"internal compiler error" on Higher-Ranked Trait Bound for impl Trait in return position

[TOETOE55]

I needed a Fn like trait, and return an impl trait with Higher-Ranked Trait Bound. But rustc panicked.

I tried this code:

trait MyFn<Arg> {
    type Output;
    fn call(&self, arg: Arg) -> Self::Output;
}

struct Wrap<F>(F);

impl<A, B, F> MyFn<A> for Wrap<F>
where
    F: Fn(A) -> B
{
    type Output = B;

    fn call(&self, arg: A) -> Self::Output {
        (self.0)(arg)
    }
}


struct A;
fn test() -> impl for<'a> MyFn<&'a A, Output=impl Iterator + 'a> {
    Wrap(|a| Some(a).into_iter())
}

I expected it was checked, but I received this compiler bug:

thread 'rustc' panicked at 'assertion failed: !erased_self_ty.has_escaping_bound_vars()', src\librustc\ty\util.rs:441:9
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: <std::io::IoSliceMut as core::fmt::Debug>::fmt
   3: std::panicking::take_hook
   4: std::panicking::take_hook
   5: rustc_driver::report_ice
   6: std::panicking::rust_panic_with_hook
   7: <rustc::ty::query::Providers as core::default::Default>::default
   8: rustc::ty::util::<impl rustc::ty::context::TyCtxt>::required_region_bounds
   9: rustc::infer::opaque_types::Instantiator::fold_opaque_ty
  10: rustc::ty::binding::BindingMode::convert
  11: rustc::infer::opaque_types::Instantiator::fold_opaque_ty
  12: <rustc_typeck::collect::find_opaque_ty_constraints::ConstraintLocator as rustc::hir::intravisit::Visitor>::visit_trait_item
  13: rustc_typeck::check::FnCtxt::apply_adjustments
  14: <rustc_typeck::check::GatherLocalsVisitor as rustc::hir::intravisit::Visitor>::visit_pat
  15: <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt
  16: <rustc_typeck::check::CheckItemTypesVisitor as rustc::hir::itemlikevisit::ItemLikeVisitor>::visit_item
  17: <rustc_typeck::check::autoderef::AutoderefKind as core::fmt::Debug>::fmt
  18: <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt
  19: <rustc_typeck::variance::test::VarianceTest as rustc::hir::itemlikevisit::ItemLikeVisitor>::visit_item
  20: <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt
  21: rustc_typeck::collect::checked_type_of
  22: <rustc_typeck::collect::has_late_bound_regions::LateBoundRegionsDetector as rustc::hir::intravisit::Visitor>::visit_lifetime
  23: rustc::middle::weak_lang_items::<impl rustc::ty::context::TyCtxt>::is_weak_lang_item
  24: rustc::dep_graph::graph::DepGraph::assert_ignored
  25: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::try_print_query_stack
  26: rustc::ty::util::Discr::checked_add
  27: rustc::ty::util::<impl rustc::ty::context::TyCtxt>::try_expand_impl_trait_type
  28: <rustc_typeck::check::check_opaque_for_inheriting_lifetimes::ProhibitOpaqueVisitor as rustc::ty::fold::TypeVisitor>::visit_region
  29: <rustc_typeck::check::autoderef::AutoderefKind as core::fmt::Debug>::fmt
  30: <rustc_typeck::check::CheckItemTypesVisitor as rustc::hir::itemlikevisit::ItemLikeVisitor>::visit_item
  31: <rustc_typeck::check::autoderef::AutoderefKind as core::fmt::Debug>::fmt
  32: <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt
  33: <rustc_typeck::variance::test::VarianceTest as rustc::hir::itemlikevisit::ItemLikeVisitor>::visit_item
  34: <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt
  35: <rustc_typeck::outlives::explicit::ExplicitPredicatesMap as core::fmt::Debug>::fmt
  36: rustc_typeck::check_crate
  37: rustc_typeck::check_crate
  38: rustc_interface::passes::BoxedResolver::to_resolver_outputs
  39: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  40: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  41: <rustc_traits::chalk_context::ChalkContext as chalk_engine::context::ContextOps<rustc_traits::chalk_context::ChalkArenas>>::canonical
  42: <syntax_pos::symbol::LocalInternedString as core::fmt::Display>::fmt
  43: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  44: rustc_interface::passes::BoxedGlobalCtxt::complete
  45: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  46: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  47: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  48: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  49: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  50: _rust_maybe_catch_panic
  51: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  52: ZN244_$LT$std..error..$LT$impl$u20$core..convert..From$LT$alloc..string..String$GT$$u20$for$u20$alloc..boxed..Box$LT$dyn$u20$std..error..Error$u2b$core..marker..Send$u2b$core..marker..Sync$GT$$GT$..from..StringError$u20$as$u20$core..fmt..Display$GT$3fmt17
  53: std::sys::windows::thread::Thread::new
  54: BaseThreadInitThunk
  55: RtlUserThreadStart
  
  
  
query stack during panic:
#0 [typeck_tables_of] processing `test`
#1 [type_of] processing `test::{{opaque}}#0`
#2 [check_mod_item_types] checking item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

Meta

rustc --version --verbose:

rustc 1.40.0 (73528e339 2019-12-16)
binary: rustc
commit-hash: 73528e339aae0f17a15ffa49a8ac608f50c6cf14
commit-date: 2019-12-16
host: x86_64-pc-windows-msvc
release: 1.40.0
LLVM version: 9.0

[TOETOE55]

hmmm, maybe it is equivalent to (no!)

fn test<'a>() -> impl MyFn<&'a A, Output=impl Iterator + 'a>

but what about:

fn test(f: impl for<'a> MyFn<&'a A, Output=impl Iterator + 'a>) {
    let x = A;
    f.call(&x);
}

it couldn't check:

fn test(_: impl for<'a> MyFn<&'a A, Output=impl Iterator + 'a>) {
                                                            ^^ undeclared lifetime

but this is OK:

fn test(_: impl for<'a> MyFn<&'a A, Output=IntoIter<&'a A>>) {
    let x = A;
    f.call(&x);
}

playground here
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f15195784e3fe07799fdb4ee84d9c049

[pnkfelix]

triage: P-high. Removing nominaton, but cc @matthewjasper and @nikomatsakis for assistance.

