Const generic ICE without feature specified #60263

Compiling playground v0.0.1 (/playground)
error[E0658]: const generics are unstable
  --> src/lib.rs:23:16
   |
23 | struct B<const I: u8>;
   |                ^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/44580
   = help: add #![feature(const_generics)] to the crate attributes to enable

warning: unused import: `std::marker::PhantomData`
  --> src/lib.rs:21:5
   |
21 | use std::marker::PhantomData;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default

thread 'rustc' panicked at 'not yet implemented', src/librustc/ty/relate.rs:707:17
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::begin_panic
   7: <rustc::ty::subst::Kind as rustc::ty::relate::Relate>::relate
   8: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
   9: <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  10: <core::result::Result<T,E> as rustc::ty::context::InternIteratorElement<T,R>>::intern_with
  11: <rustc::infer::combine::Generalizer as rustc::ty::relate::TypeRelation>::relate_item_substs
  12: rustc::ty::relate::super_relate_tys
  13: <rustc::infer::combine::Generalizer as rustc::ty::relate::TypeRelation>::tys
  14: rustc::infer::combine::CombineFields::instantiate
  15: <rustc::infer::sub::Sub as rustc::ty::relate::TypeRelation>::tys
  16: rustc::infer::InferCtxt::commit_if_ok
  17: rustc::infer::InferCtxt::commit_if_ok
  18: rustc_typeck::check::coercion::Coerce::coerce_unsized
  19: rustc::infer::InferCtxt::commit_if_ok
  20: rustc_typeck::check::coercion::Coerce::coerce
  21: rustc::infer::InferCtxt::commit_if_ok
  22: rustc_typeck::check::coercion::<impl rustc_typeck::check::FnCtxt>::try_coerce
  23: rustc_typeck::check::coercion::CoerceMany<E>::coerce_inner
  24: rustc_typeck::check::FnCtxt::check_block_with_expected
  25: rustc_typeck::check::FnCtxt::check_expr_kind
  26: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  27: rustc_typeck::check::FnCtxt::check_block_with_expected
  28: rustc_typeck::check::FnCtxt::check_expr_kind
  29: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  30: rustc_typeck::check::FnCtxt::check_return_expr
  31: rustc_typeck::check::check_fn
  32: rustc::ty::context::GlobalCtxt::enter_local
  33: rustc_typeck::check::typeck_tables_of
  34: rustc::ty::query::__query_compute::typeck_tables_of
  35: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  36: rustc::dep_graph::graph::DepGraph::with_task_impl
  37: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  38: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  39: rustc_typeck::check::typeck_item_bodies
  40: rustc::ty::query::__query_compute::typeck_item_bodies
  41: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_item_bodies>::compute
  42: rustc::dep_graph::graph::DepGraph::with_task_impl
  43: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  44: rustc::util::common::time
  45: rustc_typeck::check_crate
  46: rustc_interface::passes::analysis
  47: rustc::ty::query::__query_compute::analysis
  48: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  49: rustc::dep_graph::graph::DepGraph::with_task_impl
  50: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  51: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  52: rustc_interface::passes::create_global_ctxt::{{closure}}
  53: rustc_interface::passes::BoxedGlobalCtxt::enter
  54: rustc_interface::interface::run_compiler_in_existing_thread_pool
  55: std::thread::local::LocalKey<T>::with
  56: scoped_tls::ScopedKey<T>::set
  57: syntax::with_globals
query stack during panic:
#0 [typeck_tables_of] processing `B::<_>::bug`
#1 [typeck_item_bodies] type-checking all item bodies
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.

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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.36.0-nightly (e305df184 2019-04-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

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

error: Could not compile `playground`.

To learn more, run the command again with --verbose.