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.

33 files changed, 569 insertions, 340 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index e1703b0b02b..500737f6e80 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -60,8 +60,8 @@ use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_hir::definitions::DefPathData;
 use rustc_hir::{ConstArg, GenericArg, ItemLocalId, ParamName, TraitCandidate};
 use rustc_index::vec::{Idx, IndexVec};
+use rustc_middle::span_bug;
 use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
-use rustc_middle::{bug, span_bug};
 use rustc_session::parse::feature_err;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::source_map::DesugaringKind;
@@ -1465,17 +1465,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // frequently opened issues show.
         let opaque_ty_span = self.mark_span_with_reason(DesugaringKind::OpaqueTy, span, None);
 
-        let opaque_ty_def_id = match origin {
-            hir::OpaqueTyOrigin::TyAlias => self.create_def(
-                self.current_hir_id_owner.def_id,
-                opaque_ty_node_id,
-                DefPathData::ImplTrait,
-            ),
-            hir::OpaqueTyOrigin::FnReturn(fn_def_id) => {
-                self.create_def(fn_def_id, opaque_ty_node_id, DefPathData::ImplTrait)
-            }
-            hir::OpaqueTyOrigin::AsyncFn(..) => bug!("unreachable"),
-        };
+        let opaque_ty_def_id = self.create_def(
+            self.current_hir_id_owner.def_id,
+            opaque_ty_node_id,
+            DefPathData::ImplTrait,
+        );
         debug!(?opaque_ty_def_id);
 
         // Contains the new lifetime definitions created for the TAIT (if any).
diff --git a/compiler/rustc_borrowck/src/member_constraints.rs b/compiler/rustc_borrowck/src/member_constraints.rs
index b48f9f97daa..b5e00f471d2 100644
--- a/compiler/rustc_borrowck/src/member_constraints.rs
+++ b/compiler/rustc_borrowck/src/member_constraints.rs
@@ -11,6 +11,7 @@ use std::ops::Index;
 
 /// Compactly stores a set of `R0 member of [R1...Rn]` constraints,
 /// indexed by the region `R0`.
+#[derive(Debug)]
 pub(crate) struct MemberConstraintSet<'tcx, R>
 where
     R: Copy + Eq,
@@ -31,6 +32,7 @@ where
 }
 
 /// Represents a `R0 member of [R1..Rn]` constraint
+#[derive(Debug)]
 pub(crate) struct NllMemberConstraint<'tcx> {
     next_constraint: Option<NllMemberConstraintIndex>,
 
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 94e9e05e5d6..b35abbd107b 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -128,6 +128,7 @@ pub struct RegionInferenceContext<'tcx> {
 /// adds a new lower bound to the SCC it is analyzing: so you wind up
 /// with `'R: 'O` where `'R` is the pick-region and `'O` is the
 /// minimal viable option.
+#[derive(Debug)]
 pub(crate) struct AppliedMemberConstraint {
     /// The SCC that was affected. (The "member region".)
     ///
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index b9885952a89..d82d4cc39fb 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -1,4 +1,4 @@
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
 use rustc_data_structures::vec_map::VecMap;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::OpaqueTyOrigin;
@@ -61,17 +61,21 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         opaque_ty_decls: VecMap<OpaqueTypeKey<'tcx>, (OpaqueHiddenType<'tcx>, OpaqueTyOrigin)>,
     ) -> VecMap<LocalDefId, OpaqueHiddenType<'tcx>> {
         let mut result: VecMap<LocalDefId, OpaqueHiddenType<'tcx>> = VecMap::new();
+
+        let member_constraints: FxHashMap<_, _> = self
+            .member_constraints
+            .all_indices()
+            .map(|ci| (self.member_constraints[ci].key, ci))
+            .collect();
+        debug!(?member_constraints);
+
         for (opaque_type_key, (concrete_type, origin)) in opaque_ty_decls {
             let substs = opaque_type_key.substs;
             debug!(?concrete_type, ?substs);
 
             let mut subst_regions = vec![self.universal_regions.fr_static];
-            let universal_substs = infcx.tcx.fold_regions(substs, |region, _| {
-                if let ty::RePlaceholder(..) = region.kind() {
-                    // Higher kinded regions don't need remapping, they don't refer to anything outside of this the substs.
-                    return region;
-                }
-                let vid = self.to_region_vid(region);
+
+            let to_universal_region = |vid, subst_regions: &mut Vec<_>| {
                 trace!(?vid);
                 let scc = self.constraint_sccs.scc(vid);
                 trace!(?scc);
@@ -92,10 +96,33 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                         infcx.tcx.lifetimes.re_static
                     }
                 }
+            };
+
+            // Start by inserting universal regions from the member_constraint choice regions.
+            // This will ensure they get precedence when folding the regions in the concrete type.
+            if let Some(&ci) = member_constraints.get(&opaque_type_key) {
+                for &vid in self.member_constraints.choice_regions(ci) {
+                    to_universal_region(vid, &mut subst_regions);
+                }
+            }
+            debug!(?subst_regions);
+
+            // Next, insert universal regions from substs, so we can translate regions that appear
+            // in them but are not subject to member constraints, for instance closure substs.
+            let universal_substs = infcx.tcx.fold_regions(substs, |region, _| {
+                if let ty::RePlaceholder(..) = region.kind() {
+                    // Higher kinded regions don't need remapping, they don't refer to anything outside of this the substs.
+                    return region;
+                }
+                let vid = self.to_region_vid(region);
+                to_universal_region(vid, &mut subst_regions)
             });
+            debug!(?universal_substs);
+            debug!(?subst_regions);
 
-            subst_regions.sort();
-            subst_regions.dedup();
+            // Deduplicate the set of regions while keeping the chosen order.
+            let subst_regions = subst_regions.into_iter().collect::<FxIndexSet<_>>();
+            debug!(?subst_regions);
 
             let universal_concrete_type =
                 infcx.tcx.fold_regions(concrete_type, |region, _| match *region {
@@ -106,8 +133,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                         .unwrap_or(infcx.tcx.lifetimes.re_erased),
                     _ => region,
                 });
-
-            debug!(?universal_concrete_type, ?universal_substs);
+            debug!(?universal_concrete_type);
 
             let opaque_type_key =
                 OpaqueTypeKey { def_id: opaque_type_key.def_id, substs: universal_substs };
diff --git a/compiler/rustc_error_codes/src/error_codes/E0760.md b/compiler/rustc_error_codes/src/error_codes/E0760.md
index e1dcfefebcd..85e5faada22 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0760.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0760.md
@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 `async fn`/`impl trait` return type cannot contain a projection
 or `Self` that references lifetimes from a parent scope.
 
