ICE using unknown variable for array length in const generic function

[Michael-F-Bryan]

I found this while playing around with someone's const-generic array concatentation function on Reddit.

(playground)

#![allow(incomplete_features)]
#![feature(const_generics)]

fn concat<const N: usize, const M: usize>(
    a: &[i32; N],
    b: &[i32; M],
) -> [i32; N+M] {
    // i32 is safely zeroable.
    let mut res: [i32; NM] = unsafe { std::mem::zeroed() };  // <-- note NM, not N+M
    for i in 0..N {
        res[i] = a[i]
    }
    let mut j = N;
    for i in 0..M {
        res[j] = b[i];
        j += 1;
    }
    res
}

fn main() {
    println!("{:?}", concat(&[1, 2, 3], &[4, 5, 6]));
}

Backtrace:

 Compiling playground v0.0.1 (/playground)
error[E0425]: cannot find value `NM` in this scope
 --> src/main.rs:9:24
  |
4 | fn concat<const N: usize, const M: usize>(
  |                                 - similarly named const parameter `M` defined here
...
9 |     let mut res: [i32; NM] = unsafe { std::mem::zeroed() };
  |                        ^^ help: a const parameter with a similar name exists: `M`

error: internal compiler error: src/librustc/ty/subst.rs:650: const parameter `N/#0` (Const { ty: usize, val: Param(N/#0) }/0) out of range when substituting substs=[]

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:896:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:84
   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:1030
   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:188
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:205
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:468
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_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::span_bug_fmt
  21: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_const
  22: rustc::traits::codegen::<impl rustc::ty::context::TyCtxt>::subst_and_normalize_erasing_regions
  23: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  24: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  25: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  26: rustc_mir::const_eval::const_eval_raw_provider
  27: rustc::ty::query::__query_compute::const_eval_raw
  28: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_raw>::compute
  29: rustc::dep_graph::graph::DepGraph::with_task_impl
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  31: rustc_mir::const_eval::const_eval_provider
  32: rustc::ty::query::__query_compute::const_eval
  33: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute
  34: rustc::dep_graph::graph::DepGraph::with_task_impl
  35: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  36: rustc_mir::const_eval::const_eval_provider
  37: rustc::ty::query::__query_compute::const_eval
  38: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute
  39: rustc::dep_graph::graph::DepGraph::with_task_impl
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  41: rustc::ty::sty::Const::eval
  42: <rustc::traits::project::AssocTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_ty
  43: <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  44: rustc::ty::fold::TypeFoldable::fold_with
  45: rustc::traits::project::normalize
  46: rustc_typeck::check::FnCtxt::normalize_associated_types_in
  47: rustc::ty::context::GlobalCtxt::enter_local
  48: rustc_typeck::check::wfcheck::check_item_well_formed
  49: rustc::ty::query::__query_compute::check_item_well_formed
  50: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::check_item_well_formed>::compute
  51: rustc::dep_graph::graph::DepGraph::with_task_impl
  52: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  53: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  54: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:81
  55: rustc_data_structures::sync::par_for_each_in
  56: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:81
  57: rustc::hir::Crate::par_visit_all_item_likes
  58: rustc::util::common::time
  59: rustc_typeck::check_crate
  60: rustc_interface::passes::analysis
  61: rustc::ty::query::__query_compute::analysis
  62: rustc::dep_graph::graph::DepGraph::with_task_impl
  63: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  64: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  65: rustc_interface::passes::create_global_ctxt::{{closure}}
  66: rustc_interface::passes::BoxedGlobalCtxt::enter
  67: rustc_interface::interface::run_compiler_in_existing_thread_pool
  68: std::thread::local::LocalKey<T>::with
  69: scoped_tls::ScopedKey<T>::set
  70: 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 1.41.0-nightly (82cf3a448 2019-11-14) running on x86_64-unknown-linux-gnu

note: compiler flags: -C opt-level=3 -C codegen-units=1 --crate-type bin

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

query stack during panic:
#0 [const_eval_raw] const-evaluating `concat::{{constant}}#2`
#1 [const_eval] const-evaluating + checking `concat::{{constant}}#2`
#2 [const_eval] const-evaluating + checking `concat::{{constant}}#2`
#3 [check_item_well_formed] processing `concat`
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0425`.
error: could not compile `playground`.

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

[hellow554]

ICEs also on stable and beta

[hellow554]

probably a dup of #66205

it's not because of NM, but because of N+M in your return type. Reduced example:

#![feature(const_generics)]

fn concat<const N: usize>() -> [i32; N+1] {
    unimplemented!()
}

[Michael-F-Bryan]

Looking at your minified example I'd say it's a dupe.