[const_generics] Passing unsafe function outside `impl` crashes

[TheAifam5]

Code

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=c5d3a3135d1f5f27de7dd26bfdae9f7a

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

struct Test(*const usize);

type PassArg = ();

unsafe extern "C" fn pass(args: PassArg) {
    println!("Hello, world!");
}

impl Test {
    pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "C" fn(Args)>(&self) {
        self.0 = Self::trampiline::<Args, IDX, FN> as _
    }
    
    unsafe extern "C" fn trampiline<
        Args: Sized,
        const IDX: usize,
        const FN: unsafe extern "C" fn(Args),
    >(
        args: Args,
    ) {
        FN(args)
    }
}


fn main() {
    let x = Test();
    x.call_me::<PassArg, 30, pass>()
}

Meta

rustc --version --verbose:

1.44.0-nightly 2020-04-20 20fc02f836f3035b86b5

Error output

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

Backtrace

thread 'rustc' panicked at 'Box<Any>', /rustc/20fc02f836f3035b86b56a7cedb97c5cd4ed9612/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:515
  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::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_coercable_to_type
  28: rustc_infer::infer::InferCtxtBuilder::enter
  29: rustc_typeck::check::typeck_tables_of
  30: 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
  31: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  32: rustc_query_system::query::plumbing::get_query
  33: rustc_query_system::query::plumbing::ensure_query
  34: rustc_typeck::check::typeck_item_bodies
  35: 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
  36: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  37: rustc_query_system::query::plumbing::get_query
  38: rustc_typeck::check_crate
  39: rustc_interface::passes::analysis
  40: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  41: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  42: rustc_query_system::query::plumbing::get_query
  43: rustc_middle::ty::context::tls::enter_global
  44: rustc_interface::interface::run_compiler_in_existing_thread_pool
  45: scoped_tls::ScopedKey<T>::set
  46: 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 1.44.0-nightly (20fc02f83 2020-04-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -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 `Test::call_me::{{constant}}#1`
#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

error: could not compile `playground`.

Possible related issue

#71382

[lcnr]

Depends on const generics in type dependent paths afaict.

See #68400 (comment) for more info.

EDIT: Also depends on const generics with generic types. Merging #71154 does not close this issue

[JohnTitor]

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

[lcnr]

Adding the following as a test might still be useful, even if it only errors with "using function pointers as const parameters is forbidden":

#![feature(const_generics)]

struct Test(*const usize);

type PassArg = ();

unsafe extern "C" fn pass(args: PassArg) {
    println!("Hello, world!");
}

impl Test {
    pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "C" fn(Args)>(&self) {
        self.0 = Self::trampiline::<Args, IDX, FN> as _
    }
    
    unsafe extern "C" fn trampiline<
        Args: Sized,
        const IDX: usize,
        const FN: unsafe extern "C" fn(Args),
    >(
        args: Args,
    ) {
        FN(args)
    }
}


fn main() {
    let x = Test();
    x.call_me::<PassArg, 30, pass>()
}