GAT crash

[497e0bdf29873]

The compiler crashes with an internal error on the following MWE of a GAT workaround attempt to current const generics limitations, i.e., not being able to just do {N*N} (or rather pow(2, N) in my real code). There is no crash if T is fixed to some type, i.e, GATs are not needed, however, the idea would be to limit “trait bound bloat” by only having to repeat NodeLookup : NodeT<N> throughout the code without T and its trait bounds, etc..

Code

#![feature(generic_associated_types)]

struct Node<T, const N : usize, const P : usize> {
    data : [[T; N]; P]
}

trait NodeT<const N : usize> {
    type Node<T>;
}

struct NodeLookup;

macro_rules! impl_lookup {
    ($($n:literal)*) => { $(
        impl NodeT<$n> for NodeLookup {
            type Node<T> = Node<T, $n, {$n*$n}>;
        }
    )* }
}

impl_lookup!(1 2 3 4);

fn main() {
}

Meta

rustc --version --verbose:

rustc 1.64.0-nightly (ddcbba036 2022-06-29)
binary: rustc
commit-hash: ddcbba036aee08f0709f98a92a342a278eae5c05
commit-date: 2022-06-29
host: aarch64-apple-darwin
release: 1.64.0-nightly
LLVM version: 14.0.6

Error output

error: internal compiler error: /rustc/ddcbba036aee08f0709f98a92a342a278eae5c05/compiler/rustc_middle/src/ty/fold.rs:756:29: unexpected bound ty in binder: BoundTy { var: 0, kind: Param("T") }

