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Rollup merge of #110454 - oli-obk:limited_impl_trait_in_assoc_type, r=compiler-errors

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Require impl Trait in associated types to appear in method signatures

This implements the limited version of TAIT that was proposed in https://github.com/rust-lang/rust/issues/107645#issuecomment-1477899536

Similar to `impl Trait` in return types, `impl Trait` in associated types may only be used within the impl block which it is a part of. To make everything simpler and forward compatible to getting desugared to a plain type alias impl trait in the future, we're requiring that any associated functions or constants that want to register hidden types must be using the associated type in their signature (type of the constant or argument/return type of the associated method. Where bounds mentioning the associated type are ignored).

We have preexisting tests checking that this works transitively across multiple associated types in situations like

```rust
impl Foo for Bar {
    type A = impl Trait;
    type B = impl Iterator<Item = Self::A>;
    fn foo() -> Self::B { ...... }
}
```
This commit is contained in:
Dylan DPC 2023-05-13 11:05:32 +05:30 • committed by GitHub
commit 6cb13585d0
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36 changed files with 560 additions and 68 deletions
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4
compiler/rustc_borrowck/src/region_infer/opaque_types.rs
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@ -265,7 +265,7 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
// Only check this for TAIT. RPIT already supports `tests/ui/impl-trait/nested-return-type2.rs`
// on stable and we'd break that.
let OpaqueTyOrigin::TyAlias = origin else {
let OpaqueTyOrigin::TyAlias { .. } = origin else {
return definition_ty;
};
let def_id = opaque_type_key.def_id;
@ -360,7 +360,7 @@ fn check_opaque_type_parameter_valid(
// which would error here on all of the `'static` args.
OpaqueTyOrigin::FnReturn(..) | OpaqueTyOrigin::AsyncFn(..) => return Ok(()),
// Check these
OpaqueTyOrigin::TyAlias => {}
OpaqueTyOrigin::TyAlias { .. } => {}
}
let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
let mut seen_params: FxIndexMap<_, Vec<_>> = FxIndexMap::default();