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Handle multiple applicable projection candidates

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This commit is contained in:
Matthew Jasper 2020-07-23 21:59:20 +01:00
parent bc08b791bc
commit cfee49593d
6 changed files with 82 additions and 37 deletions
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52
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -1163,11 +1163,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
///
/// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
/// a projection, look at the bounds of `T::Bar`, see if we can find a
/// `Baz` bound and it there is one it returns it.
/// `Baz` bound. We return indexes into the list returned by
/// `tcx.item_bounds` for any applicable bounds.
fn match_projection_obligation_against_definition_bounds(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Option<ty::PolyTraitRef<'tcx>> {
) -> smallvec::SmallVec<[usize; 2]> {
let poly_trait_predicate = self.infcx().resolve_vars_if_possible(&obligation.predicate);
let placeholder_trait_predicate =
self.infcx().replace_bound_vars_with_placeholders(&poly_trait_predicate);
@ -1192,25 +1193,33 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
};
let bounds = tcx.item_bounds(def_id).subst(tcx, substs);
let matching_bound = bounds.iter().find_map(|bound| {
if let ty::PredicateAtom::Trait(pred, _) = bound.skip_binders() {
let bound = ty::Binder::bind(pred.trait_ref);
if self.infcx.probe(|_| {
self.match_projection(obligation, bound, placeholder_trait_predicate.trait_ref)
let matching_bounds = bounds
.iter()
.enumerate()
.filter_map(|(idx, bound)| {
if let ty::PredicateAtom::Trait(pred, _) = bound.skip_binders() {
let bound = ty::Binder::bind(pred.trait_ref);
if self.infcx.probe(|_| {
self.match_projection(
obligation,
bound,
placeholder_trait_predicate.trait_ref,
)
.is_ok()
}) {
return Some(bound);
}) {
return Some(idx);
}
}
}
None
});
None
})
.collect();
debug!(
"match_projection_obligation_against_definition_bounds: \
matching_bound={:?}",
matching_bound
matching_bounds={:?}",
matching_bounds
);
matching_bound
matching_bounds
}
fn match_projection(
@ -1299,14 +1308,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// clause so don't go around looking for impls.
!is_global(cand)
}
ObjectCandidate | ProjectionCandidate => {
ObjectCandidate | ProjectionCandidate(_) => {
// Arbitrarily give param candidates priority
// over projection and object candidates.
!is_global(cand)
}
ParamCandidate(..) => false,
},
ObjectCandidate | ProjectionCandidate => match victim.candidate {
ObjectCandidate | ProjectionCandidate(_) => match victim.candidate {
AutoImplCandidate(..) => {
bug!(
"default implementations shouldn't be recorded \
@ -1323,10 +1332,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| BuiltinUnsizeCandidate
| BuiltinCandidate { .. }
| TraitAliasCandidate(..) => true,
ObjectCandidate | ProjectionCandidate => {
// Arbitrarily give param candidates priority
// over projection and object candidates.
true
ObjectCandidate | ProjectionCandidate(_) => {
// Shouldn't have both an object and projection candidate,
// nor multiple object candidates. Multiple projection
// candidates are ambiguous.
false
}
ParamCandidate(ref cand) => is_global(cand),
},