 Erroneous code example:
 
-```compile_fail,E0760,edition2018
+```compile_fail,edition2018
 struct S<'a>(&'a i32);
 
 impl<'a> S<'a> {
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index ae49a76a087..9ca63c393c6 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -419,6 +419,8 @@ declare_features! (
     (active, if_let_guard, "1.47.0", Some(51114), None),
     /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.
     (active, impl_trait_in_fn_trait_return, "1.64.0", Some(99697), None),
+    /// Allows referencing `Self` and projections in impl-trait.
+    (active, impl_trait_projections, "CURRENT_RUSTC_VERSION", Some(103532), None),
     /// Allows using imported `main` function
     (active, imported_main, "1.53.0", Some(28937), None),
     /// Allows associated types in inherent impls.
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index 83b95fe0e91..e8098ea66b2 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -2777,35 +2777,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let substs = InternalSubsts::for_item(tcx, def_id, |param, _| {
             if let Some(i) = (param.index as usize).checked_sub(generics.parent_count) {
                 // Our own parameters are the resolved lifetimes.
-                if let GenericParamDefKind::Lifetime = param.kind {
-                    if let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] {
-                        self.ast_region_to_region(lifetime, None).into()
-                    } else {
-                        bug!()
-                    }
-                } else {
-                    bug!()
-                }
+                let GenericParamDefKind::Lifetime { .. } = param.kind else { bug!() };
+                let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] else { bug!() };
+                self.ast_region_to_region(lifetime, None).into()
             } else {
-                match param.kind {
-                    // For RPIT (return position impl trait), only lifetimes
-                    // mentioned in the impl Trait predicate are captured by
-                    // the opaque type, so the lifetime parameters from the
-                    // parent item need to be replaced with `'static`.
-                    //
-                    // For `impl Trait` in the types of statics, constants,
-                    // locals and type aliases. These capture all parent
-                    // lifetimes, so they can use their identity subst.
-                    GenericParamDefKind::Lifetime
-                        if matches!(
-                            origin,
-                            hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..)
-                        ) =>
-                    {
-                        tcx.lifetimes.re_static.into()
-                    }
-                    _ => tcx.mk_param_from_def(param),
-                }
+                tcx.mk_param_from_def(param)
             }
         });
         debug!("impl_trait_ty_to_ty: substs={:?}", substs);
@@ -2982,6 +2958,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         Some(tcx.liberate_late_bound_regions(fn_hir_id.expect_owner().to_def_id(), ty))
     }
 
+    #[instrument(level = "trace", skip(self, generate_err))]
     fn validate_late_bound_regions(
         &self,
         constrained_regions: FxHashSet<ty::BoundRegionKind>,
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index b4805de9618..33b9c61993a 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -10,6 +10,7 @@ use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::{ItemKind, Node, PathSegment};
+use rustc_infer::infer::opaque_types::ConstrainOpaqueTypeRegionVisitor;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{DefiningAnchor, RegionVariableOrigin, TyCtxtInferExt};
 use rustc_infer::traits::Obligation;
@@ -229,7 +230,9 @@ fn check_opaque<'tcx>(tcx: TyCtxt<'tcx>, id: hir::ItemId) {
     let substs = InternalSubsts::identity_for_item(tcx, item.owner_id.to_def_id());
     let span = tcx.def_span(item.owner_id.def_id);
 
-    check_opaque_for_inheriting_lifetimes(tcx, item.owner_id.def_id, span);
+    if !tcx.features().impl_trait_projections {
+        check_opaque_for_inheriting_lifetimes(tcx, item.owner_id.def_id, span);
+    }
     if tcx.type_of(item.owner_id.def_id).references_error() {
         return;
     }
@@ -238,6 +241,7 @@ fn check_opaque<'tcx>(tcx: TyCtxt<'tcx>, id: hir::ItemId) {
     }
     check_opaque_meets_bounds(tcx, item.owner_id.def_id, substs, span, &origin);
 }
+
 /// Checks that an opaque type does not use `Self` or `T::Foo` projections that would result
 /// in "inheriting lifetimes".
 #[instrument(level = "debug", skip(tcx, span))]
@@ -249,39 +253,11 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
     let item = tcx.hir().expect_item(def_id);
     debug!(?item, ?span);
 
-    struct FoundParentLifetime;
-    struct FindParentLifetimeVisitor<'tcx>(&'tcx ty::Generics);
-    impl<'tcx> ty::visit::TypeVisitor<'tcx> for FindParentLifetimeVisitor<'tcx> {
-        type BreakTy = FoundParentLifetime;
-
-        fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
-            debug!("FindParentLifetimeVisitor: r={:?}", r);
-            if let ty::ReEarlyBound(ty::EarlyBoundRegion { index, .. }) = *r {
-                if index < self.0.parent_count as u32 {
-                    return ControlFlow::Break(FoundParentLifetime);
-                } else {
-                    return ControlFlow::CONTINUE;
-                }
-            }
-
-            r.super_visit_with(self)
-        }
-
-        fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-            if let ty::ConstKind::Unevaluated(..) = c.kind() {
-                // FIXME(#72219) We currently don't detect lifetimes within substs
-                // which would violate this check. Even though the particular substitution is not used
-                // within the const, this should still be fixed.
-                return ControlFlow::CONTINUE;
-            }
-            c.super_visit_with(self)
-        }
-    }
-
     struct ProhibitOpaqueVisitor<'tcx> {
         tcx: TyCtxt<'tcx>,
         opaque_identity_ty: Ty<'tcx>,
-        generics: &'tcx ty::Generics,
+        parent_count: u32,
+        references_parent_regions: bool,
         selftys: Vec<(Span, Option<String>)>,
     }
 
