ICE when using type parameter inside const parameter type

[varkor]

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

fn func<A, const F: fn(inner: A)>(outer: A) {
    F(outer);
}

produces:

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

error: internal compiler error: src/librustc_middle/ty/subst.rs:573: type parameter `A/#0` (A/0) out of range when substituting (root type=Some(fn(A))) substs=[]

thread 'rustc' panicked at 'Box<Any>', /rustc/e83f7563495dbe2629b0cbc738afb0808c4482e1/src/libstd/macros.rs:13:23
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:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1504
   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:218
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:481
  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_ty
  21: rustc_middle::ty::structural_impls::fold_list
  22: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
  23: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
  24: rustc_typeck::check::FnCtxt::instantiate_value_path
  25: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  26: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  27: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::check_call
  28: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  29: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  30: rustc_typeck::check::FnCtxt::check_stmt
  31: rustc_typeck::check::FnCtxt::check_block_with_expected
  32: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  33: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  34: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  35: rustc_typeck::check::check_fn
  36: rustc_infer::infer::InferCtxtBuilder::enter
  37: rustc_typeck::check::typeck_tables_of
  38: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck_tables_of>::compute
  39: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  40: rustc_query_system::query::plumbing::get_query
  41: rustc_query_system::query::plumbing::ensure_query
  42: rustc_typeck::check::typeck_item_bodies
  43: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck_item_bodies>::compute
  44: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  45: rustc_query_system::query::plumbing::get_query
  46: rustc_typeck::check_crate
  47: rustc_interface::passes::analysis
  48: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  49: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  50: rustc_query_system::query::plumbing::get_query
  51: rustc_middle::ty::context::tls::enter_global
  52: rustc_interface::interface::run_compiler_in_existing_thread_pool
  53: scoped_tls::ScopedKey<T>::set
  54: rustc_ast::attr::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 1.45.0-nightly (e83f75634 2020-04-26) running on x86_64-unknown-linux-gnu

note: compiler flags: -C bitcode-in-rlib=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

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

query stack during panic:
#0 [typeck_tables_of] type-checking `func`
#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; 1 warning emitted

It should not be possible to access the type parameter A in the type of the const parameter.

cc @eddyb

Thanks to @aedoq for the original report.

[Cataractar]

Hi,

I am new to rust compiler development and have decided to try to fix this issue to get started.

If I understand the const-generics RFC correctly, type parameters can not be used in a const generic parameter because it cannot be ensured that the actual types have the "structural match" property. However, checking visit_ty in structural_match.rs reveals that fn pointers (as well as raw pointers and empty arrays) always support structural matching, regardless of the types used within.

My question is: is there another reason why const generic parameters are not allowed to use type parameters? If so, I think an additional check has to be added to this match arm.

[varkor]

Actually, I think I may have interpreted. It looks like this example is intended to work to me. cc @eddyb just to clarify.

[JohnTitor]

Triage: the const_compare_raw_pointers feature was removed by #73398 and the ICE no longer occurs with the latest nightly.