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Rollup merge of #119721 - compiler-errors:constness-implication, r=fee1-dead

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`~const` trait and projection bounds do not imply their non-const counterparts

This PR removes the hack where we install a non-const trait and projection bound for every `const_trait` and `~const` projection bound we have in the AST. It ends up messing up more things than it fixes, see words below.

Fixes #119718

cc `@fmease` `@fee1-dead` `@oli-obk`
r? fee1-dead or one of y'all i don't care

---

My understanding is that this hack was added to support the following code:

```rust
pub trait Owo<X = <Self as Uwu>::T> {}

#[const_trait]
pub trait Uwu: Owo {}
```

Which is concretely lifted from in the `FromResidual` and `Try` traits. Since within the param-env of `trait Uwu`, we only know that `Self: ~const Uwu` and not `Self: Uwu`, the projection `<Self as Uwu>::T` is not satsifyable.

This causes problems such as #119718, since instantiations of `FnDef` types coming from `const fn` really do **only** implement one of `FnOnce` or `const FnOnce`!

---

In the long-term, I believe that such code should really look something more like:

```rust
#[const_trait]
pub trait Owo<X = <Self as ~const Uwu>::T> {}

#[const_trait]
pub trait Uwu: Owo {}
```

... and that we should introduce some sort of `<T as ~const Foo>::Bar` bound syntax, since due to the fact that `~const` bounds can be present in item bounds, e.g.

```rust
#[const_trait] trait Foo { type Bar: ~const Destruct; }
```

It's easy to see that `<T as Foo>::Bar` and `<T as ~const Foo>::Bar` (or `<T as const Foo>::Bar`) can be distinct types with distinct item bounds!

**Admission**: I know I've said before that I don't like `~const` projection syntax, I do at this point believe they're necessary to fully express bounds and types in a maybe-const world.
This commit is contained in:
Guillaume Gomez 2024-01-09 17:52:21 +01:00 • committed by GitHub
commit f4d06256d8
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14 changed files with 227 additions and 128 deletions
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35
compiler/rustc_hir_analysis/src/astconv/mod.rs
View file

@ -1032,7 +1032,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
self.trait_defines_associated_item_named(r.def_id(), assoc_kind, assoc_name)
});
let Some(mut bound) = matching_candidates.next() else {
let Some(bound) = matching_candidates.next() else {
let reported = self.complain_about_assoc_item_not_found(
all_candidates,
&ty_param_name.to_string(),
@ -1046,38 +1046,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
};
debug!(?bound);
// look for a candidate that is not the same as our first bound, disregarding
// whether the bound is const.
let mut next_cand = matching_candidates.next();
while let Some(mut bound2) = next_cand {
debug!(?bound2);
if bound2.bound_vars() != bound.bound_vars() {
break;
}
let generics = tcx.generics_of(bound.def_id());
let Some(host_index) = generics.host_effect_index else { break };
// always return the bound that contains the host param.
if let ty::ConstKind::Param(_) = bound2.skip_binder().args.const_at(host_index).kind() {
(bound, bound2) = (bound2, bound);
}
let unconsted_args = bound
.skip_binder()
.args
.iter()
.enumerate()
.map(|(n, arg)| if host_index == n { tcx.consts.true_.into() } else { arg });
if unconsted_args.eq(bound2.skip_binder().args.iter()) {
next_cand = matching_candidates.next();
} else {
break;
}
}
if let Some(bound2) = next_cand {
if let Some(bound2) = matching_candidates.next() {
debug!(?bound2);
let assoc_kind_str = assoc_kind_str(assoc_kind);

18
compiler/rustc_hir_analysis/src/bounds.rs
View file

@ -45,24 +45,6 @@ impl<'tcx> Bounds<'tcx> {
polarity: ty::ImplPolarity,
) {
self.push_trait_bound_inner(tcx, trait_ref, span, polarity);
// push a non-const (`host = true`) version of the bound if it is `~const`.
if tcx.features().effects
&& let Some(host_effect_idx) = tcx.generics_of(trait_ref.def_id()).host_effect_index
&& trait_ref.skip_binder().args.const_at(host_effect_idx) != tcx.consts.true_
{
let generics = tcx.generics_of(trait_ref.def_id());
let Some(host_index) = generics.host_effect_index else { return };
let trait_ref = trait_ref.map_bound(|mut trait_ref| {
trait_ref.args =
tcx.mk_args_from_iter(trait_ref.args.iter().enumerate().map(|(n, arg)| {
if host_index == n { tcx.consts.true_.into() } else { arg }
}));
trait_ref
});
self.push_trait_bound_inner(tcx, trait_ref, span, polarity);
}
}
fn push_trait_bound_inner(

38
compiler/rustc_hir_analysis/src/collect/predicates_of.rs
View file

@ -11,7 +11,7 @@ use rustc_hir::intravisit::{self, Visitor};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{GenericPredicates, ImplTraitInTraitData, ToPredicate};
use rustc_span::symbol::Ident;
use rustc_span::{sym, Span, DUMMY_SP};
use rustc_span::{Span, DUMMY_SP};
/// Returns a list of all type predicates (explicit and implicit) for the definition with
/// ID `def_id`. This includes all predicates returned by `predicates_defined_on`, plus
@ -38,38 +38,12 @@ pub(super) fn predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericPredic
// an obligation and instead be skipped. Otherwise we'd use
// `tcx.def_span(def_id);`
let span = rustc_span::DUMMY_SP;
let non_const_bound = if tcx.features().effects && tcx.has_attr(def_id, sym::const_trait) {
// when `Self` is a const trait, also add `Self: Trait<.., true>` as implied bound,
// because only implementing `Self: Trait<.., false>` is currently not possible.
Some((
ty::TraitRef::new(
tcx,
def_id,
ty::GenericArgs::for_item(tcx, def_id, |param, _| {
if param.is_host_effect() {
tcx.consts.true_.into()
} else {
tcx.mk_param_from_def(param)
}
}),
)
.to_predicate(tcx),
result.predicates =
tcx.arena.alloc_from_iter(result.predicates.iter().copied().chain(std::iter::once((
ty::TraitRef::identity(tcx, def_id).to_predicate(tcx),
span,
))
} else {
None
};
result.predicates = tcx.arena.alloc_from_iter(
result
.predicates
.iter()
.copied()
.chain(std::iter::once((
ty::TraitRef::identity(tcx, def_id).to_predicate(tcx),
span,
)))
.chain(non_const_bound),
);
))));
}
debug!("predicates_of(def_id={:?}) = {:?}", def_id, result);
result