Auto merge of #103491 - cjgillot:self-rpit, r=oli-obk

Support using `Self` or projections inside an RPIT/async fn

I reuse the same idea as https://github.com/rust-lang/rust/pull/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
```rust
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.

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1539,7 +1539,6 @@ fn check_fn_or_method<'tcx>(
 
     check_return_position_impl_trait_in_trait_bounds(
         tcx,
-        wfcx,
         def_id,
         sig.output(),
         hir_decl.output.span(),
@@ -1575,9 +1574,9 @@ fn check_fn_or_method<'tcx>(
 
 /// Basically `check_associated_type_bounds`, but separated for now and should be
 /// deduplicated when RPITITs get lowered into real associated items.
+#[tracing::instrument(level = "trace", skip(tcx))]
 fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
-    wfcx: &WfCheckingCtxt<'_, 'tcx>,
     fn_def_id: LocalDefId,
     fn_output: Ty<'tcx>,
     span: Span,
@@ -1591,18 +1590,22 @@ fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
                 && tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
                 && tcx.impl_trait_in_trait_parent(proj.item_def_id) == fn_def_id.to_def_id()
             {
-                let bounds = wfcx.tcx().explicit_item_bounds(proj.item_def_id);
-                let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
-                    let normalized_bound = wfcx.normalize(span, None, bound);
-                    traits::wf::predicate_obligations(
-                        wfcx.infcx,
-                        wfcx.param_env,
-                        wfcx.body_id,
-                        normalized_bound,
-                        bound_span,
-                    )
+                // Create a new context, since we want the opaque's ParamEnv and not the parent's.
+                let span = tcx.def_span(proj.item_def_id);
+                enter_wf_checking_ctxt(tcx, span, proj.item_def_id.expect_local(), |wfcx| {
+                    let bounds = wfcx.tcx().explicit_item_bounds(proj.item_def_id);
+                    let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
+                        let normalized_bound = wfcx.normalize(span, None, bound);
+                        traits::wf::predicate_obligations(
+                            wfcx.infcx,
+                            wfcx.param_env,
+                            wfcx.body_id,
+                            normalized_bound,
+                            bound_span,
+                        )
+                    });
+                    wfcx.register_obligations(wf_obligations);
                 });
-                wfcx.register_obligations(wf_obligations);
             }
         }
     }