Trait with a lifetime parameter, associated constant, and an associated type can trigger an ICE

[ffrejaa]

The code below will trigger an ICE on the latest nightly build
This is as small as i could make it while still triggering the error

Code

#![allow(incomplete_features)]
#![feature(const_generics)]

trait Bar<const N: usize> {}

trait Foo<'a> {
    const N: usize;
    type Baz: Bar<{ Self::N }>;
}

Meta

rustc --version --verbose:

rustc 1.53.0-nightly (42816d61e 2021-04-24)
binary: rustc
commit-hash: 42816d61ead7e46d462df997958ccfd514f8c21c
commit-date: 2021-04-24
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0

Error output

error: internal compiler error: compiler/rustc_infer/src/infer/region_constraints/mod.rs:546:17: cannot relate bound region: ReLateBound(DebruijnIndex(1), BoundRegion { var: 0, kind: BrAnon(0) }) <= ReErased

Backtrace

thread 'rustc' panicked at 'Box<Any>', /rustc/42816d61ead7e46d462df997958ccfd514f8c21c/library/std/src/panic.rs:59:5
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::span_bug
   3: rustc_errors::Handler::span_bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::span_bug_fmt
   7: rustc_infer::infer::region_constraints::RegionConstraintCollector::make_subregion
   8: rustc_infer::infer::region_constraints::RegionConstraintCollector::make_eqregion
   9: <rustc_infer::infer::equate::Equate as rustc_middle::ty::relate::TypeRelation>::regions
  10: <rustc_middle::ty::subst::GenericArg as rustc_middle::ty::relate::Relate>::relate
  11: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
  12: <core::result::Result<T,E> as rustc_middle::ty::context::InternIteratorElement<T,R>>::intern_with
  13: rustc_infer::infer::InferCtxt::commit_if_ok
  14: rustc_infer::infer::InferCtxt::commit_if_ok
  15: rustc_infer::infer::InferCtxt::probe
  16: rustc_trait_selection::traits::select::candidate_assembly::<impl rustc_trait_selection::traits::select::SelectionContext>::assemble_candidates
  17: rustc_trait_selection::traits::select::candidate_assembly::<impl rustc_trait_selection::traits::select::SelectionContext>::candidate_from_obligation_no_cache
  18: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  19: rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task
  20: rustc_trait_selection::traits::select::candidate_assembly::<impl rustc_trait_selection::traits::select::SelectionContext>::candidate_from_obligation
  21: rustc_trait_selection::traits::select::SelectionContext::select
  22: rustc_infer::infer::InferCtxtBuilder::enter
  23: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
  24: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::codegen_fulfill_obligation>::compute
  25: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  26: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  27: rustc_data_structures::stack::ensure_sufficient_stack
  28: rustc_query_system::query::plumbing::force_query_with_job
  29: rustc_query_system::query::plumbing::get_query_impl
  30: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::codegen_fulfill_obligation
  31: rustc_ty_utils::instance::inner_resolve_instance
  32: rustc_ty_utils::instance::resolve_instance
  33: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::resolve_instance>::compute
  34: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_query_system::query::plumbing::force_query_with_job
  38: rustc_query_system::query::plumbing::get_query_impl
  39: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::resolve_instance
  40: rustc_middle::ty::instance::Instance::resolve_opt_const_arg
  41: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  42: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  43: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_mir_const
  44: <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize
  45: rustc_infer::infer::InferCtxtBuilder::enter
  46: core::ops::function::FnOnce::call_once
  47: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::normalize_mir_const_after_erasing_regions>::compute
  48: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  49: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  50: rustc_data_structures::stack::ensure_sufficient_stack
  51: rustc_query_system::query::plumbing::force_query_with_job
  52: rustc_query_system::query::plumbing::get_query_impl
  53: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_mir_const_after_erasing_regions
  54: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  55: rustc_mir::interpret::eval_context::InterpCx<M>::push_stack_frame
  56: rustc_mir::const_eval::eval_queries::eval_to_allocation_raw_provider
  57: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::eval_to_allocation_raw>::compute
  58: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  59: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  60: rustc_data_structures::stack::ensure_sufficient_stack
  61: rustc_query_system::query::plumbing::force_query_with_job
  62: rustc_query_system::query::plumbing::get_query_impl
  63: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_allocation_raw
  64: rustc_mir::const_eval::eval_queries::eval_to_const_value_raw_provider
  65: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::eval_to_const_value_raw>::compute
  66: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  67: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  68: rustc_data_structures::stack::ensure_sufficient_stack
  69: rustc_query_system::query::plumbing::force_query_with_job
  70: rustc_query_system::query::plumbing::get_query_impl
  71: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_const_value_raw
  72: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  73: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  74: rustc_infer::infer::InferCtxt::const_eval_resolve
  75: rustc_trait_selection::traits::const_evaluatable::is_const_evaluatable
  76: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  77: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
  78: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  79: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_all_or_error
  80: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::select_all_obligations_or_error
  81: rustc_infer::infer::InferCtxtBuilder::enter
  82: rustc_typeck::check::wfcheck::check_associated_item
  83: rustc_typeck::check::wfcheck::check_trait_item
  84: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  85: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  86: rustc_query_system::query::plumbing::force_query_with_job
  87: rustc_query_system::query::plumbing::get_query_impl
  88: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_trait_item_well_formed
  89: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::intravisit::Visitor>::visit_trait_item
  90: rustc_data_structures::sync::par_for_each_in
  91: rustc_hir::hir::Crate::par_visit_all_item_likes
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 1.53.0-nightly (42816d61e 2021-04-24) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `Foo` fulfills its obligations
#1 [resolve_instance] resolving instance `<Self as Foo>::N`
#2 [normalize_mir_const_after_erasing_regions] normalizing `<Self as Foo>::N`
#3 [eval_to_allocation_raw] const-evaluating + checking `Foo::Baz::{constant#0}`
#4 [eval_to_const_value_raw] simplifying constant for the type system `Foo::Baz::{constant#0}`
#5 [check_trait_item_well_formed] checking that `Foo::Baz` is well-formed
#6 [analysis] running analysis passes on this crate
end of query stack

