ICE with associated_const_equality: const parameter types cannot be generic

[fmease]

While working on generic consts and testing their interaction with associated const equality, I stumbled across this preexisting compiler panic. Reproducer:

#![feature(associated_const_equality)]

fn user() -> impl Owner<i32, C = 0> {}

trait Owner<K> { const C: K; }
impl<K: ConstDefault> Owner<K> for () { const C: K = K::DEFAULT; }

trait ConstDefault { const DEFAULT: Self; }
impl ConstDefault for i32 { const DEFAULT: Self = 0; }

fn main() {}

Stderr:

thread 'rustc' panicked at 'const parameter types cannot be generic', compiler/rustc_hir_analysis/src/collect/type_of.rs:79:22

Although I'm not that familiar with the implementation details of associated const equality (sth. sth. assoc const projections), I'd very much like to have a go at fixing this bug since I'm already working in this area at the moment and I'd like to see it fixed before I open the generic consts PR.
@rustbot claim

Backtrace

stack backtrace:
   0:     0x7ff103367e31 - std::backtrace_rs::backtrace::libunwind::trace::h5d192c9c54b15f60
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7ff103367e31 - std::backtrace_rs::backtrace::trace_unsynchronized::hc7d3204e467fac03
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7ff103367e31 - std::sys_common::backtrace::_print_fmt::h2af83df47bc668bf
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7ff103367e31 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hf5a4fac06076e902
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7ff1033c95df - core::fmt::rt::Argument::fmt::h1f69f816cde54b71
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/core/src/fmt/rt.rs:138:9
   5:     0x7ff1033c95df - core::fmt::write::h555ebd423ad27fc9
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/core/src/fmt/mod.rs:1094:21
   6:     0x7ff10335a967 - std::io::Write::write_fmt::ha768284cce22c0d1
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/io/mod.rs:1714:15
   7:     0x7ff103367c45 - std::sys_common::backtrace::_print::hd74993f4555d3caf
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7ff103367c45 - std::sys_common::backtrace::print::h55223f11226300d1
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7ff10336a9f3 - std::panicking::default_hook::{{closure}}::h434fb03b0a8c72cd
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/panicking.rs:269:22
  10:     0x7ff10336a784 - std::panicking::default_hook::h26d4937b6653693a
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/panicking.rs:288:9
  11:     0x7ff106601f4b - rustc_driver_impl[374d8943f7df2d38]::install_ice_hook::{closure#0}
  12:     0x7ff10336b21e - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h3bf5cd9997a69c0a
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/alloc/src/boxed.rs:2007:9
  13:     0x7ff10336b21e - std::panicking::rust_panic_with_hook::hb4f1f210fa9d7d78
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/panicking.rs:709:13
  14:     0x7ff10336afa7 - std::panicking::begin_panic_handler::{{closure}}::hf85afca6633673cc
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/panicking.rs:597:13
  15:     0x7ff103368266 - std::sys_common::backtrace::__rust_end_short_backtrace::hac73181de0e5c7e1
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/sys_common/backtrace.rs:151:18
  16:     0x7ff10336acf2 - rust_begin_unwind
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/panicking.rs:593:5
  17:     0x7ff1033c5833 - core::panicking::panic_fmt::hc9e695bf13427368
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/core/src/panicking.rs:67:14
  18:     0x7ff1033c5593 - core::panicking::panic_display::h39d8ed8855fb9673
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/core/src/panicking.rs:150:5
  19:     0x7ff1033c5593 - core::panicking::panic_str::h51f44eeb1d108324
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/core/src/panicking.rs:134:5
  20:     0x7ff1033c5593 - core::option::expect_failed::h4e46f6e39b5295d7
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/core/src/option.rs:1952:5
  21:     0x7ff10574276a - rustc_hir_analysis[12ff737570cd8dab]::collect::type_of::type_of
  22:     0x7ff1047603c3 - rustc_query_impl[6d34ccc9082d3cbf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6d34ccc9082d3cbf]::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 8usize]>>
