impl Trait in return position with lifetimes and const_evaluatable_checked gives ICE

[iliakonnov]

I did some experiments and found that reproducing ICE requires two things:

1. Incremental compilation enabled: -C incremental. I believe that's why it compiles well at playground.
2. Both const_generics and const_evaluatable_checked must be present. Otherwise, it just compiles well.

I've tried to reduce it by specifying concrete type or removing lifetimes or const generics, but had no luck. So that's why title is so long.

Code

Compile with cargo or use rustc ./ice.rs --crate-type lib -C incremental=something

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

trait SomeTrait {}    

struct SomeStruct<const S: bool>;     

impl<const S: bool> SomeTrait for SomeStruct<S> {}

fn func<'a>() -> impl 'a + SomeTrait
{
    SomeStruct::<{true}>
}

Meta

rustc --version --verbose:

rustc 1.54.0-nightly (6c2dd251b 2021-06-05)
binary: rustc
commit-hash: 6c2dd251bbff03c7a3092d43fb5b637eca0810e3
commit-date: 2021-06-05
host: x86_64-unknown-linux-gnu
release: 1.54.0-nightly
LLVM version: 12.0.1

Error output

error: internal compiler error: compiler/rustc_middle/src/ich/impls_ty.rs:94:17: StableHasher: unexpected region '_#0r

Backtrace

error: internal compiler error: compiler/rustc_middle/src/ich/impls_ty.rs:94:17: StableHasher: unexpected region '_#0r

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:1007:9
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::RegionKind>::hash_stable
   8: std::thread::local::LocalKey<T>::with
   9: <(T1,T2) as rustc_data_structures::stable_hasher::HashStable<CTX>>::hash_stable
  10: <T as rustc_query_system::dep_graph::dep_node::DepNodeParams<Ctxt>>::to_fingerprint
  11: rustc_query_system::dep_graph::dep_node::DepNode<K>::construct
  12: rustc_query_system::query::plumbing::get_query_impl
  13: rustc_query_system::query::plumbing::get_query
  14: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::try_unify_abstract_consts
  15: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  16: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
  17: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  18: rustc_infer::infer::canonical::query_response::<impl rustc_infer::infer::InferCtxt>::make_canonicalized_query_response
  19: <rustc_infer::infer::InferCtxtBuilder as rustc_trait_selection::infer::InferCtxtBuilderExt>::enter_canonical_trait_query
  20: rustc_traits::type_op::type_op_ascribe_user_type
  21: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::type_op_ascribe_user_type>::compute
  22: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  23: rustc_data_structures::stack::ensure_sufficient_stack
  24: rustc_query_system::query::plumbing::get_query_impl
  25: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::type_op_ascribe_user_type
  26: rustc_trait_selection::traits::query::type_op::ascribe_user_type::<impl rustc_trait_selection::traits::query::type_op::QueryTypeOp for rustc_middle::traits::query::type_op::AscribeUserType>::perform_query
  27: rustc_trait_selection::traits::query::type_op::QueryTypeOp::fully_perform_into
  28: <rustc_middle::ty::ParamEnvAnd<Q> as rustc_trait_selection::traits::query::type_op::TypeOp>::fully_perform
  29: rustc_mir::borrow_check::type_check::type_check
  30: rustc_mir::borrow_check::nll::compute_regions
  31: rustc_mir::borrow_check::do_mir_borrowck
  32: rustc_infer::infer::InferCtxtBuilder::enter
  33: rustc_mir::borrow_check::mir_borrowck
  34: core::ops::function::FnOnce::call_once
  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>::mir_borrowck
  40: rustc_typeck::collect::type_of::type_of
  41: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  42: rustc_data_structures::stack::ensure_sufficient_stack
  43: rustc_query_system::query::plumbing::force_query_with_job
  44: rustc_query_system::query::plumbing::get_query_impl
  45: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::type_of
  46: rustc_typeck::check::check::check_item_type
  47: rustc_middle::hir::map::Map::visit_item_likes_in_module
  48: rustc_typeck::check::check::check_mod_item_types
  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>::check_mod_item_types
  54: rustc_session::utils::<impl rustc_session::session::Session>::time
  55: rustc_typeck::check_crate
  56: rustc_interface::passes::analysis
  57: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  58: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  59: rustc_query_system::dep_graph::graph::DepGraph<K>::with_eval_always_task
  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>::analysis
  64: rustc_interface::passes::QueryContext::enter
  65: rustc_span::with_session_globals
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 1.54.0-nightly (6c2dd251b 2021-06-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C incremental --crate-type lib

query stack during panic:
#0 [type_op_ascribe_user_type] evaluating `type_op_ascribe_user_type` `Canonical { max_universe: U0, variables: [CanonicalVarInfo { kind: Region(U0) }, CanonicalVarInfo { kind: Region(U0) }], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing }, value: AscribeUserType { mir_ty: SomeStruct<{}>, def_id: DefId(0:5 ~ foo[cf51]::SomeStruct::{constructor#0}), user_substs: UserSubsts { substs: [Const { ty: (), val: Unevaluated(Unevaluated { def: WithOptConstParam { did: DefId(0:12 ~ foo[cf51]::func::{constant#0}), const_param_did: Some(DefId(0:6 ~ foo[cf51]::SomeStruct::S)) }, substs: [ReLateBound(DebruijnIndex(0), BoundRegion { var: 1, kind: BrAnon(1) })], promoted: None }) }], user_self_ty: None } } } }`
#1 [mir_borrowck] borrow-checking `func`
#2 [type_of] computing type of `func::{opaque#0}`
#3 [check_mod_item_types] checking item types in top-level module
#4 [analysis] running analysis passes on this crate
end of query stack

@rustbot label +F-const_generics +F-const_evaluatable_checked +requires-nightly +A-const-generics +A-incr-comp

[iliakonnov]

Oops, that's a duplicate of #77708. Closing this.