rustc panics on use of const_generics if #![feature(const_generics)] not enabled

[bradsolliday]

I tried compiling this code:

struct Matrix<const ROWS: usize, const COLS: usize> {
    elements: [f64; ROWS * COLS]
}

I expected to see this happen:

error[E0658]: const generics are unstable
 --> src/matrix.rs:2:21
  |
2 | struct Matrix<const ROWS: usize, const COLS: usize> {
  |                     ^^^^
  |
  = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
  = help: add `#![feature(const_generics)]` to the crate attributes to enable

error[E0658]: const generics are unstable
 --> src/matrix.rs:2:40
  |
2 | struct Matrix<const ROWS: usize, const COLS: usize> {
  |                                        ^^^^
  |
  = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
  = help: add `#![feature(const_generics)]` to the crate attributes to enable

Instead, rustc also panicked:

error[E0658]: const generics are unstable
 --> src/matrix.rs:2:21
  |
2 | struct Matrix<const ROWS: usize, const COLS: usize> {
  |                     ^^^^
  |
  = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
  = help: add `#![feature(const_generics)]` to the crate attributes to enable

error[E0658]: const generics are unstable
 --> src/matrix.rs:2:40
  |
2 | struct Matrix<const ROWS: usize, const COLS: usize> {
  |                                        ^^^^
  |
  = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
  = help: add `#![feature(const_generics)]` to the crate attributes to enable

error: internal compiler error: src/librustc_middle/ty/subst.rs:594:17: const parameter `ROWS/#0` (Const { ty: usize, val: Param(ROWS/#0) }/0) out of range when substituting substs=[]

thread 'rustc' panicked at 'Box<Any>', /rustc/346aec9b02f3c74f3fce97fd6bda24709d220e49/src/libstd/macros.rs:13:23
note: run with `RUST_BACKTRACE=1` environment variable to display a 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 1.46.0-nightly (346aec9b0 2020-07-11) 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

error: aborting due to 3 previous errors

Note: rustc doesn't panic if I add

#![feature(const_generics)]

to the top of the crate root.

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (346aec9b0 2020-07-11)
binary: rustc
commit-hash: 346aec9b02f3c74f3fce97fd6bda24709d220e49
commit-date: 2020-07-11
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

Backtrace

stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::span_bug_fmt
  20: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_const
  21: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::subst_and_normalize_erasing_regions
  22: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  23: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  24: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  25: rustc_mir::const_eval::eval_queries::const_eval_raw_provider
  26: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_raw>::compute
  27: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  28: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  29: rustc_data_structures::stack::ensure_sufficient_stack
  30: rustc_query_system::query::plumbing::get_query_impl
  31: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  32: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  33: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  34: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  35: rustc_data_structures::stack::ensure_sufficient_stack
  36: rustc_query_system::query::plumbing::get_query_impl
  37: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  38: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  39: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  40: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  41: rustc_data_structures::stack::ensure_sufficient_stack
  42: rustc_query_system::query::plumbing::get_query_impl
  43: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  44: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  45: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  46: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  47: rustc_data_structures::stack::ensure_sufficient_stack
  48: rustc_trait_selection::traits::project::normalize
  49: <rustc_infer::infer::InferCtxt as rustc_trait_selection::infer::InferCtxtExt>::partially_normalize_associated_types_in
  50: rustc_typeck::check::FnCtxt::normalize_associated_types_in
  51: rustc_typeck::check::wfcheck::<impl rustc_typeck::check::FnCtxt>::non_enum_variant::{{closure}}
  52: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  53: rustc_infer::infer::InferCtxtBuilder::enter
  54: rustc_typeck::check::wfcheck::check_item_well_formed
  55: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_item_well_formed>::compute
  56: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  57: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  58: rustc_data_structures::stack::ensure_sufficient_stack
  59: rustc_query_system::query::plumbing::get_query_impl
  60: rustc_query_system::query::plumbing::ensure_query_impl
  61: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::itemlikevisit::ParItemLikeVisitor>::visit_item
  62: rustc_data_structures::sync::par_for_each_in
  63: rustc_hir::hir::Crate::par_visit_all_item_likes
  64: rustc_session::session::Session::track_errors
  65: rustc_typeck::check_crate
  66: rustc_interface::passes::analysis
  67: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  68: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  69: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  70: rustc_query_system::query::plumbing::get_query_impl
  71: rustc_middle::ty::context::tls::enter_global
  72: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  73: rustc_span::with_source_map
  74: rustc_interface::interface::create_compiler_and_run
  75: scoped_tls::ScopedKey<T>::set
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 1.46.0-nightly (346aec9b0 2020-07-11) 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 [const_eval_raw] const-evaluating `matrix::Matrix::elements::{{constant}}#0`
#1 [const_eval_validated] const-evaluating + checking `matrix::Matrix::elements::{{constant}}#0`
#2 [const_eval_validated] const-evaluating + checking `matrix::Matrix::elements::{{constant}}#0`
#3 [check_item_well_formed] checking that `matrix::Matrix` is well-formed
#4 [analysis] running analysis passes on this crate
end of query stack

[lcnr]

Thanks for the bug report, closing as this is a duplicate of #69913 and can't really be fixed easily rn.