ICE when matching with an opaque constant pattern

[Nadrieril]

Code

playground

#[derive(PartialEq)]
struct Opaque(i32);

impl Eq for Opaque {}

const FOO: &&Opaque = &&Opaque(42);

fn main() {
    match FOO {
        FOO => {},
        Opaque(_) => {}
    }
}

This ICEs on stable and nightly. Interestingly, if the type derives Eq this doesn't fail on nightly anymore, but still on stable. The change surely has something to do with the recent change in expansion of constant patterns (#70743).
Note also that this doesn't ICE with 0 or 1 references.

From the error, this is due to an invariant being broken in thir/pattern/_match.rs. This is likely to be due do specialize_one_pattern returning the wrong number of fields after specialization. So it's the interaction of const_to_pat.rs constant expansion and _match.rs pattern specialization that does something weird. I'm currently baffled as to why this happens.

Error output

thread 'rustc' panicked at 'assertion failed: rows.iter().all(|r| r.len() == v.len())', compiler/rustc_mir_build/src/thir/pattern/_match.rs:2009:5

Backtrace

thread 'rustc' panicked at 'assertion failed: rows.iter().all(|r| r.len() == v.len())', compiler/rustc_mir_build/src/thir/pattern/_match.rs:2009:5
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_mir_build::thir::pattern::_match::is_useful
   2: rustc_mir_build::thir::pattern::_match::is_useful_specialized
   3: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
   4: rustc_mir_build::thir::pattern::_match::is_useful
   5: rustc_mir_build::thir::pattern::_match::is_useful_specialized
   6: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
   7: rustc_mir_build::thir::pattern::_match::is_useful
   8: rustc_mir_build::thir::pattern::_match::is_useful_specialized
   9: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
  10: rustc_mir_build::thir::pattern::_match::is_useful
  11: <rustc_mir_build::thir::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  12: <rustc_mir_build::thir::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  13: rustc_mir_build::thir::pattern::check_match::check_match
  14: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_match>::compute
  15: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  16: rustc_data_structures::stack::ensure_sufficient_stack
  17: rustc_query_system::query::plumbing::get_query_impl
  18: rustc_query_system::query::plumbing::ensure_query_impl
  19: rustc_session::utils::<impl rustc_session::session::Session>::time
  20: rustc_session::utils::<impl rustc_session::session::Session>::time
  21: rustc_interface::passes::analysis
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_interface::passes::QueryContext::enter
  27: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  28: rustc_span::with_source_map
  29: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Nadrieril]

@rustbot modify labels: +A-exhaustiveness-checking

[Nadrieril]

I'll self-assign while I investigate but I don't know if I can fix it alone yet.
@rustbot claim

[scottmcm]

This was an error back in 1.25,

error[E0658]: non-reference pattern used to match a reference (see issue #42640)
  --> <source>:11:9
   |
11 |         Opaque(_) => {}
   |         ^^^^^^^^^ help: consider using a reference: `&Opaque(_)`

But has been an ICE since as fast back as 1.26.

[Nadrieril]

Thanks for the investigation!