thread 'rustc' panicked at 'Box<dyn Any>', /rustc/ddcbba036aee08f0709f98a92a342a278eae5c05/compiler/rustc_errors/src/lib.rs:1391:9
stack backtrace:
   0:        0x10111ff18 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h094534642b0a9537
   1:        0x10116cab4 - core::fmt::write::he79b8cc7db6af844
   2:        0x101112f7c - std::io::Write::write_fmt::h9824dce6dc163d2c
   3:        0x101122b5c - std::panicking::default_hook::{{closure}}::h3cd2810b91c69805
   4:        0x1011228c4 - std::panicking::default_hook::hddba5976d502416b
   5:        0x1085cb5f4 - rustc_driver[f51bca0c71cdd034]::DEFAULT_HOOK::{closure#0}::{closure#0}
   6:        0x10112312c - std::panicking::rust_panic_with_hook::h94731f9d6cc315f3
   7:        0x10c0aa758 - std[ec5bd7e3f33f3027]::panicking::begin_panic::<rustc_errors[77f389aec993a0ef]::ExplicitBug>::{closure#0}
   8:        0x10c0a86cc - std[ec5bd7e3f33f3027]::sys_common::backtrace::__rust_end_short_backtrace::<std[ec5bd7e3f33f3027]::panicking::begin_panic<rustc_errors[77f389aec993a0ef]::ExplicitBug>::{closure#0}, !>
   9:        0x10c509f04 - std[ec5bd7e3f33f3027]::panicking::begin_panic::<rustc_errors[77f389aec993a0ef]::ExplicitBug>
  10:        0x10c00bc50 - std[ec5bd7e3f33f3027]::panic::panic_any::<rustc_errors[77f389aec993a0ef]::ExplicitBug>
  11:        0x10c0064d4 - <rustc_errors[77f389aec993a0ef]::HandlerInner>::bug::<&alloc[cde6e00f4e7b7126]::string::String>
  12:        0x10c0060a0 - <rustc_errors[77f389aec993a0ef]::Handler>::bug::<&alloc[cde6e00f4e7b7126]::string::String>
  13:        0x10c0be8b0 - rustc_middle[9331575e900b72f8]::ty::context::tls::with_opt::<rustc_middle[9331575e900b72f8]::util::bug::opt_span_bug_fmt<rustc_span[b8e5c2445f520dcd]::span_encoding::Span>::{closure#0}, ()>
  14:        0x10c0c0408 - rustc_middle[9331575e900b72f8]::util::bug::opt_span_bug_fmt::<rustc_span[b8e5c2445f520dcd]::span_encoding::Span>
  15:        0x10c50c650 - rustc_middle[9331575e900b72f8]::util::bug::bug_fmt
  16:        0x10be5ce1c - <rustc_middle[9331575e900b72f8]::ty::context::TyCtxt>::replace_late_bound_regions_uncached::<rustc_middle[9331575e900b72f8]::ty::PredicateKind, <rustc_middle[9331575e900b72f8]::ty::context::TyCtxt>::replace_late_bound_regions<rustc_middle[9331575e900b72f8]::ty::PredicateKind, <rustc_middle[9331575e900b72f8]::ty::context::TyCtxt>::anonymize_late_bound_regions<rustc_middle[9331575e900b72f8]::ty::PredicateKind>::{closure#0}>::{closure#0}>::{closure#0}
  17:        0x10c08da1c - <rustc_middle[9331575e900b72f8]::ty::fold::BoundVarReplacer as rustc_middle[9331575e900b72f8]::ty::fold::TypeFolder>::fold_ty
  18:        0x10be40f88 - <&rustc_middle[9331575e900b72f8]::ty::list::List<rustc_middle[9331575e900b72f8]::ty::subst::GenericArg> as rustc_middle[9331575e900b72f8]::ty::fold::TypeFoldable>::try_fold_with::<rustc_middle[9331575e900b72f8]::ty::fold::BoundVarReplacer>
  19:        0x10be7082c - <rustc_middle[9331575e900b72f8]::ty::PredicateKind as rustc_middle[9331575e900b72f8]::ty::fold::TypeFoldable>::fold_with::<rustc_middle[9331575e900b72f8]::ty::fold::BoundVarReplacer>
  20:        0x10be582cc - <rustc_middle[9331575e900b72f8]::ty::context::TyCtxt>::anonymize_late_bound_regions::<rustc_middle[9331575e900b72f8]::ty::PredicateKind>
  21:        0x10bdfe5f0 - <rustc_middle[9331575e900b72f8]::ty::erase_regions::RegionEraserVisitor as rustc_middle[9331575e900b72f8]::ty::fold::FallibleTypeFolder>::try_fold_binder::<rustc_middle[9331575e900b72f8]::ty::PredicateKind>
  22:        0x10be3effc - <&rustc_middle[9331575e900b72f8]::ty::list::List<rustc_middle[9331575e900b72f8]::ty::Predicate> as rustc_middle[9331575e900b72f8]::ty::fold::TypeFoldable>::try_fold_with::<rustc_middle[9331575e900b72f8]::ty::erase_regions::RegionEraserVisitor>
  23:        0x10bdbf7f4 - <rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer as rustc_middle[9331575e900b72f8]::ty::fold::TypeFolder>::fold_const
  24:        0x10be44734 - <&rustc_middle[9331575e900b72f8]::ty::list::List<rustc_middle[9331575e900b72f8]::ty::subst::GenericArg> as rustc_middle[9331575e900b72f8]::ty::fold::TypeFoldable>::try_fold_with::<rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer>
  25:        0x10be71208 - <rustc_middle[9331575e900b72f8]::ty::Ty as rustc_middle[9331575e900b72f8]::ty::fold::TypeSuperFoldable>::super_fold_with::<rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer>
  26:        0x10bdbb148 - <rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer as rustc_middle[9331575e900b72f8]::ty::fold::TypeFolder>::fold_ty
  27:        0x10bdc04dc - rustc_trait_selection[b4f82a16548071ea]::traits::project::opt_normalize_projection_type
  28:        0x10bdbbd54 - rustc_trait_selection[b4f82a16548071ea]::traits::project::normalize_projection_type
  29:        0x10bdbaf18 - <rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer as rustc_middle[9331575e900b72f8]::ty::fold::TypeFolder>::fold_ty
  30:        0x10ab6613c - <&rustc_middle[9331575e900b72f8]::ty::list::List<rustc_middle[9331575e900b72f8]::ty::subst::GenericArg> as rustc_middle[9331575e900b72f8]::ty::fold::TypeFoldable>::try_fold_with::<rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer>
