ICE using const_generics feature

[AuroransSolis]

hi its me again with another ice

Code

I encountered this error when working on a library for treating arrays/tuples a bit more like vectors by providing .push(val) and .pop() methods through trait impls. I implemented this on stable using a mcschnasty proc macro, but for nightly I tried spicing things up a bit and implementing it using the const_generics feature (and const_evaluatable_checked and array_map). In short, though, the code boils down to this (only OrdesDec included since I dunno if OrdesInc causes an ICE or not):

#![feature(const_generics, const_evaluatable_checked, array_map)]

pub struct ConstCheck<const CHECK: bool>;

pub trait True {}
impl True for ConstCheck<true> {}

pub trait OrdesDec {
    type Newlen;
    type Output;

    fn pop(self) -> (Self::Newlen, Self::Output);
}

impl<T, const N: usize> OrdesDec for [T; N]
where
    ConstCheck<{N > 1}>: True,
    [T; N - 1]: Sized,
{
    type Newlen = [T; N - 1];
    type Output = T;

    fn pop(self) -> (Self::Newlen, Self::Output) {
        let mut iter = IntoIter::new(self);
        let end = iter.next_back().unwrap();
        let new = [(); N - 1].map(move |()| iter.next().unwrap());
        (new, end)
    }
}

The issue occurs specifically when .pop() is used in doc tests or another crate and when it's called in a manner similar to this:

use ordes::OrdesDec;

fn main() {
    let foo = [0u8, 1, 2, 3, 4];
    let (foo, pop) = foo.pop();
}

The ICE also occurs when foo is defined like let foo = ['a', 'b', 'c', 'd', 'e']; etc. However, it works just fine in #[test] functions in the crate it's defined in.

Meta

rustc --version --verbose:

rustc 1.52.0-nightly (35dbef235 2021-03-02)
binary: rustc
commit-hash: 35dbef235048f9a2939dc20effe083ca483c37ff
commit-date: 2021-03-02
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1

Error output

error: constant expression depends on a generic parameter
  |
  = note: this may fail depending on what value the parameter takes

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:43:32: could not fully normalize `(<[T; N] as ordes::OrdesDec>::Newlen, <[T; N] as ordes::OrdesDec>::Output)`

thread 'rustc' panicked at 'Box<Any>', /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/panic.rs:59:5
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.52.0-nightly (35dbef235 2021-03-02) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C embed-bitcode=no --crate-type bin

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `(<[T; N] as ordes::OrdesDec>::Newlen, <[T; N] as ordes::OrdesDec>::Output)`
end of query stack
error: aborting due to 2 previous errors

Backtrace

Here, I even did `RUST_BACKTRACE=full cargo test` for this one:

error: constant expression depends on a generic parameter
  |
  = note: this may fail depending on what value the parameter takes

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:43:32: could not fully normalize `(<[T; N] as ordes::OrdesDec>::Newlen, <[T; N] as ordes::OrdesDec>::Output)`

