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Auto merge of #101679 - compiler-errors:rpitit-default-body, r=nikomatsakis

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Support default-body trait functions with return-position `impl Trait` in traits

Introduce a new `Trait` candidate kind for the `ImplTraitInTrait` projection candidate, which just projects an RPITIT down to its opaque type form.

This is a hack until we lower RPITITs to regular associated types, after which we will need to rework how these default bodies are type-checked, so comments are left in a few places for us to clean up later.

Fixes #101665
This commit is contained in:
bors 2022-10-12 21:03:47 +00:00
commit 0938e1680d
14 changed files with 166 additions and 129 deletions
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48
compiler/rustc_trait_selection/src/traits/project.rs
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@ -72,7 +72,15 @@ enum ProjectionCandidate<'tcx> {
/// From an "impl" (or a "pseudo-impl" returned by select)
Select(Selection<'tcx>),
ImplTraitInTrait(ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>),
ImplTraitInTrait(ImplTraitInTraitCandidate<'tcx>),
}
#[derive(PartialEq, Eq, Debug)]
enum ImplTraitInTraitCandidate<'tcx> {
// The `impl Trait` from a trait function's default body
Trait,
// A concrete type provided from a trait's `impl Trait` from an impl
Impl(ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>),
}
enum ProjectionCandidateSet<'tcx> {
@ -1319,6 +1327,19 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
let tcx = selcx.tcx();
if tcx.def_kind(obligation.predicate.item_def_id) == DefKind::ImplTraitPlaceholder {
let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.item_def_id);
// If we are trying to project an RPITIT with trait's default `Self` parameter,
// then we must be within a default trait body.
if obligation.predicate.self_ty()
== ty::InternalSubsts::identity_for_item(tcx, obligation.predicate.item_def_id)
.type_at(0)
&& tcx.associated_item(trait_fn_def_id).defaultness(tcx).has_value()
{
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
ImplTraitInTraitCandidate::Trait,
));
return;
}
let trait_def_id = tcx.parent(trait_fn_def_id);
let trait_substs =
obligation.predicate.substs.truncate_to(tcx, tcx.generics_of(trait_def_id));
@ -1330,7 +1351,9 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
let _ =
selcx.infcx().commit_if_ok(|_| match selcx.select(&obligation.with(trait_predicate)) {
Ok(Some(super::ImplSource::UserDefined(data))) => {
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(data));
candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
ImplTraitInTraitCandidate::Impl(data),
));
Ok(())
}
Ok(None) => {
@ -1792,9 +1815,18 @@ fn confirm_candidate<'cx, 'tcx>(
ProjectionCandidate::Select(impl_source) => {
confirm_select_candidate(selcx, obligation, impl_source)
}
ProjectionCandidate::ImplTraitInTrait(data) => {
ProjectionCandidate::ImplTraitInTrait(ImplTraitInTraitCandidate::Impl(data)) => {
confirm_impl_trait_in_trait_candidate(selcx, obligation, data)
}
// If we're projecting an RPITIT for a default trait body, that's just
// the same def-id, but as an opaque type (with regular RPIT semantics).
ProjectionCandidate::ImplTraitInTrait(ImplTraitInTraitCandidate::Trait) => Progress {
term: selcx
.tcx()
.mk_opaque(obligation.predicate.item_def_id, obligation.predicate.substs)
.into(),
obligations: vec![],
},
};
// When checking for cycle during evaluation, we compare predicates with
@ -2211,6 +2243,16 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
return Progress { term: tcx.ty_error().into(), obligations };
}
// Use the default `impl Trait` for the trait, e.g., for a default trait body
if leaf_def.item.container == ty::AssocItemContainer::TraitContainer {
return Progress {
term: tcx
.mk_opaque(obligation.predicate.item_def_id, obligation.predicate.substs)
.into(),
obligations,
};
}
let impl_fn_def_id = leaf_def.item.def_id;
let impl_fn_substs = obligation.predicate.substs.rebase_onto(tcx, trait_fn_def_id, data.substs);