ICE: `unexpected inference var`

[matthiaskrgr]

auto-reduced (treereduce-rust):

trait ConstChunksExactTrait<T> {
    fn const_chunks_exact<const N: usize>(&self) -> ConstChunksExact<'_, T, { N }>;
}

impl<T> ConstChunksExactTrait<T> for [T] {}

struct ConstChunksExact<'a, T: 'a, const N: usize> {}

impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, { rem }> {
    type Item = &'a [T; N];
}

fn main() {
    let slice = &[1i32, 2, 3, 4, 5, 6, 7, 7, 9, 1i32];

    let mut iter = [[1, 2, 3], [4, 5, 6], [7, 8, 9]].iter();

    for a in slice.const_chunks_exact::<3>() {
        assert_eq!(a, iter.next().unwrap());
    }
}

original:

trait ConstChunksExactTrait<T> {
    fn const_chunks_exact<const N: usize>(&self) -> ConstChunksExact<'_, T, {N}>;
}

impl <T> ConstChunksExactTrait<T> for [T] {
    
}

struct ConstChunksExact<'a, T: 'a, const N: usize> {
    
}

impl <'a, T: , const N: usize> Iterator for ConstChunksExact<'a, T, {rem}> {
    type Item = &'a [T; N];

    
}

fn main() {
    let slice = &[1i32, 2, 3, 4, 5, 6, 7, 7, 9, 1i32];

    let mut iter = [[1, 2, 3], [4, 5, 6], [7, 8 ,9]].iter();

    for a in slice.const_chunks_exact::<3>() {
        assert_eq!(a, iter.next().unwrap());
    }
}

Version information

rustc 1.79.0-nightly (d5db7fb53 2024-03-27)
binary: rustc
commit-hash: d5db7fb537c0352eec3855aa0331c271c48ac4f6
commit-date: 2024-03-27
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.2

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0425]: cannot find value `rem` in this scope
 --> /tmp/icemaker_global_tempdir.JGsE6bNlDQ7v/rustc_testrunner_tmpdir_reporting.whgKI9sRtmMz/mvce.rs:9:68
  |
9 | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, { rem }> {
  |                                                                    ^^^ not found in this scope
  |
help: you might be missing a const parameter
  |
9 | impl<'a, T, const N: usize, const rem: /* Type */> Iterator for ConstChunksExact<'a, T, { rem }> {
  |                           +++++++++++++++++++++++

error[E0046]: not all trait items implemented, missing: `const_chunks_exact`
 --> /tmp/icemaker_global_tempdir.JGsE6bNlDQ7v/rustc_testrunner_tmpdir_reporting.whgKI9sRtmMz/mvce.rs:5:1
  |
2 |     fn const_chunks_exact<const N: usize>(&self) -> ConstChunksExact<'_, T, { N }>;
  |     ------------------------------------------------------------------------------- `const_chunks_exact` from trait
...
5 | impl<T> ConstChunksExactTrait<T> for [T] {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `const_chunks_exact` in implementation

error[E0392]: lifetime parameter `'a` is never used
 --> /tmp/icemaker_global_tempdir.JGsE6bNlDQ7v/rustc_testrunner_tmpdir_reporting.whgKI9sRtmMz/mvce.rs:7:25
  |
7 | struct ConstChunksExact<'a, T: 'a, const N: usize> {}
  |                         ^^ unused lifetime parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

error[E0392]: type parameter `T` is never used
 --> /tmp/icemaker_global_tempdir.JGsE6bNlDQ7v/rustc_testrunner_tmpdir_reporting.whgKI9sRtmMz/mvce.rs:7:29
  |
7 | struct ConstChunksExact<'a, T: 'a, const N: usize> {}
  |                             ^ unused type parameter
  |
  = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`

error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
 --> /tmp/icemaker_global_tempdir.JGsE6bNlDQ7v/rustc_testrunner_tmpdir_reporting.whgKI9sRtmMz/mvce.rs:9:13
  |
9 | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, { rem }> {
  |             ^^^^^^^^^^^^^^ unconstrained const parameter
  |
  = note: expressions using a const parameter must map each value to a distinct output value
  = note: proving the result of expressions other than the parameter are unique is not supported