  23:     0x7ff104760380 - <rustc_query_impl[6d34ccc9082d3cbf]::query_impl::type_of::dynamic_query::{closure#2} as core[6907b49ead676d89]::ops::function::FnOnce<(rustc_middle[7578aedf3d8ae35a]::ty::context::TyCtxt, rustc_span[59b12f7e70a4d344]::def_id::DefId)>>::call_once
  24:     0x7ff104653075 - rustc_query_system[8ef922b68c9e8ac6]::query::plumbing::try_execute_query::<rustc_query_impl[6d34ccc9082d3cbf]::DynamicConfig<rustc_query_system[8ef922b68c9e8ac6]::query::caches::DefaultCache<rustc_span[59b12f7e70a4d344]::def_id::DefId, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[6d34ccc9082d3cbf]::plumbing::QueryCtxt, false>
  25:     0x7ff105f89963 - rustc_query_impl[6d34ccc9082d3cbf]::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7ff10546a504 - rustc_middle[7578aedf3d8ae35a]::query::plumbing::query_get_at::<rustc_query_system[8ef922b68c9e8ac6]::query::caches::DefaultCache<rustc_span[59b12f7e70a4d344]::def_id::DefId, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 8usize]>>>
  27:     0x7ff1054699ab - <rustc_middle[7578aedf3d8ae35a]::ty::consts::Const>::from_anon_const
  28:     0x7ff10596ba49 - <dyn rustc_hir_analysis[12ff737570cd8dab]::astconv::AstConv>::instantiate_poly_trait_ref_inner
  29:     0x7ff105966425 - <dyn rustc_hir_analysis[12ff737570cd8dab]::astconv::AstConv>::instantiate_poly_trait_ref
  30:     0x7ff105965f30 - <dyn rustc_hir_analysis[12ff737570cd8dab]::astconv::AstConv>::add_bounds::<core[6907b49ead676d89]::slice::iter::Iter<rustc_hir[75c69f6f15d5d6ca]::hir::GenericBound>>
  31:     0x7ff1057b1ac8 - rustc_hir_analysis[12ff737570cd8dab]::collect::item_bounds::opaque_type_bounds
  32:     0x7ff1057b05f2 - rustc_hir_analysis[12ff737570cd8dab]::collect::item_bounds::explicit_item_bounds
  33:     0x7ff1059c6c81 - rustc_query_impl[6d34ccc9082d3cbf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6d34ccc9082d3cbf]::query_impl::explicit_item_bounds::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 16usize]>>
  34:     0x7ff1059c6c55 - <rustc_query_impl[6d34ccc9082d3cbf]::query_impl::explicit_item_bounds::dynamic_query::{closure#2} as core[6907b49ead676d89]::ops::function::FnOnce<(rustc_middle[7578aedf3d8ae35a]::ty::context::TyCtxt, rustc_span[59b12f7e70a4d344]::def_id::DefId)>>::call_once
  35:     0x7ff1046baded - rustc_query_system[8ef922b68c9e8ac6]::query::plumbing::try_execute_query::<rustc_query_impl[6d34ccc9082d3cbf]::DynamicConfig<rustc_query_system[8ef922b68c9e8ac6]::query::caches::DefaultCache<rustc_span[59b12f7e70a4d344]::def_id::DefId, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[6d34ccc9082d3cbf]::plumbing::QueryCtxt, false>
  36:     0x7ff105f952a8 - rustc_query_impl[6d34ccc9082d3cbf]::query_impl::explicit_item_bounds::get_query_non_incr::__rust_end_short_backtrace
  37:     0x7ff104f4a5a5 - <rustc_hir_analysis[12ff737570cd8dab]::collect::CollectItemTypesVisitor as rustc_hir[75c69f6f15d5d6ca]::intravisit::Visitor>::visit_item
  38:     0x7ff104f45f33 - rustc_hir_analysis[12ff737570cd8dab]::collect::collect_mod_item_types
  39:     0x7ff1052be61c - rustc_query_impl[6d34ccc9082d3cbf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6d34ccc9082d3cbf]::query_impl::collect_mod_item_types::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 0usize]>>
  40:     0x7ff1052be5fe - <rustc_query_impl[6d34ccc9082d3cbf]::query_impl::collect_mod_item_types::dynamic_query::{closure#2} as core[6907b49ead676d89]::ops::function::FnOnce<(rustc_middle[7578aedf3d8ae35a]::ty::context::TyCtxt, rustc_span[59b12f7e70a4d344]::def_id::LocalDefId)>>::call_once
  41:     0x7ff104d1498f - rustc_query_system[8ef922b68c9e8ac6]::query::plumbing::try_execute_query::<rustc_query_impl[6d34ccc9082d3cbf]::DynamicConfig<rustc_query_system[8ef922b68c9e8ac6]::query::caches::VecCache<rustc_span[59b12f7e70a4d344]::def_id::LocalDefId, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[6d34ccc9082d3cbf]::plumbing::QueryCtxt, false>
  42:     0x7ff105fab3a4 - rustc_query_impl[6d34ccc9082d3cbf]::query_impl::collect_mod_item_types::get_query_non_incr::__rust_end_short_backtrace
  43:     0x7ff105aaf117 - <rustc_middle[7578aedf3d8ae35a]::hir::map::Map>::for_each_module::<rustc_hir_analysis[12ff737570cd8dab]::check_crate::{closure#0}::{closure#0}::{closure#0}>
  44:     0x7ff105aaecf2 - <rustc_session[6ee1fe53d65fb5ae]::session::Session>::track_errors::<rustc_hir_analysis[12ff737570cd8dab]::check_crate::{closure#0}, ()>
  45:     0x7ff105aad501 - rustc_hir_analysis[12ff737570cd8dab]::check_crate