@@ -289,12 +265,25 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
         type BreakTy = Ty<'tcx>;
 
         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
-            debug!("check_opaque_for_inheriting_lifetimes: (visit_ty) t={:?}", t);
+            debug!(?t, "root_visit_ty");
             if t == self.opaque_identity_ty {
                 ControlFlow::CONTINUE
             } else {
-                t.super_visit_with(&mut FindParentLifetimeVisitor(self.generics))
-                    .map_break(|FoundParentLifetime| t)
+                t.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
+                    tcx: self.tcx,
+                    op: |region| {
+                        if let ty::ReEarlyBound(ty::EarlyBoundRegion { index, .. }) = *region
+                            && index < self.parent_count
+                        {
+                            self.references_parent_regions= true;
+                        }
+                    },
+                });
+                if self.references_parent_regions {
+                    ControlFlow::Break(t)
+                } else {
+                    ControlFlow::CONTINUE
+                }
             }
         }
     }
@@ -327,15 +316,20 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
 
     if let ItemKind::OpaqueTy(hir::OpaqueTy {
         origin: hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::FnReturn(..),
+        in_trait,
         ..
     }) = item.kind
     {
+        let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
+        let opaque_identity_ty = if in_trait {
+            tcx.mk_projection(def_id.to_def_id(), substs)
+        } else {
+            tcx.mk_opaque(def_id.to_def_id(), substs)
+        };
         let mut visitor = ProhibitOpaqueVisitor {
-            opaque_identity_ty: tcx.mk_opaque(
-                def_id.to_def_id(),
-                InternalSubsts::identity_for_item(tcx, def_id.to_def_id()),
-            ),
-            generics: tcx.generics_of(def_id),
+            opaque_identity_ty,
+            parent_count: tcx.generics_of(def_id).parent_count as u32,
+            references_parent_regions: false,
             tcx,
             selftys: vec![],
         };
@@ -343,10 +337,6 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
             .explicit_item_bounds(def_id)
             .iter()
             .try_for_each(|(predicate, _)| predicate.visit_with(&mut visitor));
-        debug!(
-            "check_opaque_for_inheriting_lifetimes: prohibit_opaque={:?}, visitor.opaque_identity_ty={:?}, visitor.generics={:?}",
-            prohibit_opaque, visitor.opaque_identity_ty, visitor.generics
-        );
 
         if let Some(ty) = prohibit_opaque.break_value() {
             visitor.visit_item(&item);
@@ -357,15 +347,16 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
                 _ => unreachable!(),
             };
 
-            let mut err = struct_span_err!(
-                tcx.sess,
+            let mut err = feature_err(
+                &tcx.sess.parse_sess,
+                sym::impl_trait_projections,
                 span,
-                E0760,
-                "`{}` return type cannot contain a projection or `Self` that references lifetimes from \
-                 a parent scope",
-                if is_async { "async fn" } else { "impl Trait" },
+                &format!(
+                    "`{}` return type cannot contain a projection or `Self` that references \
+                    lifetimes from a parent scope",
+                    if is_async { "async fn" } else { "impl Trait" },
+                ),
             );
-
             for (span, name) in visitor.selftys {
                 err.span_suggestion(
                     span,
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 1d7ceda725a..b81f9d7a6d2 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/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);
             }
         }
     }
diff --git a/compiler/rustc_hir_analysis/src/collect/predicates_of.rs b/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
index e2da580de0c..c3f1bb457f7 100644
--- a/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
@@ -84,60 +84,30 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
 
         Node::ImplItem(item) => item.generics,
 
-        Node::Item(item) => {
-            match item.kind {
-                ItemKind::Impl(ref impl_) => {
-                    if impl_.defaultness.is_default() {
-                        is_default_impl_trait = tcx.impl_trait_ref(def_id).map(ty::Binder::dummy);
-                    }
-                    &impl_.generics
+        Node::Item(item) => match item.kind {
+            ItemKind::Impl(ref impl_) => {
+                if impl_.defaultness.is_default() {
+                    is_default_impl_trait = tcx.impl_trait_ref(def_id).map(ty::Binder::dummy);
                 }
-                ItemKind::Fn(.., ref generics, _)
-                | ItemKind::TyAlias(_, ref generics)
-                | ItemKind::Enum(_, ref generics)
-                | ItemKind::Struct(_, ref generics)
-                | ItemKind::Union(_, ref generics) => *generics,
-
-                ItemKind::Trait(_, _, ref generics, ..) => {
-                    is_trait = Some(ty::TraitRef::identity(tcx, def_id));
-                    *generics
-                }
-                ItemKind::TraitAlias(ref generics, _) => {
-                    is_trait = Some(ty::TraitRef::identity(tcx, def_id));
-                    *generics
-                }
-                ItemKind::OpaqueTy(OpaqueTy {
-                    origin: hir::OpaqueTyOrigin::AsyncFn(..) | hir::OpaqueTyOrigin::FnReturn(..),
-                    ..
-                }) => {
-                    // return-position impl trait
-                    //
-                    // We don't inherit predicates from the parent here:
-                    // If we have, say `fn f<'a, T: 'a>() -> impl Sized {}`
-                    // then the return type is `f::<'static, T>::{{opaque}}`.
-                    //
-                    // If we inherited the predicates of `f` then we would
-                    // require that `T: 'static` to show that the return
-                    // type is well-formed.
-                    //
-                    // The only way to have something with this opaque type
-                    // is from the return type of the containing function,
-                    // which will ensure that the function's predicates
-                    // hold.
-                    return ty::GenericPredicates { parent: None, predicates: &[] };
-                }
-                ItemKind::OpaqueTy(OpaqueTy {
-                    ref generics,
-                    origin: hir::OpaqueTyOrigin::TyAlias,
-                    ..
-                }) => {
-                    // type-alias impl trait
-                    generics
-                }
-
-                _ => NO_GENERICS,
+                &impl_.generics
             }
-        }
+            ItemKind::Fn(.., ref generics, _)
+            | ItemKind::TyAlias(_, ref generics)
+            | ItemKind::Enum(_, ref generics)
+            | ItemKind::Struct(_, ref generics)
+            | ItemKind::Union(_, ref generics) => *generics,
+
+            ItemKind::Trait(_, _, ref generics, ..) => {
+                is_trait = Some(ty::TraitRef::identity(tcx, def_id));
+                *generics
+            }
+            ItemKind::TraitAlias(ref generics, _) => {
+                is_trait = Some(ty::TraitRef::identity(tcx, def_id));
+                *generics
+            }
+            ItemKind::OpaqueTy(OpaqueTy { ref generics, .. }) => generics,
+            _ => NO_GENERICS,
+        },
 
