assertion failed: `(left == right)` in relate_tys.rs

[sffc]

This is a bug potentially related to #86702 or #84937, possibly relating to lifetime resolution in a function argument that involves HRTBs.

I did my best to reduce the example, but I couldn't do it completely in isolation from the rest of the ICU4X project code.

To reproduce:

1. Check out sffc/omnicu@30eaa22
2. cd to provider/core
3. Run cargo +nightly-2021-08-27 check --all-targets

Code

// This depends on DataPayload from ICU4X (see link above)

struct Options {}

fn demo(options: &Options) {
    let p1: DataPayload<HelloWorldV1Marker> = todo!();
    let p2: DataPayload<HelloWorldV1MessageMarker> =
        p1.map_project_with_capture(options, |obj, options, _| todo!());
}

Here is the definition of the function map_project_with_capture, which you can also find in the linked branch:

    pub fn map_project_with_capture<M2, T>(
        self,
        capture: T,
        f: for<'a> fn(
            <M::Yokeable as Yokeable<'a>>::Output,
            capture: T,
            PhantomData<&'a ()>,
        ) -> <M2::Yokeable as Yokeable<'a>>::Output,
    ) -> DataPayload<'data, M2>
    where
        M2: DataMarker<'data, Cart = M::Cart>,
    {
        use DataPayloadInner::*;
        match self.inner {
            RcStruct(yoke) => DataPayload {
                inner: RcStruct(yoke.project_with_capture(capture, f)),
            },
            Owned(yoke) => DataPayload {
                inner: Owned(yoke.project_with_capture(capture, f)),
            },
            RcBuf(yoke) => DataPayload {
                inner: RcBuf(yoke.project_with_capture(capture, f)),
            },
        }
    }

Yoke::project_with_capture can be found in the yoke crate on crates.io:

https://docs.rs/yoke/0.2.3/yoke/struct.Yoke.html#method.project_with_capture

Meta

rustc --version --verbose:

$ cargo +nightly-2021-08-27 rustc -- --version --verbose
rustc 1.56.0-nightly (ad02dc46b 2021-08-26)
binary: rustc
commit-hash: ad02dc46badee510bd3a2c093edf80fcaade91b1
commit-date: 2021-08-26
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 13.0.0

Error output

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `U3`,
 right: `U4`', compiler/rustc_mir/src/borrow_check/type_check/relate_tys.rs:86:9
note: run with `RUST_BACKTRACE=1` environment variable to display a 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 1.56.0-nightly (ad02dc46b 2021-08-26) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `demo::{closure#0}`
#1 [analysis] running analysis passes on this crate
end of query stack

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/ad02dc46badee510bd3a2c093edf80fcaade91b1/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/ad02dc46badee510bd3a2c093edf80fcaade91b1/library/core/src/panicking.rs:101:14
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed
   4: <rustc_mir::borrow_check::type_check::relate_tys::NllTypeRelatingDelegate as rustc_infer::infer::nll_relate::TypeRelatingDelegate>::create_next_universe
   5: rustc_infer::infer::nll_relate::TypeRelating<D>::create_scope::{{closure}}
   6: <rustc_infer::infer::nll_relate::ScopeInstantiator as rustc_middle::ty::fold::TypeVisitor>::visit_region
   7: <core::iter::adapters::copied::Copied<I> as core::iter::traits::iterator::Iterator>::try_fold
   8: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_visit_with
   9: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_visit_with
  10: <core::iter::adapters::copied::Copied<I> as core::iter::traits::iterator::Iterator>::try_fold
  11: <rustc_infer::infer::nll_relate::TypeRelating<D> as rustc_middle::ty::relate::TypeRelation>::binders
  12: rustc_middle::ty::relate::super_relate_tys
  13: rustc_infer::infer::combine::<impl rustc_infer::infer::InferCtxt>::super_combine_tys
  14: rustc_middle::ty::relate::TypeRelation::relate
  15: <rustc_middle::ty::subst::GenericArg as rustc_middle::ty::relate::Relate>::relate
  16: <smallvec::SmallVec<A> as core::iter::traits::collect::Extend<<A as smallvec::Array>::Item>>::extend
  17: <core::result::Result<T,E> as rustc_middle::ty::context::InternIteratorElement<T,R>>::intern_with
  18: rustc_middle::ty::relate::super_relate_tys
  19: rustc_infer::infer::combine::<impl rustc_infer::infer::InferCtxt>::super_combine_tys
  20: rustc_middle::ty::relate::TypeRelation::relate
  21: rustc_middle::ty::relate::super_relate_tys
  22: rustc_infer::infer::combine::<impl rustc_infer::infer::InferCtxt>::super_combine_tys
  23: rustc_middle::ty::relate::TypeRelation::relate
  24: rustc_mir::borrow_check::type_check::TypeChecker::relate_types
  25: rustc_mir::borrow_check::type_check::input_output::<impl rustc_mir::borrow_check::type_check::TypeChecker>::equate_normalized_input_or_output
  26: rustc_mir::borrow_check::type_check::type_check
  27: rustc_mir::borrow_check::nll::compute_regions
  28: rustc_mir::borrow_check::do_mir_borrowck
  29: rustc_infer::infer::InferCtxtBuilder::enter
  30: rustc_mir::borrow_check::mir_borrowck
  31: core::ops::function::FnOnce::call_once
  32: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  33: rustc_data_structures::stack::ensure_sufficient_stack
  34: rustc_query_system::query::plumbing::force_query_with_job
  35: rustc_query_system::query::plumbing::get_query_impl
  36: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
  37: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  38: rustc_interface::passes::analysis
  39: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  40: rustc_data_structures::stack::ensure_sufficient_stack
  41: rustc_query_system::query::plumbing::force_query_with_job
  42: rustc_query_system::query::plumbing::get_query_impl
  43: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  44: rustc_interface::passes::QueryContext::enter
  45: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  46: rustc_span::with_source_map
  47: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

CC @Manishearth

[hellow554]

MCVE:

trait Yokeable<'a> {
    type Output: 'a;
}
impl<'a> Yokeable<'a> for () {
    type Output = ();
}

trait DataMarker<'data> {
    type Yokeable: for<'a> Yokeable<'a>;
}
impl<'data> DataMarker<'data> for () {
    type Yokeable = ();
}

struct DataPayload<'data, M>(&'data M);

impl DataPayload<'static, ()> {
    pub fn map_project_with_capture<M2, T>(
        _: for<'a> fn(
            capture: T,
            std::marker::PhantomData<&'a ()>,
        ) -> <M2::Yokeable as Yokeable<'a>>::Output,
    ) -> DataPayload<'static, M2>
    where
        M2: DataMarker<'static>,
    {
        todo!()
    }
}

fn main() {
    let _: DataPayload<()> = DataPayload::<()>::map_project_with_capture::<_, &()>(|_, _| todo!());
}

Bisecting now

[hellow554]

searched nightlies: from nightly-2021-07-21 to nightly-2021-08-30
regressed nightly: nightly-2021-08-26
searched commits: from b03ccac to 0afc208
regressed commit: 0afc208

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --end=2021-08-30 --regress ice --access=github

---

cc #85499 @jackh726