  31:        0x10abe2fe8 - <rustc_middle[9331575e900b72f8]::ty::PredicateKind as rustc_middle[9331575e900b72f8]::ty::fold::TypeFoldable>::try_fold_with::<rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer>
  32:        0x10abf0724 - <rustc_middle[9331575e900b72f8]::ty::Predicate as rustc_middle[9331575e900b72f8]::ty::fold::TypeSuperFoldable>::super_fold_with::<rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer>
  33:        0x10ac5a1f4 - <rustc_trait_selection[b4f82a16548071ea]::traits::project::AssocTypeNormalizer>::fold::<rustc_middle[9331575e900b72f8]::ty::Predicate>
  34:        0x10ac5e55c - rustc_trait_selection[b4f82a16548071ea]::traits::project::normalize::<rustc_middle[9331575e900b72f8]::ty::Predicate>
  35:        0x10abb0a68 - <rustc_infer[45d40a8feb40e3fa]::infer::InferCtxtBuilder>::enter::<core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>, rustc_typeck[67235f765db4edde]::check::compare_method::check_type_bounds::{closure#2}>
  36:        0x10ac2b1f4 - rustc_typeck[67235f765db4edde]::check::compare_method::check_type_bounds
  37:        0x10ac20290 - rustc_typeck[67235f765db4edde]::check::compare_method::compare_ty_impl
  38:        0x10aac793c - rustc_typeck[67235f765db4edde]::check::check::check_item_type
  39:        0x10aacaca0 - rustc_typeck[67235f765db4edde]::check::check::check_mod_item_types
  40:        0x10b386704 - rustc_query_system[e247dafbc3a4ff98]::query::plumbing::try_execute_query::<rustc_query_impl[90b84e005b379a12]::plumbing::QueryCtxt, rustc_query_system[e247dafbc3a4ff98]::query::caches::DefaultCache<rustc_span[b8e5c2445f520dcd]::def_id::LocalDefId, ()>>
  41:        0x10b42c6a0 - rustc_query_system[e247dafbc3a4ff98]::query::plumbing::get_query::<rustc_query_impl[90b84e005b379a12]::queries::check_mod_item_types, rustc_query_impl[90b84e005b379a12]::plumbing::QueryCtxt>
  42:        0x10abdb8c8 - <rustc_session[24b5e5485ce21942]::session::Session>::time::<(), rustc_typeck[67235f765db4edde]::check_crate::{closure#6}>
  43:        0x10ab8dfc4 - rustc_typeck[67235f765db4edde]::check_crate
  44:        0x108656888 - rustc_interface[5cb818030c646550]::passes::analysis
  45:        0x10b3b96f8 - rustc_query_system[e247dafbc3a4ff98]::query::plumbing::try_execute_query::<rustc_query_impl[90b84e005b379a12]::plumbing::QueryCtxt, rustc_query_system[e247dafbc3a4ff98]::query::caches::DefaultCache<(), core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>>
  46:        0x10b457934 - rustc_query_system[e247dafbc3a4ff98]::query::plumbing::get_query::<rustc_query_impl[90b84e005b379a12]::queries::analysis, rustc_query_impl[90b84e005b379a12]::plumbing::QueryCtxt>
  47:        0x10858d7e8 - <rustc_interface[5cb818030c646550]::passes::QueryContext>::enter::<rustc_driver[f51bca0c71cdd034]::run_compiler::{closure#1}::{closure#2}::{closure#3}, core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>
  48:        0x1085717a4 - <rustc_interface[5cb818030c646550]::interface::Compiler>::enter::<rustc_driver[f51bca0c71cdd034]::run_compiler::{closure#1}::{closure#2}, core[c82fdd3b01836480]::result::Result<core[c82fdd3b01836480]::option::Option<rustc_interface[5cb818030c646550]::queries::Linker>, rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>
  49:        0x1085bd41c - rustc_span[b8e5c2445f520dcd]::with_source_map::<core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>, rustc_interface[5cb818030c646550]::interface::create_compiler_and_run<core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>, rustc_driver[f51bca0c71cdd034]::run_compiler::{closure#1}>::{closure#1}>
  50:        0x108571f9c - <scoped_tls[c0f860eaab270d6e]::ScopedKey<rustc_span[b8e5c2445f520dcd]::SessionGlobals>>::set::<rustc_interface[5cb818030c646550]::interface::run_compiler<core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>, rustc_driver[f51bca0c71cdd034]::run_compiler::{closure#1}>::{closure#0}, core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>
  51:        0x10858fdd8 - std[ec5bd7e3f33f3027]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[5cb818030c646550]::util::run_in_thread_pool_with_globals<rustc_interface[5cb818030c646550]::interface::run_compiler<core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>, rustc_driver[f51bca0c71cdd034]::run_compiler::{closure#1}>::{closure#0}, core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>::{closure#0}, core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>
  52:        0x1085904a4 - <<std[ec5bd7e3f33f3027]::thread::Builder>::spawn_unchecked_<rustc_interface[5cb818030c646550]::util::run_in_thread_pool_with_globals<rustc_interface[5cb818030c646550]::interface::run_compiler<core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>, rustc_driver[f51bca0c71cdd034]::run_compiler::{closure#1}>::{closure#0}, core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>::{closure#0}, core[c82fdd3b01836480]::result::Result<(), rustc_errors[77f389aec993a0ef]::ErrorGuaranteed>>::{closure#1} as core[c82fdd3b01836480]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  53:        0x10112b58c - std::sys::unix::thread::Thread::new::thread_start::h46c89a4ac1d16d95
  54:        0x1a242226c - __pthread_deallocate