         Node::ForeignItem(item) => match item.kind {
             ForeignItemKind::Static(..) => NO_GENERICS,
@@ -181,6 +151,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
 
     trace!(?predicates);
     trace!(?ast_generics);
+    trace!(?generics);
 
     // Collect the predicates that were written inline by the user on each
     // type parameter (e.g., `<T: Foo>`).
@@ -299,6 +270,54 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
         );
     }
 
+    // Opaque types duplicate some of their generic parameters.
+    // We create bi-directional Outlives predicates between the original
+    // and the duplicated parameter, to ensure that they do not get out of sync.
+    if let Node::Item(&Item { kind: ItemKind::OpaqueTy(..), .. }) = node {
+        let opaque_ty_id = tcx.hir().get_parent_node(hir_id);
+        let opaque_ty_node = tcx.hir().get(opaque_ty_id);
+        let Node::Ty(&Ty { kind: TyKind::OpaqueDef(_, lifetimes, _), .. }) = opaque_ty_node else {
+            bug!("unexpected {opaque_ty_node:?}")
+        };
+        debug!(?lifetimes);
+        for (arg, duplicate) in std::iter::zip(lifetimes, ast_generics.params) {
+            let hir::GenericArg::Lifetime(arg) = arg else { bug!() };
+            let orig_region = <dyn AstConv<'_>>::ast_region_to_region(&icx, &arg, None);
+            if !matches!(orig_region.kind(), ty::ReEarlyBound(..)) {
+                // Only early-bound regions can point to the original generic parameter.
+                continue;
+            }
+
+            let hir::GenericParamKind::Lifetime { .. } = duplicate.kind else { continue };
+            let dup_def = tcx.hir().local_def_id(duplicate.hir_id).to_def_id();
+
+            let Some(dup_index) = generics.param_def_id_to_index(tcx, dup_def) else { bug!() };
+
+            let dup_region = tcx.mk_region(ty::ReEarlyBound(ty::EarlyBoundRegion {
+                def_id: dup_def,
+                index: dup_index,
+                name: duplicate.name.ident().name,
+            }));
+            predicates.push((
+                ty::Binder::dummy(ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(
+                    orig_region,
+                    dup_region,
+                )))
+                .to_predicate(icx.tcx),
+                duplicate.span,
+            ));
+            predicates.push((
+                ty::Binder::dummy(ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(
+                    dup_region,
+                    orig_region,
+                )))
+                .to_predicate(icx.tcx),
+                duplicate.span,
+            ));
+        }
+        debug!(?predicates);
+    }
+
     ty::GenericPredicates {
         parent: generics.parent,
         predicates: tcx.arena.alloc_from_iter(predicates),
diff --git a/compiler/rustc_hir_analysis/src/variance/mod.rs b/compiler/rustc_hir_analysis/src/variance/mod.rs
index 82103c5a03b..b6137f89cad 100644
--- a/compiler/rustc_hir_analysis/src/variance/mod.rs
+++ b/compiler/rustc_hir_analysis/src/variance/mod.rs
@@ -5,9 +5,10 @@
 
 use rustc_arena::DroplessArena;
 use rustc_hir::def::DefKind;
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::{self, CrateVariancesMap, TyCtxt};
+use rustc_middle::ty::{self, CrateVariancesMap, TyCtxt, TypeSuperVisitable, TypeVisitable};
+use std::ops::ControlFlow;
 
 /// Defines the `TermsContext` basically houses an arena where we can
 /// allocate terms.
@@ -50,6 +51,9 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: DefId) -> &[ty::Variance] {
         | DefKind::Union
         | DefKind::Variant
         | DefKind::Ctor(..) => {}
+        DefKind::OpaqueTy | DefKind::ImplTraitPlaceholder => {
+            return variance_of_opaque(tcx, item_def_id.expect_local());
+        }
         _ => {
             // Variance not relevant.
             span_bug!(tcx.def_span(item_def_id), "asked to compute variance for wrong kind of item")
@@ -61,3 +65,89 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: DefId) -> &[ty::Variance] {
     let crate_map = tcx.crate_variances(());
     crate_map.variances.get(&item_def_id).copied().unwrap_or(&[])
 }
+
+#[instrument(level = "trace", skip(tcx), ret)]
+fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
+    let generics = tcx.generics_of(item_def_id);
+
+    // Opaque types may only use regions that are bound. So for
+    // ```rust
+    // type Foo<'a, 'b, 'c> = impl Trait<'a> + 'b;
+    // ```
+    // we may not use `'c` in the hidden type.
+    struct OpaqueTypeLifetimeCollector {
+        variances: Vec<ty::Variance>,
+    }
+
+    impl<'tcx> ty::TypeVisitor<'tcx> for OpaqueTypeLifetimeCollector {
+        #[instrument(level = "trace", skip(self), ret)]
+        fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::RegionKind::ReEarlyBound(ebr) = r.kind() {
+                self.variances[ebr.index as usize] = ty::Invariant;
+            }
+            r.super_visit_with(self)
+        }
+    }
+
+    // By default, RPIT are invariant wrt type and const generics, but they are bivariant wrt
+    // lifetime generics.
+    let mut variances: Vec<_> = std::iter::repeat(ty::Invariant).take(generics.count()).collect();
+
+    // Mark all lifetimes from parent generics as unused (Bivariant).
+    // This will be overridden later if required.
+    {
+        let mut generics = generics;
+        while let Some(def_id) = generics.parent {
+            generics = tcx.generics_of(def_id);
+            for param in &generics.params {
+                match param.kind {
+                    ty::GenericParamDefKind::Lifetime => {
+                        variances[param.index as usize] = ty::Bivariant;
+                    }
+                    ty::GenericParamDefKind::Type { .. }
+                    | ty::GenericParamDefKind::Const { .. } => {}
+                }
+            }
+        }
+    }
+
+    let mut collector = OpaqueTypeLifetimeCollector { variances };
+    let id_substs = ty::InternalSubsts::identity_for_item(tcx, item_def_id.to_def_id());
+    for pred in tcx.bound_explicit_item_bounds(item_def_id.to_def_id()).transpose_iter() {
+        let pred = pred.map_bound(|(pred, _)| *pred).subst(tcx, id_substs);
+        debug!(?pred);
+
+        // We only ignore opaque type substs if the opaque type is the outermost type.
+        // The opaque type may be nested within itself via recursion in e.g.
+        // type Foo<'a> = impl PartialEq<Foo<'a>>;
+        // which thus mentions `'a` and should thus accept hidden types that borrow 'a
+        // instead of requiring an additional `+ 'a`.
+        match pred.kind().skip_binder() {
+            ty::PredicateKind::Trait(ty::TraitPredicate {
+                trait_ref: ty::TraitRef { def_id: _, substs },
+                constness: _,
+                polarity: _,
+            }) => {
+                for subst in &substs[1..] {
+                    subst.visit_with(&mut collector);
+                }
+            }
+            ty::PredicateKind::Projection(ty::ProjectionPredicate {
+                projection_ty: ty::ProjectionTy { substs, item_def_id: _ },
+                term,
+            }) => {
+                for subst in &substs[1..] {
+                    subst.visit_with(&mut collector);
+                }
+                term.visit_with(&mut collector);
+            }
+            ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(_, region)) => {
+                region.visit_with(&mut collector);
+            }
+            _ => {
+                pred.visit_with(&mut collector);
+            }
+        }
+    }
+    tcx.arena.alloc_from_iter(collector.variances.into_iter())
+}
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index 37f071a19ac..c2552561c42 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -564,6 +564,7 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                 &opt_variances,
                 a_subst,
                 b_subst,
+                true,
             )
         }
     }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 22f32251f6d..d773aa5f1fc 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -261,6 +261,7 @@ fn label_msg_span(
     }
 }
 
