Support using Self or projections inside an RPIT/async fn

[cjgillot]

I reuse the same idea as #103449 to use variances to encode whether a lifetime parameter is captured by impl-trait.

The current implementation of async and RPIT replace all lifetimes from the parent generics by 'static. This PR changes the scheme

impl<'a> Foo<'a> {
    fn foo<'b, T>() -> impl Into<Self> + 'b { ... }
}

opaque Foo::<'_a>::foo::<'_b, T>::opaque<'b>: Into<Foo<'_a>> + 'b;
impl<'a> Foo<'a> {
    // OLD
    fn foo<'b, T>() -> Foo::<'static>::foo::<'static, T>::opaque::<'b> { ... }
                             ^^^^^^^ the `Self` becomes `Foo<'static>`

    // NEW
    fn foo<'b, T>() -> Foo::<'a>::foo::<'b, T>::opaque::<'b> { ... }
                             ^^ the `Self` stays `Foo<'a>`
}

There is the same issue with projections. In the example, substitute Self by <T as Trait<'b>>::Assoc in the sugared version, and Foo<'_a> by <T as Trait<'_b>>::Assoc in the desugared one.

This allows to support Self in impl-trait, since we do not replace lifetimes by 'static any more. The same trick allows to use projections like T::Assoc where Self is allowed. The feature is gated behind a impl_trait_projections feature gate.

The implementation relies on 2 tweaking rules for opaques in 2 places:

• we only relate substs that correspond to captured lifetimes during TypeRelation;
• we only list captured lifetimes in choice region computation.

For simplicity, I encoded the "capturedness" of lifetimes as a variance, Bivariant vs Invariant for unused vs captured lifetimes. The variances_of query used to ICE for opaques.

Impl-trait that do not reference Self or projections will have their variances as:

• o (invariant) for each parent type or const;
• * (bivariant) for each parent lifetime --> will not participate in borrowck;
• o (invariant) for each own lifetime.

Impl-trait that does reference Self and/or projections will have some parent lifetimes marked as o (as the example above), and participate in type relation and borrowck. In the example above, variances_of(opaque) = ['_a: o, '_b: *, T: o, 'b: o].

r? types
cc @compiler-errors , as you asked about the issue with Self and projections.

[rustbot]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[GuillaumeGomez]

Never comment an error code which has an associated error explanation.

[GuillaumeGomez]

Looks good for the error code explanation part. Thanks!

[oli-obk]

r? @oli-obk

[bors]

☔ The latest upstream changes (presumably #103530) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-timer]

Finished benchmarking commit (7fe6f36): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.7%	[0.4%, 1.3%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.1%	[4.1%, 4.1%]	1
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.0%	[-1.0%, -1.0%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.0%	[1.9%, 2.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

[oli-obk]

This perf regression is expected. We are generating obligations that were missing before