bjoernager/rust
bjoernager/rust
1
Fork 0
Code Activity

Auto merge of #86264 - crlf0710:trait_upcasting_part1, r=nikomatsakis

Browse source 
Trait upcasting coercion (part1)

This revives the first part of earlier PR #60900 .

It's not very clear to me which parts of that pr was design decisions, so i decide to cut it into pieces and land them incrementally. This allows more eyes on the details.

This is the first part, it adds feature gates, adds feature gates tests, and implemented the unsize conversion part.
(I hope i have dealt with the `ExistentialTraitRef` values correctly...)

The next part will be implementing the pointer casting.
This commit is contained in:
bors 2021-07-31 07:46:14 +00:00
commit 7069a8c2b7
20 changed files with 504 additions and 27 deletions
Download patch file Download diff file Expand all files Collapse all files

22
compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
View file

@ -693,22 +693,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
let may_apply = match (source.kind(), target.kind()) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
// Upcasts permit two things:
//
// 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
// 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
//
// Note that neither of these changes requires any
// change at runtime. Eventually this will be
// generalized.
//
// We always upcast when we can because of reason
// #2 (region bounds).
data_a.principal_def_id() == data_b.principal_def_id()
&& data_b
.auto_traits()
// All of a's auto traits need to be in b's auto traits.
.all(|b| data_a.auto_traits().any(|a| a == b))
// See `confirm_builtin_unsize_candidate` for more info.
let auto_traits_compatible = data_b
.auto_traits()
// All of a's auto traits need to be in b's auto traits.
.all(|b| data_a.auto_traits().any(|a| a == b));
auto_traits_compatible
}
// `T` -> `Trait`

54
compiler/rustc_trait_selection/src/traits/select/confirmation.rs
View file

@ -703,10 +703,56 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
match (source.kind(), target.kind()) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
// See `assemble_candidates_for_unsizing` for more info.
let iter = data_a
.principal()
.map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
// Upcast coercions permit several things:
//
// 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
// 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
// 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
//
// Note that neither of the first two of these changes requires any
// change at runtime. The third needs to change pointer metadata at runtime.
//
// We always perform upcasting coercions when we can because of reason
// #2 (region bounds).
// We already checked the compatiblity of auto traits within `assemble_candidates_for_unsizing`.
let principal_a = data_a.principal();
let principal_def_id_b = data_b.principal_def_id();
let existential_predicate = if let Some(principal_a) = principal_a {
let source_trait_ref = principal_a.with_self_ty(tcx, source);
let target_trait_did = principal_def_id_b.ok_or_else(|| Unimplemented)?;
let upcast_idx = util::supertraits(tcx, source_trait_ref)
.position(|upcast_trait_ref| upcast_trait_ref.def_id() == target_trait_did)
.ok_or_else(|| Unimplemented)?;
// FIXME(crlf0710): This is less than ideal, for example,
// if the trait is defined as `trait Foo: Bar<u32> + Bar<i32>`,
// the coercion from Box<Foo> to Box<dyn Bar<_>> is actually ambiguous.
// We currently make this coercion fail for now.
//
// see #65991 for more information.
if util::supertraits(tcx, source_trait_ref)
.skip(upcast_idx + 1)
.any(|upcast_trait_ref| upcast_trait_ref.def_id() == target_trait_did)
{
return Err(Unimplemented);
}
let target_trait_ref =
util::supertraits(tcx, source_trait_ref).nth(upcast_idx).unwrap();
let existential_predicate = target_trait_ref.map_bound(|trait_ref| {
ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
tcx, trait_ref,
))
});
Some(existential_predicate)
} else if principal_def_id_b.is_none() {
None
} else {
return Err(Unimplemented);
};
let iter = existential_predicate
.into_iter()
.chain(
data_a