ICE with const generics and chalk: "var types encountered in super_relate_consts"

[LukasKalbertodt]

Compiling the following code with rustc 1.40.0-nightly (7979016af 2019-10-20) with -Z chalk leads to an ICE. Without chalk, it works fine.

Possible related (because of similar error message) to #62579?

#![feature(const_generics)]

pub struct Foo<T, const N: usize>([T; N]);
impl<T, const N: usize> Foo<T, {N}> {}

fn main() {}

Output:

warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> const-bug.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default

error: internal compiler error: src/librustc/ty/relate.rs:567: var types encountered in super_relate_consts: Const { ty: _, val: Infer(Var(_#0c)) } Const { ty: usize, val: Param(N/#1) }

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:925:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.37/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.37/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:61
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1028
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1412
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:65
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:50
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:189
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:206
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:473
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_context_opt
  18: rustc::ty::context::tls::with_opt
  19: rustc::util::bug::opt_span_bug_fmt
  20: rustc::util::bug::bug_fmt
  21: rustc::ty::relate::super_relate_consts
  22: <rustc::ty::subst::GenericArg as rustc::ty::relate::Relate>::relate
  23: <core::result::Result<T,E> as rustc::ty::context::InternIteratorElement<T,R>>::intern_with
  24: rustc::ty::relate::super_relate_tys
  25: rustc::infer::combine::<impl rustc::infer::InferCtxt>::super_combine_tys
  26: <rustc::infer::nll_relate::TypeRelating<D> as rustc::ty::relate::TypeRelation>::tys
  27: <rustc::traits::DomainGoal as rustc::ty::relate::Relate>::relate
  28: rustc_traits::chalk_context::unify::unify
  29: rustc::infer::InferCtxt::probe
  30: rustc_traits::chalk_context::resolvent_ops::<impl chalk_engine::context::ResolventOps<rustc_traits::chalk_context::ChalkArenas,rustc_traits::chalk_context::ChalkArenas> for rustc_traits::chalk_context::ChalkInferenceContext>::resolvent_clause
  31: chalk_engine::logic::<impl chalk_engine::forest::Forest<C,CO>>::push_initial_strands_instantiated
  32: rustc::ty::context::GlobalCtxt::enter_local
  33: <rustc_traits::chalk_context::ChalkContext as chalk_engine::context::ContextOps<rustc_traits::chalk_context::ChalkArenas>>::instantiate_ucanonical_goal
  34: chalk_engine::logic::<impl chalk_engine::forest::Forest<C,CO>>::get_or_create_table_for_ucanonical_goal
  35: chalk_engine::logic::<impl chalk_engine::forest::Forest<C,CO>>::pursue_strand
  36: rustc::ty::context::GlobalCtxt::enter_local
  37: <rustc_traits::chalk_context::ChalkContext as chalk_engine::context::ContextOps<rustc_traits::chalk_context::ChalkArenas>>::instantiate_ex_clause
  38: chalk_engine::logic::<impl chalk_engine::forest::Forest<C,CO>>::ensure_answer_recursively
  39: chalk_engine::logic::<impl chalk_engine::forest::Forest<C,CO>>::pursue_strand
  40: rustc::ty::context::GlobalCtxt::enter_local
  41: <rustc_traits::chalk_context::ChalkContext as chalk_engine::context::ContextOps<rustc_traits::chalk_context::ChalkArenas>>::instantiate_ex_clause
  42: chalk_engine::logic::<impl chalk_engine::forest::Forest<C,CO>>::ensure_answer_recursively
  43: <chalk_engine::forest::ForestSolver<C,CO> as chalk_engine::context::AnswerStream<C>>::peek_answer
  44: <rustc_traits::chalk_context::ChalkContext as chalk_engine::context::AggregateOps<rustc_traits::chalk_context::ChalkArenas>>::make_solution
  45: rustc_traits::chalk_context::evaluate_goal
  46: rustc::ty::query::__query_compute::evaluate_goal
  47: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::evaluate_goal>::compute
  48: rustc::dep_graph::graph::DepGraph::with_task_impl
  49: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  50: <rustc::traits::chalk_fulfill::FulfillmentContext as rustc::traits::engine::TraitEngine>::select_where_possible
  51: <rustc::traits::chalk_fulfill::FulfillmentContext as rustc::traits::engine::TraitEngine>::select_all_or_error
  52: rustc_typeck::check::FnCtxt::select_all_obligations_or_error
  53: rustc::ty::context::GlobalCtxt::enter_local
  54: rustc_typeck::check::wfcheck::check_item_well_formed
  55: rustc::ty::query::__query_compute::check_item_well_formed
  56: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::check_item_well_formed>::compute
  57: rustc::dep_graph::graph::DepGraph::with_task_impl
  58: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  59: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  60: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:80
  61: rustc_data_structures::sync::par_for_each_in
  62: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:80
  63: rustc::hir::Crate::par_visit_all_item_likes
  64: rustc::util::common::time
  65: rustc_typeck::check_crate
  66: rustc_interface::passes::analysis
  67: rustc::ty::query::__query_compute::analysis
  68: rustc::dep_graph::graph::DepGraph::with_task_impl
  69: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  70: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  71: rustc_interface::passes::create_global_ctxt::{{closure}}
  72: rustc_interface::passes::BoxedGlobalCtxt::enter
  73: rustc_interface::interface::run_compiler_in_existing_thread_pool
  74: std::thread::local::LocalKey<T>::with
  75: scoped_tls::ScopedKey<T>::set
  76: syntax::with_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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.40.0-nightly (7979016af 2019-10-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z chalk

query stack during panic:
#0 [evaluate_goal] evaluating trait selection obligation `T: std::marker::Sized`
#1 [check_item_well_formed] processing `Foo<T, N>`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

Code responsible (already has a FIXME):

rust/src/librustc/ty/relate.rs

Lines 560 to 568 in 10f12fe

	// Currently, the values that can be unified are those that
	// implement both `PartialEq` and `Eq`, corresponding to
	// `structural_match` types.
	// FIXME(const_generics): check for `structural_match` synthetic attribute.
	let new_const_val = match (eagerly_eval(a), eagerly_eval(b)) {
	(ConstValue::Infer(_), _) | (_, ConstValue::Infer(_)) => {
	// The caller should handle these cases!
	bug!("var types encountered in super_relate_consts: {:?} {:?}", a, b)
	}

CC @varkor @Skinny121

[eddyb]

The problem is an inconsistency in the callers of this method.
The bug! also exists for type inference variables.
I assume chalk integration is missing something it does for types, for consts.

[BenLewis-Seequent]

The caller(rustc::infer::nll_relate::TypeRelating) methods for consts and types look totally different from each other. The ICE happens when it is trying to relate an inference variable to a generic parameter, the method for types does handle this situation but the consts method doesn't.

[varkor]

We shouldn't be calling super_relate_consts here:

rust/src/librustc/infer/nll_relate/mod.rs

Line 627 in 50ffa79

relate::super_relate_consts(self, a, b)

as we may still have inference variables around (as in the type case).