+#[instrument(level = "trace", skip(tcx))]
 pub fn unexpected_hidden_region_diagnostic<'tcx>(
     tcx: TyCtxt<'tcx>,
     span: Span,
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index 1fa95f8d62a..8d3d002cb27 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -332,32 +332,11 @@ impl<'tcx> InferCtxt<'tcx> {
         concrete_ty: Ty<'tcx>,
         span: Span,
     ) {
-        let def_id = opaque_type_key.def_id;
-
-        let tcx = self.tcx;
-
         let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
-
         debug!(?concrete_ty);
 
-        let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
-            hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
-                // We lower
-                //
-                // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
-                //
-                // into
-                //
-                // type foo::<'p0..'pn>::Foo<'q0..'qm>
-                // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
-                //
-                // For these types we only iterate over `'l0..lm` below.
-                tcx.generics_of(def_id).parent_count
-            }
-            // These opaque type inherit all lifetime parameters from their
-            // parent, so we have to check them all.
-            hir::OpaqueTyOrigin::TyAlias => 0,
-        };
+        let variances = self.tcx.variances_of(opaque_type_key.def_id);
+        debug!(?variances);
 
         // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
         // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
@@ -370,9 +349,12 @@ impl<'tcx> InferCtxt<'tcx> {
         // type can be equal to any of the region parameters of the
         // opaque type definition.
         let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
-            opaque_type_key.substs[first_own_region..]
+            opaque_type_key
+                .substs
                 .iter()
-                .filter_map(|arg| match arg.unpack() {
+                .enumerate()
+                .filter(|(i, _)| variances[*i] == ty::Variance::Invariant)
+                .filter_map(|(_, arg)| match arg.unpack() {
                     GenericArgKind::Lifetime(r) => Some(r),
                     GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
                 })
@@ -381,6 +363,7 @@ impl<'tcx> InferCtxt<'tcx> {
         );
 
         concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
+            tcx: self.tcx,
             op: |r| self.member_constraint(opaque_type_key, span, concrete_ty, r, &choice_regions),
         });
     }
@@ -440,11 +423,12 @@ impl<'tcx> InferCtxt<'tcx> {
 //
 // We ignore any type parameters because impl trait values are assumed to
 // capture all the in-scope type parameters.
-struct ConstrainOpaqueTypeRegionVisitor<OP> {
-    op: OP,
+pub struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP: FnMut(ty::Region<'tcx>)> {
+    pub tcx: TyCtxt<'tcx>,
+    pub op: OP,
 }
 
-impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
+impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
 where
     OP: FnMut(ty::Region<'tcx>),
 {
@@ -490,6 +474,31 @@ where
                 substs.as_generator().yield_ty().visit_with(self);
                 substs.as_generator().resume_ty().visit_with(self);
             }
+
+            ty::Opaque(def_id, ref substs) => {
+                // Skip lifetime paramters that are not captures.
+                let variances = self.tcx.variances_of(*def_id);
+
+                for (v, s) in std::iter::zip(variances, substs.iter()) {
+                    if *v != ty::Variance::Bivariant {
+                        s.visit_with(self);
+                    }
+                }
+            }
+
+            ty::Projection(proj)
+                if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
+            {
+                // Skip lifetime paramters that are not captures.
+                let variances = self.tcx.variances_of(proj.item_def_id);
+
+                for (v, s) in std::iter::zip(variances, proj.substs.iter()) {
+                    if *v != ty::Variance::Bivariant {
+                        s.visit_with(self);
+                    }
+                }
+            }
+
             _ => {
                 ty.super_visit_with(self);
             }
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index e09ac968b60..76a7fb41e6c 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -928,6 +928,8 @@ fn should_encode_variances(def_kind: DefKind) -> bool {
         | DefKind::Union
         | DefKind::Enum
         | DefKind::Variant
+        | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Fn
         | DefKind::Ctor(..)
         | DefKind::AssocFn => true,
@@ -941,8 +943,6 @@ fn should_encode_variances(def_kind: DefKind) -> bool {
         | DefKind::Const
         | DefKind::ForeignMod
         | DefKind::TyAlias
-        | DefKind::OpaqueTy
-        | DefKind::ImplTraitPlaceholder
         | DefKind::Impl
         | DefKind::Trait
         | DefKind::TraitAlias
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index a770c6a2e99..ffb4ad4db88 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -1257,7 +1257,7 @@ impl<'tcx> InstantiatedPredicates<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable, Lift)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TyEncodable, TyDecodable, Lift)]
 #[derive(TypeFoldable, TypeVisitable)]
 pub struct OpaqueTypeKey<'tcx> {
     pub def_id: LocalDefId,
@@ -1332,6 +1332,9 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
         let id_substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
         debug!(?id_substs);
 