thread 'rustc' panicked at compiler/rustc_borrowck/src/type_check/relate_tys.rs:362:9:
unexpected inference var std::option::Option<&'?16 [i32; ?0c: usize]>
stack backtrace:
   0:     0x70ec8827b235 - std::backtrace_rs::backtrace::libunwind::trace::h35c05b60de86c082
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x70ec8827b235 - std::backtrace_rs::backtrace::trace_unsynchronized::h716cc93b6616466b
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x70ec8827b235 - std::sys_common::backtrace::_print_fmt::h2eafab78e5405086
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x70ec8827b235 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h782eb93aef6bbfdc
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x70ec882cc3fb - core::fmt::rt::Argument::fmt::h423c1825313972bd
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/core/src/fmt/rt.rs:142:9
   5:     0x70ec882cc3fb - core::fmt::write::h8bf0eb0713660507
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/core/src/fmt/mod.rs:1153:17
   6:     0x70ec8826fe4f - std::io::Write::write_fmt::h1869b88304121496
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/io/mod.rs:1843:15
   7:     0x70ec8827b00e - std::sys_common::backtrace::_print::h3f71a60e8eec22db
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x70ec8827b00e - std::sys_common::backtrace::print::hbc0f4a4435261b96
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x70ec8827dc89 - std::panicking::default_hook::{{closure}}::hcb90344a23466037
  10:     0x70ec8827d9f3 - std::panicking::default_hook::h4e7402b852825f26
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/panicking.rs:292:9
  11:     0x70ec84bf7eee - std[ee38f4c3e7db4cb4]::panicking::update_hook::<alloc[5f51a9fa993ddee8]::boxed::Box<rustc_driver_impl[b62da54691605f75]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x70ec8827e38c - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h41b4c40829d8b80c
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/alloc/src/boxed.rs:2032:9
  13:     0x70ec8827e38c - std::panicking::rust_panic_with_hook::h34319b28877d2524
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/panicking.rs:793:13
  14:     0x70ec8827e136 - std::panicking::begin_panic_handler::{{closure}}::h0eb0df2dd2d8dc83
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/panicking.rs:658:13
  15:     0x70ec8827b6f9 - std::sys_common::backtrace::__rust_end_short_backtrace::h78e6ce0174ef8f2f
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x70ec8827de67 - rust_begin_unwind
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/panicking.rs:646:5
  17:     0x70ec882c8896 - core::panicking::panic_fmt::hb9147ccf68681889
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/core/src/panicking.rs:72:14
  18:     0x70ec864d1c29 - <rustc_borrowck[87c1aa11a50066c]::type_check::relate_tys::NllTypeRelating as rustc_middle[edfcb02b6a244f2]::ty::relate::TypeRelation>::tys
  19:     0x70ec864d639b - <rustc_borrowck[87c1aa11a50066c]::type_check::TypeChecker>::typeck_mir
  20:     0x70ec8369047a - rustc_borrowck[87c1aa11a50066c]::type_check::type_check
  21:     0x70ec836ca0aa - rustc_borrowck[87c1aa11a50066c]::nll::compute_regions
  22:     0x70ec86fe5af2 - rustc_borrowck[87c1aa11a50066c]::do_mir_borrowck
  23:     0x70ec861def26 - rustc_borrowck[87c1aa11a50066c]::mir_borrowck
  24:     0x70ec861dea17 - rustc_query_impl[2d2f12ae863522a2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[2d2f12ae863522a2]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[edfcb02b6a244f2]::query::erase::Erased<[u8; 8usize]>>
  25:     0x70ec861e0971 - rustc_query_system[1181c72fefabd0ae]::query::plumbing::try_execute_query::<rustc_query_impl[2d2f12ae863522a2]::DynamicConfig<rustc_query_system[1181c72fefabd0ae]::query::caches::VecCache<rustc_span[b9a3f8dff5ad8c72]::def_id::LocalDefId, rustc_middle[edfcb02b6a244f2]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[2d2f12ae863522a2]::plumbing::QueryCtxt, false>