thread 'rustc' panicked at 'Box<Any>', /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/panic.rs:59:5
stack backtrace:
   0:     0x7f4cd4c2eeb0 - std::backtrace_rs::backtrace::libunwind::trace::h9d49145f95eb5894
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5
   1:     0x7f4cd4c2eeb0 - std::backtrace_rs::backtrace::trace_unsynchronized::hab1d020365bb6864
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f4cd4c2eeb0 - std::sys_common::backtrace::_print_fmt::h7659588431e304bd
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f4cd4c2eeb0 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h09f4a9e3befae3c7
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x7f4cd4c9f5dc - core::fmt::write::hf3fdfde304b9a088
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/core/src/fmt/mod.rs:1092:17
   5:     0x7f4cd4c221f2 - std::io::Write::write_fmt::h1cb850689c7116f0
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/io/mod.rs:1567:15
   6:     0x7f4cd4c32d55 - std::sys_common::backtrace::_print::hdbccd5aa093ba544
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x7f4cd4c32d55 - std::sys_common::backtrace::print::hc639c4f320222558
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x7f4cd4c32d55 - std::panicking::default_hook::{{closure}}::hdb012dd7a485bb5d
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/panicking.rs:208:50
   9:     0x7f4cd4c32803 - std::panicking::default_hook::h75facbce77b6ba91
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/panicking.rs:225:9
  10:     0x7f4cd5425d4b - rustc_driver::report_ice::h01b4629c250edc77
  11:     0x7f4cd4c33656 - std::panicking::rust_panic_with_hook::hbcaa5de2cb5e22d5
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/panicking.rs:595:17
  12:     0x7f4cd663dede - std::panicking::begin_panic::{{closure}}::h63e7681549a10239
  13:     0x7f4cd663de39 - std::sys_common::backtrace::__rust_end_short_backtrace::h55cc8bd5fcc0db45
  14:     0x7f4cd663de82 - std::panicking::begin_panic::h86a3b23efd0bb448
  15:     0x7f4cd661f100 - std::panic::panic_any::hb1b2c5d1a3cfa2c4
  16:     0x7f4cd662244a - rustc_errors::HandlerInner::bug::hcfa29b039dbceeb3
  17:     0x7f4cd6621f23 - rustc_errors::Handler::bug::h42757b5aa01edd2a
  18:     0x7f4cd6565d1c - rustc_middle::ty::context::tls::with_opt::h2f706dbac3fd19ed
  19:     0x7f4cd7bb7370 - rustc_middle::util::bug::opt_span_bug_fmt::h877f93abd5e75e9d
  20:     0x7f4cd6565ec6 - rustc_middle::util::bug::bug_fmt::h8668d235609a7e98
  21:     0x7f4cd67e47f2 - rustc_infer::infer::InferCtxtBuilder::enter::h941ddd8cd42a23f3
  22:     0x7f4cd67e946a - rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions::h2f2bca8f8f195236
  23:     0x7f4cd6ab8387 - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl::h78ffdcf789152c46
  24:     0x7f4cd6a9a1b1 - rustc_data_structures::stack::ensure_sufficient_stack::ha5c2c6bf605d7ba3
  25:     0x7f4cd6a73d4e - rustc_query_system::query::plumbing::force_query_with_job::h2ef22bfd2f0dd140
  26:     0x7f4cd6a63b6c - rustc_query_system::query::plumbing::get_query_impl::hc0e55cbf7636eade
  27:     0x7f4cd6acd1c9 - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_generic_arg_after_erasing_regions::h6a1e8c2a1ab72f4a
  28:     0x7f4cd709cc52 - <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty::h997f94575c7076ec
  29:     0x7f4cd702fe2b - rustc_middle::ty::fold::TypeFoldable::fold_with::hd3808b12bd935bb9
  30:     0x7f4cd709bd1e - rustc_middle::ty::layout::<impl rustc_middle::ty::instance::Instance>::fn_sig_for_fn_abi::h05e54f8e6b014b8a
  31:     0x7f4cd67a7c12 - <rustc_target::abi::call::FnAbi<&rustc_middle::ty::TyS> as rustc_middle::ty::layout::FnAbiExt<C>>::of_instance::hb5a6d4852d31ff73
  32:     0x7f4cd679d63e - rustc_codegen_llvm::mono_item::<impl rustc_codegen_ssa::traits::declare::PreDefineMethods for rustc_codegen_llvm::context::CodegenCx>::predefine_fn::hdecae27e6d220717
  33:     0x7f4cd67a4a16 - rustc_codegen_llvm::base::compile_codegen_unit::module_codegen::h7eaf0dacc0c82f75
  34:     0x7f4cd71d968d - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task::hcef8469810a45dd1
  35:     0x7f4cd7213784 - rustc_codegen_llvm::base::compile_codegen_unit::hed73fb3f05d56057
  36:     0x7f4cd71e785f - <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate::h3fe294a8f0735589
  37:     0x7f4cd7177ac9 - rustc_interface::queries::Queries::ongoing_codegen::hea0e157f127bf881
  38:     0x7f4cd712b72e - rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter::h0f61eb416f22a566
  39:     0x7f4cd715c621 - rustc_span::with_source_map::h4a0dc647038ad76f
  40:     0x7f4cd712c6da - rustc_interface::interface::create_compiler_and_run::h4b294c33113a1bc7
  41:     0x7f4cd712a888 - scoped_tls::ScopedKey<T>::set::h3d50179f272e16d1
  42:     0x7f4cd712aee4 - std::sys_common::backtrace::__rust_begin_short_backtrace::h7cf87b5fc59a6baa
  43:     0x7f4cd714a0fa - core::ops::function::FnOnce::call_once{{vtable.shim}}::h8b483ac32d974368
  44:     0x7f4cd4c4383a - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hdc51fe7e73bc86bf
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/alloc/src/boxed.rs:1546:9
  45:     0x7f4cd4c4383a - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::he605738a76b56d9d
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/alloc/src/boxed.rs:1546:9
  46:     0x7f4cd4c4383a - std::sys::unix::thread::Thread::new::thread_start::he44b12fd83e74919
                               at /rustc/35dbef235048f9a2939dc20effe083ca483c37ff/library/std/src/sys/unix/thread.rs:71:17
  47:     0x7f4cd4b3df5e - start_thread
  48:     0x7f4cd4a631ef - clone
  49:                0x0 - <unknown>

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.52.0-nightly (35dbef235 2021-03-02) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C embed-bitcode=no --crate-type bin

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `(<[T; N] as ordes::OrdesDec>::Newlen, <[T; N] as ordes::OrdesDec>::Output)`
end of query stack
error: aborting due to 2 previous errors

Notes

I believe this may be somewhat related to #79018 or #82835, but I'm not certain, since the issue I'm having only occurs when OrdesDec::pop() is used outside of the ordes crate.

[Alexendoo]

No longer ICEs since #86993

[AuroransSolis]

I know it's not a proper test, but all tests in the crate mentioned in the opening message for this issue pass now.