+        // This zip may have several times the same lifetime in `substs` paired with a different
+        // lifetime from `id_substs`.  Simply `collect`ing the iterator is the correct behaviour:
+        // it will pick the last one, which is the one we introduced in the impl-trait desugaring.
         let map = substs.iter().zip(id_substs);
 
         let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> = match origin {
@@ -1345,61 +1348,13 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
                 // type Foo<'a, 'b, 'c> = impl Trait<'a> + 'b;
                 // ```
                 // we may not use `'c` in the hidden type.
-                struct OpaqueTypeLifetimeCollector<'tcx> {
-                    lifetimes: FxHashSet<ty::Region<'tcx>>,
-                }
-
-                impl<'tcx> ty::TypeVisitor<'tcx> for OpaqueTypeLifetimeCollector<'tcx> {
-                    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
-                        self.lifetimes.insert(r);
-                        r.super_visit_with(self)
-                    }
-                }
+                let variances = tcx.variances_of(def_id);
+                debug!(?variances);
 
-                let mut collector = OpaqueTypeLifetimeCollector { lifetimes: Default::default() };
-
-                for pred in tcx.bound_explicit_item_bounds(def_id.to_def_id()).transpose_iter() {
-                    let pred = pred.map_bound(|(pred, _)| *pred).subst(tcx, id_substs);
-
-                    trace!(pred=?pred.kind());
-
-                    // We only ignore opaque type substs if the opaque type is the outermost type.
-                    // The opaque type may be nested within itself via recursion in e.g.
-                    // type Foo<'a> = impl PartialEq<Foo<'a>>;
-                    // which thus mentions `'a` and should thus accept hidden types that borrow 'a
-                    // instead of requiring an additional `+ 'a`.
-                    match pred.kind().skip_binder() {
-                        ty::PredicateKind::Trait(TraitPredicate {
-                            trait_ref: ty::TraitRef { def_id: _, substs },
-                            constness: _,
-                            polarity: _,
-                        }) => {
-                            trace!(?substs);
-                            for subst in &substs[1..] {
-                                subst.visit_with(&mut collector);
-                            }
-                        }
-                        ty::PredicateKind::Projection(ty::ProjectionPredicate {
-                            projection_ty: ty::ProjectionTy { substs, item_def_id: _ },
-                            term,
-                        }) => {
-                            for subst in &substs[1..] {
-                                subst.visit_with(&mut collector);
-                            }
-                            term.visit_with(&mut collector);
-                        }
-                        _ => {
-                            pred.visit_with(&mut collector);
-                        }
-                    }
-                }
-                let lifetimes = collector.lifetimes;
-                trace!(?lifetimes);
                 map.filter(|(_, v)| {
-                    let ty::GenericArgKind::Lifetime(lt) = v.unpack() else {
-                        return true;
-                    };
-                    lifetimes.contains(&lt)
+                    let ty::GenericArgKind::Lifetime(lt) = v.unpack() else { return true };
+                    let ty::ReEarlyBound(ebr) = lt.kind() else { bug!() };
+                    variances[ebr.index as usize] == ty::Variance::Invariant
                 })
                 .collect()
             }
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
index c083a405e3c..6d02551716e 100644
--- a/compiler/rustc_middle/src/ty/relate.rs
+++ b/compiler/rustc_middle/src/ty/relate.rs
@@ -60,7 +60,7 @@ pub trait TypeRelation<'tcx>: Sized {
 
         let tcx = self.tcx();
         let opt_variances = tcx.variances_of(item_def_id);
-        relate_substs_with_variances(self, item_def_id, opt_variances, a_subst, b_subst)
+        relate_substs_with_variances(self, item_def_id, opt_variances, a_subst, b_subst, true)
     }
 
     /// Switch variance for the purpose of relating `a` and `b`.
@@ -151,13 +151,14 @@ pub fn relate_substs_with_variances<'tcx, R: TypeRelation<'tcx>>(
     variances: &[ty::Variance],
     a_subst: SubstsRef<'tcx>,
     b_subst: SubstsRef<'tcx>,
+    fetch_ty_for_diag: bool,
 ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
     let tcx = relation.tcx();
 
     let mut cached_ty = None;
     let params = iter::zip(a_subst, b_subst).enumerate().map(|(i, (a, b))| {
         let variance = variances[i];
-        let variance_info = if variance == ty::Invariant {
+        let variance_info = if variance == ty::Invariant && fetch_ty_for_diag {
             let ty =
                 *cached_ty.get_or_insert_with(|| tcx.bound_type_of(ty_def_id).subst(tcx, a_subst));
             ty::VarianceDiagInfo::Invariant { ty, param_index: i.try_into().unwrap() }
@@ -561,7 +562,15 @@ pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
         (&ty::Opaque(a_def_id, a_substs), &ty::Opaque(b_def_id, b_substs))
             if a_def_id == b_def_id =>
         {
-            let substs = relate_substs(relation, a_substs, b_substs)?;
+            let opt_variances = tcx.variances_of(a_def_id);
+            let substs = relate_substs_with_variances(
+                relation,
+                a_def_id,
+                opt_variances,
+                a_substs,
+                b_substs,
+                false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
+            )?;
             Ok(tcx.mk_opaque(a_def_id, substs))
         }
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 1b109ca6b11..29312a21b4d 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -787,6 +787,7 @@ symbols! {
         impl_lint_pass,
         impl_trait_in_bindings,
         impl_trait_in_fn_trait_return,
+        impl_trait_projections,
         implied_by,
         import,
         import_name_type,
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
index c60ade360df..bd7b64ad7c1 100644
--- a/compiler/rustc_ty_utils/src/ty.rs
+++ b/compiler/rustc_ty_utils/src/ty.rs
@@ -108,12 +108,7 @@ fn adt_sized_constraint(tcx: TyCtxt<'_>, def_id: DefId) -> &[Ty<'_>] {
 
