Repository: rust
Issue: ICE: `const variables should not be hashed: ?0c`
Labels: I-ICE,E-needs-test,T-compiler,A-incr-comp,C-bug

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### Code

```Rust
#![feature(impl_trait_in_assoc_type, const_precise_live_drops)]

trait Trait {
    type Opaque1;
}

impl<const B: Word> Trait for &'a () {
    type Opaque1 = impl Sized;

    fn constrain(self) -> (Self::Opaque1,) {}
}
```


### Meta
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If you're using the stable version of the compiler, you should also check if the
bug also exists in the beta or nightly versions.
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`rustc --version --verbose`:
```
rustc 1.78.0-nightly (a655e648a 2024-03-08)
binary: rustc
commit-hash: a655e648a9f94d74263108366b83e677af56e35d
commit-date: 2024-03-08
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0
```

### Error output
`rustc -Cincremental=. file.rs`
```
error[E0261]: use of undeclared lifetime name `'a`
 --> ice.rs:7:32
  |
7 | impl<const B: Word> Trait for &'a () {
  |      -                         ^^ undeclared lifetime
  |      |
  |      help: consider introducing lifetime `'a` here: `'a,`

error[E0407]: method `constrain` is not a member of trait `Trait`
  --> ice.rs:10:5
   |
10 |     fn constrain(self) -> (Self::Opaque1,) {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a member of trait `Trait`

error[E0412]: cannot find type `Word` in this scope
 --> ice.rs:7:15
  |
7 | impl<const B: Word> Trait for &'a () {
  |               ^^^^ not found in this scope

error[E0601]: `main` function not found in crate `ice`
  --> ice.rs:11:2
   |
11 | }
   |  ^ consider adding a `main` function to `ice.rs`

error[E0207]: the const parameter `B` is not constrained by the impl trait, self type, or predicates
 --> ice.rs:7:6
  |
7 | impl<const B: Word> Trait for &'a () {
  |      ^^^^^^^^^^^^^ 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

error[E0308]: mismatched types
  --> ice.rs:10:27
   |
10 |     fn constrain(self) -> (Self::Opaque1,) {}
   |        ---------          ^^^^^^^^^^^^^^^^ expected `(_,)`, found `()`
   |        |
   |        implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:  expected tuple `(_,)`
           found unit type `()`
```

