Auto merge of #115659 - compiler-errors:itp, r=cjgillot

Stabilize `impl_trait_projections`

Closes #115659

## TL;DR:

This allows us to mention `Self` and `T::Assoc` in async fn and return-position `impl Trait`, as you would expect you'd be able to.

Some examples:
```rust
#![feature(return_position_impl_trait_in_trait, async_fn_in_trait)]
// (just needed for final tests below)

// ---------------------------------------- //

struct Wrapper<'a, T>(&'a T);

impl Wrapper<'_, ()> {
    async fn async_fn() -> Self {
        //^ Previously rejected because it returns `-> Self`, not `-> Wrapper<'_, ()>`.
        Wrapper(&())
    }

    fn impl_trait() -> impl Iterator<Item = Self> {
        //^ Previously rejected because it mentions `Self`, not `Wrapper<'_, ()>`.
        std::iter::once(Wrapper(&()))
    }
}

// ---------------------------------------- //

trait Trait<'a> {
    type Assoc;
    fn new() -> Self::Assoc;
}
impl Trait<'_> for () {
    type Assoc = ();
    fn new() {}
}

impl<'a, T: Trait<'a>> Wrapper<'a, T> {
    async fn mk_assoc() -> T::Assoc {
        //^ Previously rejected because `T::Assoc` doesn't mention `'a` in the HIR,
        //  but ends up resolving to `<T as Trait<'a>>::Assoc`, which does rely on `'a`.
        // That's the important part -- the elided trait.
        T::new()
    }

    fn a_few_assocs() -> impl Iterator<Item = T::Assoc> {
        //^ Previously rejected for the same reason
        [T::new(), T::new(), T::new()].into_iter()
    }
}

// ---------------------------------------- //

trait InTrait {
    async fn async_fn() -> Self;

    fn impl_trait() -> impl Iterator<Item = Self>;
}

impl InTrait for &() {
    async fn async_fn() -> Self { &() }
    //^ Previously rejected just like inherent impls

    fn impl_trait() -> impl Iterator<Item = Self> {
        //^ Previously rejected just like inherent impls
        [&()].into_iter()
    }
}
```

## Technical:

Lifetimes in return-position `impl Trait` (and `async fn`) are duplicated as early-bound generics local to the opaque in order to make sure we are able to substitute any late-bound lifetimes from the function in the opaque's hidden type. (The [dev guide](https://rustc-dev-guide.rust-lang.org/return-position-impl-trait-in-trait.html#aside-opaque-lifetime-duplication) has a small section about why this is necessary -- this was written for RPITITs, but it applies to all RPITs)

Prior to #103491, all of the early-bound lifetimes not local to the opaque were replaced with `'static` to avoid issues where relating opaques caused their *non-captured* lifetimes to be related. This `'static` replacement led to strange and possibly unsound behaviors (rust-lang/rust#61949 (comment)) (rust-lang/rust#53613) when referencing the `Self` type alias in an impl or indirectly referencing a lifetime parameter via a projection type (via a `T::Assoc` projection without an explicit trait), since lifetime resolution is performed on the HIR, when neither `T::Assoc`-style projections or `Self` in impls are expanded.

Therefore an error was implemented in #62849 to deny this subtle behavior as a known limitation of the compiler. It was attempted by `@cjgillot` to fix this in #91403, which was subsequently unlanded. Then it was re-attempted to much success (🎉) in #103491, which is where we currently are in the compiler.

The PR above (#103491) fixed this issue technically by *not* replacing the opaque's parent lifetimes with `'static`, but instead using variance to properly track which lifetimes are captured and are not. The PR gated any of the "side-effects" of the PR behind a feature gate (`impl_trait_projections`) presumably to avoid having to involve T-lang or T-types in the PR as well. `@cjgillot` can clarify this if I'm misunderstanding what their intention was with the feature gate.

Since we're not replacing (possibly *invariant*!) lifetimes with `'static` anymore, there are no more soundness concerns here. Therefore, this PR removes the feature gate.

Tests:
* `tests/ui/async-await/feature-self-return-type.rs`
* `tests/ui/impl-trait/feature-self-return-type.rs`
* `tests/ui/async-await/issues/issue-78600.rs`
* `tests/ui/impl-trait/capture-lifetime-not-in-hir.rs`

---

r? cjgillot on the impl (not much, just removing the feature gate)

I'm gonna mark this as FCP for T-lang and T-types.