[smmalis37]

Minimized further

trait Empty {
}

trait T<'l> {
    type Assoc;
}

fn foo() -> impl for<'a> T<'a, Assoc = impl Empty + 'a> {
    todo!()
}

It seems like pretty much any trait will work in place of Empty, but I wanted one guaranteed to have no special treatment.

[smmalis37]

This is the exact same stack trace and error message as #54895. Should one of these be duped to the other?

[matthiaskrgr]

This is a regression since 1.28.
@rustbot modify labels: +regression-from-stable-to-stable

[smmalis37]

As of the latest nightly this still ICEs, but the stack has changed thanks to #85499 :

thread 'rustc' panicked at 'Normalizing [Binder(TraitPredicate(<impl for<'a> Empty as std::marker::Sized>), []), Binder(OutlivesPredicate(impl for<'a> Empty, ReLateBound(DebruijnIndex(1), BoundRegion { var: 0, kind: BrNamed(DefId(0:9 ~ playground[14a5]::foo::{opaque#0}::'a), 'a) })), []), Binder(TraitPredicate(<impl for<'a> Empty as Empty>), [])] without wrapping in a `Binder`', compiler/rustc_trait_selection/src/traits/project.rs:332:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/50171c310cd15e1b2d3723766ce64e2e4d6696fc/library/std/src/panicking.rs:517:5
   1: std::panicking::begin_panic_fmt
             at /rustc/50171c310cd15e1b2d3723766ce64e2e4d6696fc/library/std/src/panicking.rs:460:5
   2: rustc_trait_selection::traits::project::AssocTypeNormalizer::fold
   3: rustc_trait_selection::traits::project::normalize
   4: <rustc_infer::infer::InferCtxt as rustc_trait_selection::infer::InferCtxtExt>::partially_normalize_associated_types_in
   5: rustc_trait_selection::opaque_types::Instantiator::fold_opaque_ty
   6: <rustc_middle::ty::fold::BottomUpFolder<F,G,H> as rustc_middle::ty::fold::TypeFolder>::fold_ty
   7: rustc_middle::ty::fold::TypeFoldable::fold_with
   8: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for rustc_middle::ty::sty::Binder<T>>::super_fold_with
   9: rustc_trait_selection::opaque_types::Instantiator::fold_opaque_ty
  10: <rustc_middle::ty::fold::BottomUpFolder<F,G,H> as rustc_middle::ty::fold::TypeFolder>::fold_ty
  11: <rustc_infer::infer::InferCtxt as rustc_trait_selection::opaque_types::InferCtxtExt>::instantiate_opaque_types
  12: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::instantiate_opaque_types_from_value
  13: rustc_typeck::check::check::check_fn
  14: rustc_infer::infer::InferCtxtBuilder::enter
  15: rustc_typeck::check::typeck
  16: rustc_query_system::query::plumbing::get_query_impl
  17: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck
  18: rustc_middle::ty::context::TyCtxt::typeck_opt_const_arg
  19: rustc_mir_build::build::mir_built
  20: rustc_query_system::query::plumbing::get_query_impl
  21: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_built
  22: rustc_mir::transform::check_unsafety::unsafety_check_result
  23: core::ops::function::FnOnce::call_once
  24: rustc_query_system::query::plumbing::get_query_impl
  25: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::unsafety_check_result
  26: rustc_mir::transform::mir_const
  27: rustc_query_system::query::plumbing::get_query_impl
  28: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_const
  29: rustc_mir::transform::mir_promoted
  30: rustc_query_system::query::plumbing::get_query_impl
  31: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_promoted
  32: rustc_mir::borrow_check::mir_borrowck
  33: core::ops::function::FnOnce::call_once
  34: rustc_query_system::query::plumbing::get_query_impl
  35: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
  36: rustc_typeck::collect::type_of::type_of
  37: rustc_query_system::query::plumbing::get_query_impl
  38: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::type_of
  39: rustc_typeck::check::check::check_item_type
  40: rustc_middle::hir::map::Map::visit_item_likes_in_module
  41: rustc_typeck::check::check::check_mod_item_types
  42: rustc_query_system::query::plumbing::get_query_impl
  43: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_item_types
  44: rustc_session::utils::<impl rustc_session::session::Session>::time
  45: rustc_typeck::check_crate
  46: rustc_interface::passes::analysis
  47: rustc_query_system::query::plumbing::get_query_impl
  48: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  49: rustc_interface::passes::QueryContext::enter
  50: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  51: rustc_span::with_source_map
  52: scoped_tls::ScopedKey<T>::set
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 1.56.0-nightly (50171c310 2021-09-01) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

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

query stack during panic:
#0 [typeck] type-checking `foo`
#1 [mir_built] building MIR for `foo`
#2 [unsafety_check_result] unsafety-checking `foo`
#3 [mir_const] processing MIR for `foo`
#4 [mir_promoted] processing `foo`
#5 [mir_borrowck] borrow-checking `foo`
#6 [type_of] computing type of `foo::{opaque#0}`
#7 [check_mod_item_types] checking item types in top-level module
#8 [analysis] running analysis passes on this crate
end of query stack
For more information about this error, try `rustc --explain E0601`.