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Rollup merge of #117992 - compiler-errors:sound-but-not-complete, r=lcnr,aliemjay

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Don't require intercrate mode for negative coherence

Negative coherence needs to be *sound*, but does not need to be *complete*, since it's looking for the *existence* of a negative goal, not the non-existence of a positive goal.

This removes some trivial and annoying ambiguities when a negative impl has region constraints.

r? lcnr idk if this needs an fcp but if it does, pls kick it off
This commit is contained in:
Matthias Krüger 2023-11-20 20:56:42 +01:00 • committed by GitHub
commit 0270afee31
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10 changed files with 32 additions and 50 deletions
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11
compiler/rustc_infer/src/infer/at.rs
View file

@ -65,8 +65,15 @@ impl<'tcx> InferCtxt<'tcx> {
/// Forks the inference context, creating a new inference context with the same inference
/// variables in the same state. This can be used to "branch off" many tests from the same
/// common state. Used in coherence.
/// common state.
pub fn fork(&self) -> Self {
self.fork_with_intercrate(self.intercrate)
}
/// Forks the inference context, creating a new inference context with the same inference
/// variables in the same state, except possibly changing the intercrate mode. This can be
/// used to "branch off" many tests from the same common state. Used in negative coherence.
pub fn fork_with_intercrate(&self, intercrate: bool) -> Self {
Self {
tcx: self.tcx,
defining_use_anchor: self.defining_use_anchor,
@ -81,7 +88,7 @@ impl<'tcx> InferCtxt<'tcx> {
tainted_by_errors: self.tainted_by_errors.clone(),
err_count_on_creation: self.err_count_on_creation,
universe: self.universe.clone(),
intercrate: self.intercrate,
intercrate,
next_trait_solver: self.next_trait_solver,
}
}

22
compiler/rustc_trait_selection/src/traits/coherence.rs
View file

@ -397,6 +397,8 @@ fn impl_intersection_has_negative_obligation(
) -> bool {
debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
// N.B. We need to unify impl headers *with* intercrate mode, even if proving negative predicates
// do not need intercrate mode enabled.
let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
let root_universe = infcx.universe();
assert_eq!(root_universe, ty::UniverseIndex::ROOT);
@ -415,13 +417,6 @@ fn impl_intersection_has_negative_obligation(
return false;
};
plug_infer_with_placeholders(
infcx,
root_universe,
(impl1_header.impl_args, impl2_header.impl_args),
);
let param_env = infcx.resolve_vars_if_possible(param_env);
// 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.
@ -429,6 +424,13 @@ fn impl_intersection_has_negative_obligation(
drop(infcx.take_registered_region_obligations());
drop(infcx.take_and_reset_region_constraints());
plug_infer_with_placeholders(
infcx,
root_universe,
(impl1_header.impl_args, impl2_header.impl_args),
);
let param_env = infcx.resolve_vars_if_possible(param_env);
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))
}
@ -554,7 +556,11 @@ fn try_prove_negated_where_clause<'tcx>(
return false;
};
let ref infcx = root_infcx.fork();
// N.B. We don't need to use intercrate mode here because we're trying to prove
// the *existence* of a negative goal, not the non-existence of a positive goal.
// Without this, we over-eagerly register coherence ambiguity candidates when
// impl candidates do exist.
let ref infcx = root_infcx.fork_with_intercrate(false);
let ocx = ObligationCtxt::new(infcx);
ocx.register_obligation(Obligation::new(