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use rustc_hir::def_id::DefId;
use rustc_infer::infer::{InferCtxt, LateBoundRegionConversionTime};
use rustc_infer::traits::util::elaborate_predicates_with_span;
use rustc_infer::traits::{Obligation, ObligationCause, TraitObligation};
use rustc_middle::ty;
use rustc_span::{Span, DUMMY_SP};
use crate::traits::ObligationCtxt;
pub enum Ambiguity {
DefId(DefId),
ParamEnv(Span),
}
pub fn recompute_applicable_impls<'tcx>(
infcx: &InferCtxt<'tcx>,
obligation: &TraitObligation<'tcx>,
) -> Vec<Ambiguity> {
let tcx = infcx.tcx;
let param_env = obligation.param_env;
let impl_may_apply = |impl_def_id| {
let ocx = ObligationCtxt::new_in_snapshot(infcx);
let placeholder_obligation =
infcx.replace_bound_vars_with_placeholders(obligation.predicate);
let obligation_trait_ref =
ocx.normalize(&ObligationCause::dummy(), param_env, placeholder_obligation.trait_ref);
let impl_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().subst(tcx, impl_substs);
let impl_trait_ref = ocx.normalize(&ObligationCause::dummy(), param_env, impl_trait_ref);
if let Err(_) =
ocx.eq(&ObligationCause::dummy(), param_env, obligation_trait_ref, impl_trait_ref)
{
return false;
}
let impl_predicates = tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs);
ocx.register_obligations(impl_predicates.predicates.iter().map(|&predicate| {
Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
}));
ocx.select_where_possible().is_empty()
};
let param_env_candidate_may_apply = |poly_trait_predicate: ty::PolyTraitPredicate<'tcx>| {
let ocx = ObligationCtxt::new_in_snapshot(infcx);
let placeholder_obligation =
infcx.replace_bound_vars_with_placeholders(obligation.predicate);
let obligation_trait_ref =
ocx.normalize(&ObligationCause::dummy(), param_env, placeholder_obligation.trait_ref);
let param_env_predicate = infcx.replace_bound_vars_with_fresh_vars(
DUMMY_SP,
LateBoundRegionConversionTime::HigherRankedType,
poly_trait_predicate,
);
let param_env_trait_ref =
ocx.normalize(&ObligationCause::dummy(), param_env, param_env_predicate.trait_ref);
if let Err(_) =
ocx.eq(&ObligationCause::dummy(), param_env, obligation_trait_ref, param_env_trait_ref)
{
return false;
}
ocx.select_where_possible().is_empty()
};
let mut ambiguities = Vec::new();
tcx.for_each_relevant_impl(
obligation.predicate.def_id(),
obligation.predicate.skip_binder().trait_ref.self_ty(),
|impl_def_id| {
if infcx.probe(|_| impl_may_apply(impl_def_id)) {
ambiguities.push(Ambiguity::DefId(impl_def_id))
}
},
);
let predicates =
tcx.predicates_of(obligation.cause.body_id.owner.to_def_id()).instantiate_identity(tcx);
for obligation in elaborate_predicates_with_span(tcx, predicates.into_iter()) {
let kind = obligation.predicate.kind();
if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = kind.skip_binder()
&& param_env_candidate_may_apply(kind.rebind(trait_pred))
{
if kind.rebind(trait_pred.trait_ref) == ty::TraitRef::identity(tcx, trait_pred.def_id()) {
ambiguities.push(Ambiguity::ParamEnv(tcx.def_span(trait_pred.def_id())))
} else {
ambiguities.push(Ambiguity::ParamEnv(obligation.cause.span))
}
}
}
ambiguities
}
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