

trait Tr<A> { type B; type C; }
struct S<A, B, T: Tr<A, B = B>>(<T as Tr<A>>::C);

Does this pattern exist in practice? People would use PhantomData and this should avoid this problem (I hope).

[compiler-errors]

Is this a thing in current rust?

Yep, see https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_infer/infer/combine.rs.html#506-530. This wf trick just ends up not working for higher-kinded types due to strange caveats...

[compiler-errors]

Well, either a caveat or just a bug I cannot find.

[compiler-errors]

I honestly don't think I understand enough about variance and the Generalizer type relation to fix this issue. I think this would technically be solved by lazy projection normalization, since this is due to a inference variable created by normalizing a projection type.

I will at least add a known-bug test for this. Sorry!

@rustbot release-assignment