ICE: Next coherence: called `Result::unwrap()` on an `Err` value: Sorts(ExpectedFound { expected: (), found: str })

[fmease]

Context

This was minimized from library/alloc. Relevant to #121848 (the stabilization of -Znext-solver=coherence).

This is related to the open issue #124702 and the closed issue #124422.
However, contrary to those, this issue represents a pass→ICE regression, not just an error→ICE one.
Furthermore, it only uses the feature min_specialization (and not the more general specialization)
as it solely relies on assoc fn specialization (and not assoc type specialization).

Reproducer

rustc allok.rs -Znext-solver=coherence where allok.rs contains:

// #![no_std] // optionally
#![crate_type = "lib"]
#![feature(min_specialization)]
#![allow(dead_code)]

trait Display {}

trait ToOwned {
    type Owned;
}

impl<T> ToOwned for T {
    type Owned = T;
}

struct Cow<B: ?Sized>(B);

impl<B: ?Sized> Display for Cow<B>
where
    B: ToOwned,
    B::Owned: Display,
{
}

impl Display for () {}

trait ToString {
    fn to_string();
}

impl<T: Display + ?Sized> ToString for T {
    default fn to_string() {}
}

impl ToString for Cow<str> {
    fn to_string() {}
}

impl ToOwned for str {
    type Owned = ();
}

Compiler Output

thread 'rustc' panicked at compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs:354:18:
called `Result::unwrap()` on an `Err` value: Sorts(ExpectedFound { expected: (), found: str })

Full compiler output (incl. backtrace)

