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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
parent 3da96aed94 760673e97d
commit f4d06256d8
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);