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 1.64.0-nightly (ddcbba036 2022-06-29) running on aarch64-apple-darwin

query stack during panic:
#0 [check_mod_item_types] checking item types in top-level module
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

Backtrace

$ RUST_BACKTRACE=1 rustc crash.rs 
error: internal compiler error: /rustc/ddcbba036aee08f0709f98a92a342a278eae5c05/compiler/rustc_middle/src/ty/fold.rs:756:29: unexpected bound ty in binder: BoundTy { var: 0, kind: Param("T") }

thread 'rustc' panicked at 'Box<dyn Any>', /rustc/ddcbba036aee08f0709f98a92a342a278eae5c05/compiler/rustc_errors/src/lib.rs:1391:9
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner>::bug::<&alloc::string::String>
   3: <rustc_errors::Handler>::bug::<&alloc::string::String>
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, ()>
   5: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>
   6: rustc_middle::util::bug::bug_fmt
   7: <rustc_middle::ty::context::TyCtxt>::replace_late_bound_regions_uncached::<rustc_middle::ty::PredicateKind, <rustc_middle::ty::context::TyCtxt>::replace_late_bound_regions<rustc_middle::ty::PredicateKind, <rustc_middle::ty::context::TyCtxt>::anonymize_late_bound_regions<rustc_middle::ty::PredicateKind>::{closure#0}>::{closure#0}>::{closure#0}
   8: <rustc_middle::ty::fold::BoundVarReplacer as rustc_middle::ty::fold::TypeFolder>::fold_ty
   9: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_middle::ty::fold::BoundVarReplacer>
  10: <rustc_middle::ty::PredicateKind as rustc_middle::ty::fold::TypeFoldable>::fold_with::<rustc_middle::ty::fold::BoundVarReplacer>
  11: <rustc_middle::ty::context::TyCtxt>::anonymize_late_bound_regions::<rustc_middle::ty::PredicateKind>
  12: <rustc_middle::ty::erase_regions::RegionEraserVisitor as rustc_middle::ty::fold::FallibleTypeFolder>::try_fold_binder::<rustc_middle::ty::PredicateKind>
  13: <&rustc_middle::ty::list::List<rustc_middle::ty::Predicate> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_middle::ty::erase_regions::RegionEraserVisitor>
  14: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  15: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_trait_selection::traits::project::AssocTypeNormalizer>
  16: <rustc_middle::ty::Ty as rustc_middle::ty::fold::TypeSuperFoldable>::super_fold_with::<rustc_trait_selection::traits::project::AssocTypeNormalizer>
  17: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  18: rustc_trait_selection::traits::project::opt_normalize_projection_type
  19: rustc_trait_selection::traits::project::normalize_projection_type
  20: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  21: <&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_trait_selection::traits::project::AssocTypeNormalizer>
  22: <rustc_middle::ty::PredicateKind as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_trait_selection::traits::project::AssocTypeNormalizer>
  23: <rustc_middle::ty::Predicate as rustc_middle::ty::fold::TypeSuperFoldable>::super_fold_with::<rustc_trait_selection::traits::project::AssocTypeNormalizer>
  24: <rustc_trait_selection::traits::project::AssocTypeNormalizer>::fold::<rustc_middle::ty::Predicate>
  25: rustc_trait_selection::traits::project::normalize::<rustc_middle::ty::Predicate>
  26: <rustc_infer::infer::InferCtxtBuilder>::enter::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_typeck::check::compare_method::check_type_bounds::{closure#2}>
  27: rustc_typeck::check::compare_method::check_type_bounds
  28: rustc_typeck::check::compare_method::compare_ty_impl
  29: rustc_typeck::check::check::check_item_type
  30: rustc_typeck::check::check::check_mod_item_types
  31: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::LocalDefId, ()>>
  32: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::check_mod_item_types, rustc_query_impl::plumbing::QueryCtxt>
  33: <rustc_session::session::Session>::time::<(), rustc_typeck::check_crate::{closure#6}>
  34: rustc_typeck::check_crate
  35: rustc_interface::passes::analysis
  36: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), core::result::Result<(), rustc_errors::ErrorGuaranteed>>>
  37: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
  38: <rustc_interface::passes::QueryContext>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}::{closure#3}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
  39: <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>>
  40: 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}>
  41: <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.