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Include a backtrace in the code block by setting `RUST_BACKTRACE=1` in your
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<details><summary><strong>Backtrace</strong></summary>
<p>

```
thread 'rustc' panicked at /rustc/a655e648a9f94d74263108366b83e677af56e35d/compiler/rustc_type_ir/src/const_kind.rs:168:17:
const variables should not be hashed: ?0c
stack backtrace:
   0:     0x7f337cb8ca52 - std::backtrace_rs::backtrace::libunwind::trace::h96388202534bc8b4
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/../../backtrace/src/backtrace/libunwind.rs:104:5
   1:     0x7f337cb8ca52 - std::backtrace_rs::backtrace::trace_unsynchronized::hd24fda5ee1171687
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f337cb8ca52 - std::sys_common::backtrace::_print_fmt::h7fc6da6ac76efe1a
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7f337cb8ca52 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h20b3a1c18b9ce13e
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f337cbdda5c - core::fmt::rt::Argument::fmt::h715695219e3d5daa
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/core/src/fmt/rt.rs:142:9
   5:     0x7f337cbdda5c - core::fmt::write::hdcbaf4dd41b0e397
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/core/src/fmt/mod.rs:1153:17
   6:     0x7f337cb814cf - std::io::Write::write_fmt::ha86dec8b961be9fe
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/io/mod.rs:1846:15
   7:     0x7f337cb8c824 - std::sys_common::backtrace::_print::he2e628b8a344c918
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f337cb8c824 - std::sys_common::backtrace::print::h2d03e07df3cbc968
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f337cb8f52b - std::panicking::default_hook::{{closure}}::h70d1a774a5c00714
  10:     0x7f337cb8f287 - std::panicking::default_hook::h29bee0cc1d7f8ef9
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/panicking.rs:292:9
  11:     0x7f337fa57fdc - std[b22d021cf1654e3a]::panicking::update_hook::<alloc[24b8088a11102544]::boxed::Box<rustc_driver_impl[b8de2d3efc1fcf4f]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f337cb8fc90 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h40bee4bed6b10263
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/alloc/src/boxed.rs:2030:9
  13:     0x7f337cb8fc90 - std::panicking::rust_panic_with_hook::he3cf6200ea961b48
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/panicking.rs:783:13
  14:     0x7f337cb8f9d2 - std::panicking::begin_panic_handler::{{closure}}::h10c391556907277b
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/panicking.rs:657:13
  15:     0x7f337cb8cf26 - std::sys_common::backtrace::__rust_end_short_backtrace::hcaa45d1b2c613d86
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x7f337cb8f704 - rust_begin_unwind
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/panicking.rs:645:5
  17:     0x7f337cbd9f75 - core::panicking::panic_fmt::h99fd2851b9dbc03c
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/core/src/panicking.rs:72:14
  18:     0x7f33810afb6c - <rustc_middle[fdfb78edc2009106]::ty::consts::ConstData as rustc_data_structures[412ae8551ed91436]::stable_hasher::HashStable<rustc_query_system[689e62d1a5f987e2]::ich::hcx::StableHashingContext>>::hash_stable
  19:     0x7f33810af503 - <rustc_type_ir[9c7832cd21fb2102]::ty_info::WithCachedTypeInfo<rustc_middle[fdfb78edc2009106]::ty::consts::ConstData> as rustc_data_structures[412ae8551ed91436]::stable_hasher::HashStable<rustc_query_system[689e62d1a5f987e2]::ich::hcx::StableHashingContext>>::hash_stable
  20:     0x7f3380fb1412 - <&rustc_middle[fdfb78edc2009106]::ty::list::List<rustc_middle[fdfb78edc2009106]::ty::generic_args::GenericArg> as rustc_data_structures[412ae8551ed91436]::stable_hasher::HashStable<rustc_query_system[689e62d1a5f987e2]::ich::hcx::StableHashingContext>>::hash_stable
  21:     0x7f3380fb1cd3 - <rustc_type_ir[9c7832cd21fb2102]::ty_info::WithCachedTypeInfo<rustc_type_ir[9c7832cd21fb2102]::ty_kind::TyKind<rustc_middle[fdfb78edc2009106]::ty::context::TyCtxt>> as rustc_data_structures[412ae8551ed91436]::stable_hasher::HashStable<rustc_query_system[689e62d1a5f987e2]::ich::hcx::StableHashingContext>>::hash_stable
  22:     0x7f338174ba5b - rustc_query_system[689e62d1a5f987e2]::query::plumbing::try_execute_query::<rustc_query_impl[efcbf58886c52e8a]::DynamicConfig<rustc_query_system[689e62d1a5f987e2]::query::caches::DefaultCache<rustc_middle[fdfb78edc2009106]::ty::ParamEnvAnd<rustc_middle[fdfb78edc2009106]::ty::generic_args::GenericArg>, rustc_middle[fdfb78edc2009106]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[efcbf58886c52e8a]::plumbing::QueryCtxt, true>
  23:     0x7f338174b1dc - rustc_query_impl[efcbf58886c52e8a]::query_impl::try_normalize_generic_arg_after_erasing_regions::get_query_incr::__rust_end_short_backtrace
  24:     0x7f33812a68d4 - <rustc_middle[fdfb78edc2009106]::ty::normalize_erasing_regions::TryNormalizeAfterErasingRegionsFolder as rustc_type_ir[9c7832cd21fb2102]::fold::FallibleTypeFolder<rustc_middle[fdfb78edc2009106]::ty::context::TyCtxt>>::try_fold_ty