[hellow554]

Also occurs on stable! Regression from 1.50 to 1.51

@rustbot modify labels: -requires-nightly +regression-from-stable-to-stable

[hameerabbasi]

Bisected:

searched nightlies: from nightly-2020-01-01 to nightly-2021-04-29
regressed nightly: nightly-2021-01-18
searched commits: from 8a65184 to 4253153
regressed commit: 4253153

bisected with cargo-bisect-rustc v0.6.0

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

cargo bisect-rustc --test-dir=. --start=2020-01-01 --regress=ice

Regressed PR is #80679, cc @lcnr @jackh726.

[hameerabbasi]

@hellow554 Would you mind expanding on how to reproduce it on stable, because it seems feature gated to me. See #84659 (comment) and #84659 (comment)

[hellow554]

@hameerabbasi just because something is gated doesn't mean it can't crash on stable ;)
Use the above code and try to build it on the playground or compile it with a stable compiler, it will ICE.

[apiraino]

We briefly discussed this on Zulip.

@hellow554 I'd revert some labels here because we think this is a nightly-only issue due to the unstable const_generics feature. The fact that it compiles and reproduces on stable is kind of misleading because it's the incomplete_features that allows that. Makes sense?

@rustbot modify labels: +requires-nightly -regression-from-stable-to-stable +P-medium

[hellow554]

@apiraino sure enough ;)

but just to be clear, even without #![allow(incomplete_features)] this results in an ICE on stable

[jackh726]

Okay so this is a bit complicated. Because while this does has feature(const_generics), is still ICEs on stable. That said, not including the feature gate doesn't ICE.

Given that, I think the current labels are appropriate, except maybe requires-nightly. It is technically a stable to stable regression, but it requires a feature to ICE, and I'm not sure that's the spirit of what that label is intended to track.

[apiraino]

@jackh726 agreed on the labels, I've got the context partially wrong here.

@rustbot modify labels: -requires-nightly

[BoxyUwU]

With #![feature(const_evaluatable_checked)] this also doesn't ICE

[Alexendoo]

No longer ICEs since #88369 as far as I could tell with various combinations of the new feature gates