  46:     0x7ff105aa613a - rustc_interface[25e9143dc71174ae]::passes::analysis
  47:     0x7ff105aedf3a - rustc_query_impl[6d34ccc9082d3cbf]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[6d34ccc9082d3cbf]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 1usize]>>
  48:     0x7ff105aedf29 - <rustc_query_impl[6d34ccc9082d3cbf]::query_impl::analysis::dynamic_query::{closure#2} as core[6907b49ead676d89]::ops::function::FnOnce<(rustc_middle[7578aedf3d8ae35a]::ty::context::TyCtxt, ())>>::call_once
  49:     0x7ff105c7bfa8 - rustc_query_system[8ef922b68c9e8ac6]::query::plumbing::try_execute_query::<rustc_query_impl[6d34ccc9082d3cbf]::DynamicConfig<rustc_query_system[8ef922b68c9e8ac6]::query::caches::SingleCache<rustc_middle[7578aedf3d8ae35a]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[6d34ccc9082d3cbf]::plumbing::QueryCtxt, false>
  50:     0x7ff105c7bd79 - rustc_query_impl[6d34ccc9082d3cbf]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  51:     0x7ff105c34465 - <rustc_middle[7578aedf3d8ae35a]::ty::context::GlobalCtxt>::enter::<rustc_driver_impl[374d8943f7df2d38]::run_compiler::{closure#1}::{closure#2}::{closure#4}, core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>
  52:     0x7ff1057b7c9b - <rustc_interface[25e9143dc71174ae]::interface::Compiler>::enter::<rustc_driver_impl[374d8943f7df2d38]::run_compiler::{closure#1}::{closure#2}, core[6907b49ead676d89]::result::Result<core[6907b49ead676d89]::option::Option<rustc_interface[25e9143dc71174ae]::queries::Linker>, rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>
  53:     0x7ff1057b3ae9 - rustc_span[59b12f7e70a4d344]::set_source_map::<core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>, rustc_interface[25e9143dc71174ae]::interface::run_compiler<core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>, rustc_driver_impl[374d8943f7df2d38]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  54:     0x7ff1057b3615 - <scoped_tls[6d925012d1f68df7]::ScopedKey<rustc_span[59b12f7e70a4d344]::SessionGlobals>>::set::<rustc_interface[25e9143dc71174ae]::interface::run_compiler<core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>, rustc_driver_impl[374d8943f7df2d38]::run_compiler::{closure#1}>::{closure#0}, core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>
  55:     0x7ff1057b2c36 - std[dcf3549276dd02fc]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[25e9143dc71174ae]::util::run_in_thread_pool_with_globals<rustc_interface[25e9143dc71174ae]::interface::run_compiler<core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>, rustc_driver_impl[374d8943f7df2d38]::run_compiler::{closure#1}>::{closure#0}, core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>
  56:     0x7ff105e7aff5 - <<std[dcf3549276dd02fc]::thread::Builder>::spawn_unchecked_<rustc_interface[25e9143dc71174ae]::util::run_in_thread_pool_with_globals<rustc_interface[25e9143dc71174ae]::interface::run_compiler<core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>, rustc_driver_impl[374d8943f7df2d38]::run_compiler::{closure#1}>::{closure#0}, core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[6907b49ead676d89]::result::Result<(), rustc_span[59b12f7e70a4d344]::ErrorGuaranteed>>::{closure#1} as core[6907b49ead676d89]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  57:     0x7ff103375695 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h7bd6d94c21941047
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/alloc/src/boxed.rs:1993:9
  58:     0x7ff103375695 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h16cbf2f6802b7a97
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/alloc/src/boxed.rs:1993:9
  59:     0x7ff103375695 - std::sys::unix::thread::Thread::new::thread_start::h654b71fdae4d62c8
                               at /rustc/f7ca9df69549470541fbf542f87a03eb9ed024b6/library/std/src/sys/unix/thread.rs:108:17
  60:     0x7ff1030e6bb5 - <unknown>
  61:     0x7ff103168d90 - <unknown>
  62:                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 1.72.0-nightly (f7ca9df69 2023-06-24) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [type_of] computing type of `user::{constant#0}`
#1 [explicit_item_bounds] finding item bounds for `user::{opaque#0}`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

@rustbot label C-bug T-compiler I-ICE F-associated_const_equality

[fmease]

Oops, duplicate of #108271. Somehow that didn't show up in search earlier 😅

[fmease]

@rustbot release-assignment