 /// See `ParamEnv` struct definition for details.
 fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
-    // The param_env of an impl Trait type is its defining function's param_env
-    if let Some(parent) = ty::is_impl_trait_defn(tcx, def_id) {
-        return param_env(tcx, parent.to_def_id());
-    }
     // Compute the bounds on Self and the type parameters.
-
     let ty::InstantiatedPredicates { mut predicates, .. } =
         tcx.predicates_of(def_id).instantiate_identity(tcx);
 
diff --git a/src/test/ui/async-await/feature-self-return-type.rs b/src/test/ui/async-await/feature-self-return-type.rs
new file mode 100644
index 00000000000..41f887430c1
--- /dev/null
+++ b/src/test/ui/async-await/feature-self-return-type.rs
@@ -0,0 +1,28 @@
+// edition:2018
+#![feature(impl_trait_projections)]
+
+// This test checks that we emit the correct borrowck error when `Self` is used as a return type.
+// See #61949 for context.
+
+pub struct Foo<'a> {
+    pub bar: &'a i32,
+}
+
+impl<'a> Foo<'a> {
+    pub async fn new(_bar: &'a i32) -> Self {
+        Foo {
+            bar: &22
+        }
+    }
+}
+
+pub async fn foo() {
+    let x = {
+        let bar = 22;
+        Foo::new(&bar).await
+        //~^ ERROR `bar` does not live long enough
+    };
+    drop(x);
+}
+
+fn main() { }
diff --git a/src/test/ui/async-await/feature-self-return-type.stderr b/src/test/ui/async-await/feature-self-return-type.stderr
new file mode 100644
index 00000000000..8924683684f
--- /dev/null
+++ b/src/test/ui/async-await/feature-self-return-type.stderr
@@ -0,0 +1,15 @@
+error[E0597]: `bar` does not live long enough
+  --> $DIR/feature-self-return-type.rs:22:18
+   |
+LL |     let x = {
+   |         - borrow later stored here
+LL |         let bar = 22;
+LL |         Foo::new(&bar).await
+   |                  ^^^^ borrowed value does not live long enough
+LL |
+LL |     };
+   |     - `bar` dropped here while still borrowed
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/src/test/ui/async-await/in-trait/async-associated-types.rs b/src/test/ui/async-await/in-trait/async-associated-types.rs
index a6f928f3b1b..974f5aaff83 100644
--- a/src/test/ui/async-await/in-trait/async-associated-types.rs
+++ b/src/test/ui/async-await/in-trait/async-associated-types.rs
@@ -1,8 +1,8 @@
-// check-fail
-// known-bug: #102682
+// check-pass
 // edition: 2021
 
 #![feature(async_fn_in_trait)]
+#![feature(impl_trait_projections)]
 #![allow(incomplete_features)]
 
 use std::fmt::Debug;
diff --git a/src/test/ui/async-await/in-trait/async-associated-types.stderr b/src/test/ui/async-await/in-trait/async-associated-types.stderr
deleted file mode 100644
index 0985150eee0..00000000000
--- a/src/test/ui/async-await/in-trait/async-associated-types.stderr
+++ /dev/null
@@ -1,57 +0,0 @@
-error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
-  --> $DIR/async-associated-types.rs:19:43
-   |
-LL |     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
-   |                                           ^^^^^^^^^^^^^^
-   |
-note: first, the lifetime cannot outlive the lifetime `'a` as defined here...
-  --> $DIR/async-associated-types.rs:16:6
-   |
-LL | impl<'a, 'b, T: Debug + Sized + 'b, U: 'a> MyTrait<'a, 'b, T> for U {
-   |      ^^
-note: ...so that the types are compatible
-  --> $DIR/async-associated-types.rs:19:43
-   |
-LL |     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
-   |                                           ^^^^^^^^^^^^^^
-   = note: expected `(&'a U, &'b T)`
-              found `(&U, &T)`
-   = note: but, the lifetime must be valid for the static lifetime...
-note: ...so that the types are compatible
-  --> $DIR/async-associated-types.rs:19:43
-   |
-LL |     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
-   |                                           ^^^^^^^^^^^^^^
-   = note: expected `MyTrait<'static, 'static, T>`
-              found `MyTrait<'_, '_, T>`
-
-error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'b` due to conflicting requirements
-  --> $DIR/async-associated-types.rs:19:43
-   |
-LL |     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
-   |                                           ^^^^^^^^^^^^^^
-   |
-note: first, the lifetime cannot outlive the lifetime `'b` as defined here...
-  --> $DIR/async-associated-types.rs:16:10
-   |
-LL | impl<'a, 'b, T: Debug + Sized + 'b, U: 'a> MyTrait<'a, 'b, T> for U {
-   |          ^^
-note: ...so that the types are compatible
-  --> $DIR/async-associated-types.rs:19:43
-   |
-LL |     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
-   |                                           ^^^^^^^^^^^^^^
-   = note: expected `(&'a U, &'b T)`
-              found `(&U, &T)`
-   = note: but, the lifetime must be valid for the static lifetime...
-note: ...so that the types are compatible
-  --> $DIR/async-associated-types.rs:19:43
-   |
-LL |     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
-   |                                           ^^^^^^^^^^^^^^
-   = note: expected `MyTrait<'static, 'static, T>`
-              found `MyTrait<'_, '_, T>`
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0495`.
diff --git a/src/test/ui/async-await/issue-61949-self-return-type.rs b/src/test/ui/async-await/issue-61949-self-return-type.rs
index 43429ba2329..d73dbc6e828 100644
--- a/src/test/ui/async-await/issue-61949-self-return-type.rs
+++ b/src/test/ui/async-await/issue-61949-self-return-type.rs
@@ -1,4 +1,5 @@
 // edition:2018
+// gate-test-impl_trait_projections
 
 // This test checks that `Self` is prohibited as a return type. See #61949 for context.
 