thread 'rustc' panicked at compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs:354:18:
called `Result::unwrap()` on an `Err` value: Sorts(ExpectedFound { expected: (), found: str })
stack backtrace:
   0:     0x7f3966b8cb55 - std::backtrace_rs::backtrace::libunwind::trace::h31c27f5cd8890956
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x7f3966b8cb55 - std::backtrace_rs::backtrace::trace_unsynchronized::ha07b1f1ddca65445
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f3966b8cb55 - std::sys_common::backtrace::_print_fmt::h483b2f8f14ecd78e
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7f3966b8cb55 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h90ddf6aa5ba26796
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f3966bdbd6b - core::fmt::rt::Argument::fmt::h37457b6759783b1e
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/core/src/fmt/rt.rs:165:63
   5:     0x7f3966bdbd6b - core::fmt::write::h7a84d347dc46f1df
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/core/src/fmt/mod.rs:1157:21
   6:     0x7f3966b8189f - std::io::Write::write_fmt::h75548018278e1e2d
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/io/mod.rs:1835:15
   7:     0x7f3966b8c92e - std::sys_common::backtrace::_print::hfd2cbf6f93bd2979
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f3966b8c92e - std::sys_common::backtrace::print::h977548a342545762
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f3966b8f299 - std::panicking::default_hook::{{closure}}::hf83accd896299866
  10:     0x7f3966b8efdd - std::panicking::default_hook::h461a18bec2aea8c7
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/panicking.rs:298:9
  11:     0x7f3969cb06ec - std[54c5d3e34c069c08]::panicking::update_hook::<alloc[6626c39146c6add4]::boxed::Box<rustc_driver_impl[cb5cb60ec49302dc]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f3966b8f996 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h30d290371b8f08f7
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/alloc/src/boxed.rs:2036:9
  13:     0x7f3966b8f996 - std::panicking::rust_panic_with_hook::h811d7f93fb5b56a8
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/panicking.rs:799:13
  14:     0x7f3966b8f744 - std::panicking::begin_panic_handler::{{closure}}::h2c83d37cb400439f
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/panicking.rs:664:13
  15:     0x7f3966b8d019 - std::sys_common::backtrace::__rust_end_short_backtrace::hd5859b7327062f1c
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x7f3966b8f477 - rust_begin_unwind
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/panicking.rs:652:5
  17:     0x7f3966bd8333 - core::panicking::panic_fmt::he7d3af46f65769b3
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/core/src/panicking.rs:72:14
  18:     0x7f3966bd8976 - core::result::unwrap_failed::h4fa3196e36a2fbb4
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/core/src/result.rs:1654:5
  19:     0x7f396bc4b61b - <rustc_trait_selection[6bd5f21d8d80d48f]::solve::eval_ctxt::EvalCtxt>::unify_query_var_values
  20:     0x7f396bc3f741 - <rustc_trait_selection[6bd5f21d8d80d48f]::solve::inspect::analyse::InspectCandidate>::instantiate_nested_goals
  21:     0x7f396a7e975e - <rustc_trait_selection[6bd5f21d8d80d48f]::traits::coherence::AmbiguityCausesVisitor as rustc_trait_selection[6bd5f21d8d80d48f]::solve::inspect::analyse::ProofTreeVisitor>::visit_goal
  22:     0x7f396a7e97f3 - <rustc_trait_selection[6bd5f21d8d80d48f]::traits::coherence::AmbiguityCausesVisitor as rustc_trait_selection[6bd5f21d8d80d48f]::solve::inspect::analyse::ProofTreeVisitor>::visit_goal
  23:     0x7f396a7e97f3 - <rustc_trait_selection[6bd5f21d8d80d48f]::traits::coherence::AmbiguityCausesVisitor as rustc_trait_selection[6bd5f21d8d80d48f]::solve::inspect::analyse::ProofTreeVisitor>::visit_goal
  24:     0x7f396a7e97f3 - <rustc_trait_selection[6bd5f21d8d80d48f]::traits::coherence::AmbiguityCausesVisitor as rustc_trait_selection[6bd5f21d8d80d48f]::solve::inspect::analyse::ProofTreeVisitor>::visit_goal
  25:     0x7f396b57201a - rustc_trait_selection[6bd5f21d8d80d48f]::traits::coherence::overlap
  26:     0x7f396bc8a248 - <rustc_middle[152581898470156b]::traits::specialization_graph::Children as rustc_trait_selection[6bd5f21d8d80d48f]::traits::specialize::specialization_graph::ChildrenExt>::insert
  27:     0x7f396867a488 - <rustc_middle[152581898470156b]::traits::specialization_graph::Graph as rustc_trait_selection[6bd5f21d8d80d48f]::traits::specialize::specialization_graph::GraphExt>::insert
  28:     0x7f396b08447c - rustc_trait_selection[6bd5f21d8d80d48f]::traits::specialize::specialization_graph_provider
  29:     0x7f396b083fdf - rustc_query_impl[b4be295015ca8fb2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b4be295015ca8fb2]::query_impl::specialization_graph_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[152581898470156b]::query::erase::Erased<[u8; 8usize]>>
  30:     0x7f396b0869d7 - rustc_query_system[e6c56b7dd6267367]::query::plumbing::try_execute_query::<rustc_query_impl[b4be295015ca8fb2]::DynamicConfig<rustc_query_system[e6c56b7dd6267367]::query::caches::DefIdCache<rustc_middle[152581898470156b]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[b4be295015ca8fb2]::plumbing::QueryCtxt, false>
  31:     0x7f396b7580f2 - rustc_query_impl[b4be295015ca8fb2]::query_impl::specialization_graph_of::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7f396b757a53 - <rustc_middle[152581898470156b]::ty::trait_def::TraitDef>::ancestors
  33:     0x7f396b61eff9 - rustc_hir_analysis[5c82ccef5ad3385e]::check::wfcheck::check_well_formed
