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Rollup merge of #117994 - compiler-errors:throw-away-regions-in-coherence, r=lcnr

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Ignore but do not assume region obligations from unifying headers in negative coherence

Partly addresses a FIXME that was added in #112875. Just as we can throw away the nested trait/projection obligations from unifying two impl headers, we can also just throw away the region obligations too.

I removed part of the FIXME that was incorrect, namely:
> Given that the only region constraints we get are involving inference regions in the root, it shouldn't matter, but still sus.

This is not true when unifying `fn(A)` and `for<'b> fn(&'b B)` which ends up with placeholder region outlives from non-root universes. I'm pretty sure this is okay, though it would be nice if we were to use them as assumptions. See the `explicit` revision of the test I committed, which still fails.

Fixes #117986

r? lcnr, feel free to reassign tho.
This commit is contained in:
Michael Goulet 2023-11-19 19:14:35 -08:00 • committed by GitHub
commit 40a781b179
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3 changed files with 52 additions and 9 deletions
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17
compiler/rustc_trait_selection/src/traits/coherence.rs
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@ -408,7 +408,7 @@ fn impl_intersection_has_negative_obligation(
// Equate the headers to find their intersection (the general type, with infer vars,
// that may apply both impls).
let Some(_equate_obligations) =
let Some(equate_obligations) =
equate_impl_headers(infcx, param_env, &impl1_header, &impl2_header)
else {
return false;
@ -416,6 +416,13 @@ fn impl_intersection_has_negative_obligation(
plug_infer_with_placeholders(infcx, universe, (impl1_header.impl_args, impl2_header.impl_args));
// FIXME(with_negative_coherence): the infcx has constraints from equating
// the impl headers. We should use these constraints as assumptions, not as
// requirements, when proving the negated where clauses below.
drop(equate_obligations);
drop(infcx.take_registered_region_obligations());
drop(infcx.take_and_reset_region_constraints());
util::elaborate(tcx, tcx.predicates_of(impl2_def_id).instantiate(tcx, impl2_header.impl_args))
.any(|(clause, _)| try_prove_negated_where_clause(infcx, clause, param_env))
}
@ -541,14 +548,6 @@ fn try_prove_negated_where_clause<'tcx>(
return false;
};
// FIXME(with_negative_coherence): the infcx has region contraints from equating
// the impl headers as requirements. Given that the only region constraints we
// get are involving inference regions in the root, it shouldn't matter, but
// still sus.
//
// We probably should just throw away the region obligations registered up until
// now, or ideally use them as assumptions when proving the region obligations
// that we get from proving the negative predicate below.
let ref infcx = root_infcx.fork();
let ocx = ObligationCtxt::new(infcx);