  25:     0x7f3381308328 - <rustc_middle[fdfb78edc2009106]::ty::Ty>::needs_drop
  26:     0x7f33800a0dd1 - <rustc_mir_transform[c65367db1eeb27d3]::remove_uninit_drops::RemoveUninitDrops as rustc_middle[fdfb78edc2009106]::mir::MirPass>::run_pass
  27:     0x7f3380e1341b - rustc_mir_transform[c65367db1eeb27d3]::pass_manager::run_passes_inner
  28:     0x7f33814e7f9f - rustc_mir_transform[c65367db1eeb27d3]::mir_drops_elaborated_and_const_checked
  29:     0x7f33814e72a1 - rustc_query_impl[efcbf58886c52e8a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[efcbf58886c52e8a]::query_impl::mir_drops_elaborated_and_const_checked::dynamic_query::{closure#2}::{closure#0}, rustc_middle[fdfb78edc2009106]::query::erase::Erased<[u8; 8usize]>>
  30:     0x7f338129897d - rustc_query_system[689e62d1a5f987e2]::query::plumbing::try_execute_query::<rustc_query_impl[efcbf58886c52e8a]::DynamicConfig<rustc_query_system[689e62d1a5f987e2]::query::caches::VecCache<rustc_span[708bf0c68405aa3d]::def_id::LocalDefId, rustc_middle[fdfb78edc2009106]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[efcbf58886c52e8a]::plumbing::QueryCtxt, true>
  31:     0x7f3380e427a7 - rustc_query_impl[efcbf58886c52e8a]::query_impl::mir_drops_elaborated_and_const_checked::get_query_incr::__rust_end_short_backtrace
  32:     0x7f33817206d6 - rustc_interface[401370fc7cb5d2e1]::passes::analysis
  33:     0x7f338171fda9 - rustc_query_impl[efcbf58886c52e8a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[efcbf58886c52e8a]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[fdfb78edc2009106]::query::erase::Erased<[u8; 1usize]>>
  34:     0x7f3381b78be2 - rustc_query_system[689e62d1a5f987e2]::query::plumbing::try_execute_query::<rustc_query_impl[efcbf58886c52e8a]::DynamicConfig<rustc_query_system[689e62d1a5f987e2]::query::caches::SingleCache<rustc_middle[fdfb78edc2009106]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[efcbf58886c52e8a]::plumbing::QueryCtxt, true>
  35:     0x7f3381b787dc - rustc_query_impl[efcbf58886c52e8a]::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  36:     0x7f3381b8746f - rustc_interface[401370fc7cb5d2e1]::interface::run_compiler::<core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>, rustc_driver_impl[b8de2d3efc1fcf4f]::run_compiler::{closure#0}>::{closure#0}
  37:     0x7f3381dbc585 - std[b22d021cf1654e3a]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[401370fc7cb5d2e1]::util::run_in_thread_with_globals<rustc_interface[401370fc7cb5d2e1]::util::run_in_thread_pool_with_globals<rustc_interface[401370fc7cb5d2e1]::interface::run_compiler<core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>, rustc_driver_impl[b8de2d3efc1fcf4f]::run_compiler::{closure#0}>::{closure#0}, core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>>::{closure#0}, core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>>
  38:     0x7f3381dbc3b2 - <<std[b22d021cf1654e3a]::thread::Builder>::spawn_unchecked_<rustc_interface[401370fc7cb5d2e1]::util::run_in_thread_with_globals<rustc_interface[401370fc7cb5d2e1]::util::run_in_thread_pool_with_globals<rustc_interface[401370fc7cb5d2e1]::interface::run_compiler<core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>, rustc_driver_impl[b8de2d3efc1fcf4f]::run_compiler::{closure#0}>::{closure#0}, core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>>::{closure#0}, core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[9bbbf43e76ab1d4d]::result::Result<(), rustc_span[708bf0c68405aa3d]::ErrorGuaranteed>>::{closure#1} as core[9bbbf43e76ab1d4d]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  39:     0x7f337cb99435 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hcb78242ba64783e0
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/alloc/src/boxed.rs:2016:9
  40:     0x7f337cb99435 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hbcd67c6e19068517
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/alloc/src/boxed.rs:2016:9
  41:     0x7f337cb99435 - std::sys::pal::unix::thread::Thread::new::thread_start::h8e93ba7fecc401fc
                               at /rustc/a655e648a9f94d74263108366b83e677af56e35d/library/std/src/sys/pal/unix/thread.rs:108:17
  42:     0x7f337c98155a - <unknown>
  43:     0x7f337c9fea3c - <unknown>
  44:                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: please attach the file at `/tmp/im3/rustc-ice-2024-03-08T23_03_48-805648.txt` to your bug report

note: compiler flags: -C incremental=[REDACTED]

query stack during panic:
#0 [try_normalize_generic_arg_after_erasing_regions] normalizing `<&() as Trait>::Opaque1`
#1 [mir_drops_elaborated_and_const_checked] elaborating drops for `<impl at ice.rs:7:1: 7:37>::constrain`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 6 previous errors

Some errors have detailed explanations: E0207, E0261, E0308, E0407, E0412, E0601.
For more information about an error, try `rustc --explain E0207`.
```

</p>
</details>