  26:     0x70ec861e048c - rustc_query_impl[2d2f12ae863522a2]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  27:     0x70ec8654b49d - rustc_interface[8a3ca12de370bc6d]::passes::analysis
  28:     0x70ec8654a659 - rustc_query_impl[2d2f12ae863522a2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[2d2f12ae863522a2]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[edfcb02b6a244f2]::query::erase::Erased<[u8; 1usize]>>
  29:     0x70ec86d94da5 - rustc_query_system[1181c72fefabd0ae]::query::plumbing::try_execute_query::<rustc_query_impl[2d2f12ae863522a2]::DynamicConfig<rustc_query_system[1181c72fefabd0ae]::query::caches::SingleCache<rustc_middle[edfcb02b6a244f2]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[2d2f12ae863522a2]::plumbing::QueryCtxt, false>
  30:     0x70ec86d94b09 - rustc_query_impl[2d2f12ae863522a2]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  31:     0x70ec86c87299 - rustc_interface[8a3ca12de370bc6d]::interface::run_compiler::<core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>, rustc_driver_impl[b62da54691605f75]::run_compiler::{closure#0}>::{closure#0}
  32:     0x70ec86d665e3 - std[ee38f4c3e7db4cb4]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[8a3ca12de370bc6d]::util::run_in_thread_with_globals<rustc_interface[8a3ca12de370bc6d]::util::run_in_thread_pool_with_globals<rustc_interface[8a3ca12de370bc6d]::interface::run_compiler<core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>, rustc_driver_impl[b62da54691605f75]::run_compiler::{closure#0}>::{closure#0}, core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>>::{closure#0}, core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>>
  33:     0x70ec86d66410 - <<std[ee38f4c3e7db4cb4]::thread::Builder>::spawn_unchecked_<rustc_interface[8a3ca12de370bc6d]::util::run_in_thread_with_globals<rustc_interface[8a3ca12de370bc6d]::util::run_in_thread_pool_with_globals<rustc_interface[8a3ca12de370bc6d]::interface::run_compiler<core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>, rustc_driver_impl[b62da54691605f75]::run_compiler::{closure#0}>::{closure#0}, core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>>::{closure#0}, core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[852c89a24d64efc7]::result::Result<(), rustc_span[b9a3f8dff5ad8c72]::ErrorGuaranteed>>::{closure#1} as core[852c89a24d64efc7]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  34:     0x70ec88287ca9 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hc7f4be48b61584a0
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/alloc/src/boxed.rs:2018:9
  35:     0x70ec88287ca9 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h98f2eb6153ee7231
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/alloc/src/boxed.rs:2018:9
  36:     0x70ec88287ca9 - std::sys::pal::unix::thread::Thread::new::thread_start::h5b05a8117b189654
                               at /rustc/d5db7fb537c0352eec3855aa0331c271c48ac4f6/library/std/src/sys/pal/unix/thread.rs:108:17
  37:     0x70ec81aa955a - <unknown>
  38:     0x70ec81b26a3c - <unknown>
  39:                0x0 - <unknown>

error: 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 1.79.0-nightly (d5db7fb53 2024-03-27) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_borrowck] borrow-checking `main`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 5 previous errors

Some errors have detailed explanations: E0046, E0207, E0392, E0425.
For more information about an error, try `rustc --explain E0046`.

[matthiaskrgr]

#121154

[compiler-errors]

This could be minimized further

@rustbot label: E-needs-mcve

[Qwanve]

Minimized a bit more

struct Foo<const N: usize>;

trait Bar {
    type Item;
    fn c() -> Self::Item;
}

impl<const N: usize> Bar for Foo<{ rem }> {
    type Item = Foo<N>;
}

fn main() {
    let _: Foo<0> = <Foo<0> as Bar>::c();
}

[BoxyUwU]

the same cause as #122638