Type with StructuralPartialEq impl with unsatisfied lifetime bounds can be used as const pattern

[theemathas]

I tried this code:

#![feature(structural_match)]

struct Thing;

trait IsStatic {}
impl<'a: 'static> IsStatic for &'a () {}

impl std::marker::StructuralPartialEq for Thing where for<'a> &'a (): IsStatic {}

impl PartialEq for Thing
where
    for<'a> &'a (): IsStatic,
{
    fn eq(&self, _: &Thing) -> bool {
        panic!()
    }
}

const A: Thing = Thing;

fn main() {
    let A = Thing;
}

In this code, the for<'a> &'a (): IsStatic bound is unsatisfied. Therefore, the Thing type implements neither StructuralPartialEq nor PartialEq. Therefore, this code should not compile. However, the code compiles without errors, which seems wrong.

There's this comment in the compiler which seems relevant:

rust/compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

Lines 503 to 512 in 2e667b0

	// This *could* accept a type that isn't actually `PartialEq`, because region bounds get
	// ignored. However that should be pretty much impossible since consts that do not depend on
	// generics can only mention the `'static` lifetime, and how would one have a type that's
	// `PartialEq` for some lifetime but *not* for `'static`? If this ever becomes a problem
	// we'll need to leave some sort of trace of this requirement in the MIR so that borrowck
	// can ensure that the type really implements `PartialEq`.
	// We also do *not* require `const PartialEq`, not even in `const fn`. This violates the model
	// that patterns can only do things that the code could also do without patterns, but it is
	// needed for backwards compatibility. The actual pattern matching compares primitive values,
	// `PartialEq::eq` never gets invoked, so there's no risk of us running non-const code.

See also #147714, which involves an incorrect StructuralPartialEq impl from derive(PartialEq).

Meta

Reproducible on the playground with version 1.94.0-nightly (2025-12-09 c61a3a44d1a5bee35914)

[lcnr]

makes sense. Think we should switch to a proper ObligationCtxt here and check region constraints 😁 should not be reachable on stable luckily :3