  34:     0x7f396b61c26d - rustc_query_impl[b4be295015ca8fb2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b4be295015ca8fb2]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[152581898470156b]::query::erase::Erased<[u8; 1usize]>>
  35:     0x7f396b61ba00 - rustc_query_system[e6c56b7dd6267367]::query::plumbing::try_execute_query::<rustc_query_impl[b4be295015ca8fb2]::DynamicConfig<rustc_query_system[e6c56b7dd6267367]::query::caches::VecCache<rustc_hir[b66080818c936735]::hir_id::OwnerId, rustc_middle[152581898470156b]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[b4be295015ca8fb2]::plumbing::QueryCtxt, false>
  36:     0x7f396b61b77f - rustc_query_impl[b4be295015ca8fb2]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  37:     0x7f396b6196c1 - rustc_hir_analysis[5c82ccef5ad3385e]::check::wfcheck::check_mod_type_wf
  38:     0x7f396b619507 - rustc_query_impl[b4be295015ca8fb2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b4be295015ca8fb2]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[152581898470156b]::query::erase::Erased<[u8; 1usize]>>
  39:     0x7f396bdc0786 - rustc_query_system[e6c56b7dd6267367]::query::plumbing::try_execute_query::<rustc_query_impl[b4be295015ca8fb2]::DynamicConfig<rustc_query_system[e6c56b7dd6267367]::query::caches::DefaultCache<rustc_span[431df025478c194f]::def_id::LocalModDefId, rustc_middle[152581898470156b]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[b4be295015ca8fb2]::plumbing::QueryCtxt, false>
  40:     0x7f396bdc053f - rustc_query_impl[b4be295015ca8fb2]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7f396b3071dc - rustc_hir_analysis[5c82ccef5ad3385e]::check_crate
  42:     0x7f396b2fdc87 - rustc_interface[6cfcedd897ebfba3]::passes::analysis
  43:     0x7f396b2fd7c7 - rustc_query_impl[b4be295015ca8fb2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b4be295015ca8fb2]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[152581898470156b]::query::erase::Erased<[u8; 1usize]>>
  44:     0x7f396bdbb025 - rustc_query_system[e6c56b7dd6267367]::query::plumbing::try_execute_query::<rustc_query_impl[b4be295015ca8fb2]::DynamicConfig<rustc_query_system[e6c56b7dd6267367]::query::caches::SingleCache<rustc_middle[152581898470156b]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[b4be295015ca8fb2]::plumbing::QueryCtxt, false>
  45:     0x7f396bdbad89 - rustc_query_impl[b4be295015ca8fb2]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  46:     0x7f396bc19bce - rustc_interface[6cfcedd897ebfba3]::interface::run_compiler::<core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>, rustc_driver_impl[cb5cb60ec49302dc]::run_compiler::{closure#0}>::{closure#1}
  47:     0x7f396bc06889 - std[54c5d3e34c069c08]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[6cfcedd897ebfba3]::util::run_in_thread_with_globals<rustc_interface[6cfcedd897ebfba3]::util::run_in_thread_pool_with_globals<rustc_interface[6cfcedd897ebfba3]::interface::run_compiler<core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>, rustc_driver_impl[cb5cb60ec49302dc]::run_compiler::{closure#0}>::{closure#1}, core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>>::{closure#0}, core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>>
  48:     0x7f396bc06636 - <<std[54c5d3e34c069c08]::thread::Builder>::spawn_unchecked_<rustc_interface[6cfcedd897ebfba3]::util::run_in_thread_with_globals<rustc_interface[6cfcedd897ebfba3]::util::run_in_thread_pool_with_globals<rustc_interface[6cfcedd897ebfba3]::interface::run_compiler<core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>, rustc_driver_impl[cb5cb60ec49302dc]::run_compiler::{closure#0}>::{closure#1}, core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>>::{closure#0}, core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d365900a307aefd6]::result::Result<(), rustc_span[431df025478c194f]::ErrorGuaranteed>>::{closure#2} as core[d365900a307aefd6]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  49:     0x7f3966b997eb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h82bad4f6994c816b
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/alloc/src/boxed.rs:2022:9
  50:     0x7f3966b997eb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h8259f63a3422ebdf
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/alloc/src/boxed.rs:2022:9
  51:     0x7f3966b997eb - std::sys::pal::unix::thread::Thread::new::thread_start::hb7744739c9820032
                               at /rustc/e82c861d7e5ecd766cb0dab0bf622445dec999dc/library/std/src/sys/pal/unix/thread.rs:108:17
  52:     0x7f39669809eb - <unknown>
  53:     0x7f3966a047cc - <unknown>
  54:                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 make sure that you have updated to the latest nightly

note: please attach the file at `/home/fmease/programming/rust/rustc-ice-2024-05-06T06_53_45-318190.txt` to your bug report

note: compiler flags: -Z next-solver=coherence

query stack during panic:
#0 [specialization_graph_of] building specialization graph of trait `ToString`
#1 [check_well_formed] checking that `<impl at allok.rs:31:1: 31:41>` is well-formed
#2 [check_mod_type_wf] checking that types are well-formed in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

Meta

1. Reproduces with e27af29 (i.e., HEAD~1 wrt. to an earlier iteration of PR #121848)
2. Reproduces on nightly (9c9b568)
3. Reproduces on master (96f1da8)

[fmease]

I think this was fixed at some point, presumably by #124566.
However, it regressed again in #124418 (cc @compiler-errors) similar to #124702.

[fmease]

I presume #121848 will fail to build library/alloc again after a rebase onto the latest master.