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 1.64.0-nightly (ddcbba036 2022-06-29) running on aarch64-apple-darwin

query stack during panic:
#0 [check_mod_item_types] checking item types in top-level module
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

[Rageking8]

@rustbot label +requires-nightly

[voidc]

Minimized:

#![feature(generic_associated_types)]

trait Foo {
    type Assoc<T>;
}

impl Foo for () {
    type Assoc<T> = [T; 2*2];
}

Playground

[jackh726]

Caused by this change: https://github.com/rust-lang/rust/pull/97648/files#diff-79cd877dd2dfb44945f291684ef33b7d715853b694fcaa153e0d849712e9439cR770

[jackh726]

@lcnr ^

[jackh726]

The problem is we end up with for<T> <() as Foo>::Assoc<T> = [T; 2*2] in the param_env.

When folding consts, we eval them (here:

rust/compiler/rustc_trait_selection/src/traits/project.rs

Line 618 in 9a6fa4f

constant.eval(self.selcx.tcx(), self.param_env)

)

try_eval_inner calls erase_regions here:

rust/compiler/rustc_middle/src/ty/consts/kind.rs

Line 187 in 9a6fa4f

.erase_regions(param_env)

Part of erasing regions is anonymizing late-bound regions (here:

rust/compiler/rustc_middle/src/ty/erase_regions.rs

Line 51 in 9a6fa4f

let u = self.tcx.anonymize_late_bound_regions(t);

)

In anonymize_late_bound_regions (

rust/compiler/rustc_middle/src/ty/fold.rs

Line 920 in 9a6fa4f

pub fn anonymize_late_bound_regions<T>(self, sig: Binder<'tcx, T>) -> Binder<'tcx, T>

), we delegate to replace_late_bound_regions, which returns T instead of Binder<T>, which "makes sense" if we assume that there are no bound tys or consts.

It's a bit of a tug of war game between wanting to catch unexpected bound tys/consts, and "delegating" bound variable binding higher up. Probably the right thing to do for now would be to revert the bound variable handling to just not panic if we encounter bound tys or consts.

[lcnr]

Part of erasing regions is anonymizing late-bound regions here

I guess alternatively, just anonymize all bound variables at this point? don't think regions should be special, going to try that