@@ -19,6 +20,7 @@ async fn foo() {
     let x = {
         let bar = 22;
         Foo::new(&bar).await
+        //~^ ERROR `bar` does not live long enough
     };
     drop(x);
 }
diff --git a/src/test/ui/async-await/issue-61949-self-return-type.stderr b/src/test/ui/async-await/issue-61949-self-return-type.stderr
index 52b726e186e..638b197bc02 100644
--- a/src/test/ui/async-await/issue-61949-self-return-type.stderr
+++ b/src/test/ui/async-await/issue-61949-self-return-type.stderr
@@ -1,9 +1,25 @@
-error[E0760]: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
-  --> $DIR/issue-61949-self-return-type.rs:10:40
+error[E0658]: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
+  --> $DIR/issue-61949-self-return-type.rs:11:40
    |
 LL |     pub async fn new(_bar: &'a i32) -> Self {
    |                                        ^^^^ help: consider spelling out the type instead: `Foo<'a>`
+   |
+   = note: see issue #103532 <https://github.com/rust-lang/rust/issues/103532> for more information
+   = help: add `#![feature(impl_trait_projections)]` to the crate attributes to enable
+
+error[E0597]: `bar` does not live long enough
+  --> $DIR/issue-61949-self-return-type.rs:22:18
+   |
+LL |     let x = {
+   |         - borrow later stored here
+LL |         let bar = 22;
+LL |         Foo::new(&bar).await
+   |                  ^^^^ borrowed value does not live long enough
+LL |
+LL |     };
+   |     - `bar` dropped here while still borrowed
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0760`.
+Some errors have detailed explanations: E0597, E0658.
+For more information about an error, try `rustc --explain E0597`.
diff --git a/src/test/ui/async-await/issues/issue-78600.stderr b/src/test/ui/async-await/issues/issue-78600.stderr
index 92b66147106..37eafa996c5 100644
--- a/src/test/ui/async-await/issues/issue-78600.stderr
+++ b/src/test/ui/async-await/issues/issue-78600.stderr
@@ -1,11 +1,14 @@
-error[E0760]: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
+error[E0658]: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
   --> $DIR/issue-78600.rs:6:33
    |
 LL |     async fn new(i: &'a i32) -> Result<Self, ()> {
    |                                 ^^^^^^^----^^^^^
    |                                        |
    |                                        help: consider spelling out the type instead: `S<'a>`
+   |
+   = note: see issue #103532 <https://github.com/rust-lang/rust/issues/103532> for more information
+   = help: add `#![feature(impl_trait_projections)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0760`.
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/impl-trait/bound-normalization-fail.stderr b/src/test/ui/impl-trait/bound-normalization-fail.stderr
index bd8d3d3d24e..a9fa2da569f 100644
--- a/src/test/ui/impl-trait/bound-normalization-fail.stderr
+++ b/src/test/ui/impl-trait/bound-normalization-fail.stderr
@@ -19,17 +19,20 @@ help: consider constraining the associated type `<T as impl_trait::Trait>::Assoc
 LL |     fn foo_fail<T: Trait<Assoc = ()>>() -> impl FooLike<Output = T::Assoc> {
    |                         ++++++++++++
 
-error[E0760]: `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
+error[E0658]: `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
   --> $DIR/bound-normalization-fail.rs:41:41
    |
 LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output = T::Assoc> {
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #103532 <https://github.com/rust-lang/rust/issues/103532> for more information
+   = help: add `#![feature(impl_trait_projections)]` to the crate attributes to enable
 
-error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lifetimes::Trait<'static>>::Assoc`
+error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lifetimes::Trait<'a>>::Assoc`
   --> $DIR/bound-normalization-fail.rs:41:41
    |
 LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output = T::Assoc> {
-   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lifetimes::Trait<'static>>::Assoc`
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lifetimes::Trait<'a>>::Assoc`
 ...
 LL |         Foo(())
    |         ------- return type was inferred to be `Foo<()>` here
@@ -40,13 +43,13 @@ note: expected this to be `()`
 LL |     type Output = T;
    |                   ^
    = note:    expected unit type `()`
-           found associated type `<T as lifetimes::Trait<'static>>::Assoc`
-help: consider constraining the associated type `<T as lifetimes::Trait<'static>>::Assoc` to `()`
+           found associated type `<T as lifetimes::Trait<'a>>::Assoc`
+help: consider constraining the associated type `<T as lifetimes::Trait<'a>>::Assoc` to `()`
    |
 LL |     fn foo2_fail<'a, T: Trait<'a, Assoc = ()>>() -> impl FooLike<Output = T::Assoc> {
    |                                 ++++++++++++
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0271, E0760.
+Some errors have detailed explanations: E0271, E0658.
 For more information about an error, try `rustc --explain E0271`.
diff --git a/src/test/ui/impl-trait/feature-self-return-type.rs b/src/test/ui/impl-trait/feature-self-return-type.rs
new file mode 100644
index 00000000000..51877e9cc3c
--- /dev/null
+++ b/src/test/ui/impl-trait/feature-self-return-type.rs
@@ -0,0 +1,102 @@
+// edition:2018
+#![feature(impl_trait_projections)]
+
+// This test checks that we emit the correct borrowck error when `Self` or a projection is used as
+// a return type.  See #61949 for context.
+
+mod with_self {
+    pub struct Foo<'a> {
+        pub bar: &'a i32,
+    }
+
+    impl<'a> Foo<'a> {
+        pub fn new(_bar: &'a i32) -> impl Into<Self> {
+            Foo {
+                bar: &22
+            }
+        }
+    }
+
+    fn foo() {
+        let x = {
+            let bar = 22;
+            Foo::new(&bar).into()
+            //~^ ERROR `bar` does not live long enough
+        };
+        drop(x);
+    }
+}
+
+struct Foo<T>(T);
+
+trait FooLike {
+    type Output;
+}
+
+impl<T> FooLike for Foo<T> {
+    type Output = T;
+}
+
+mod impl_trait {
+    use super::*;
+
+    trait Trait {
+        type Assoc;
+
+        fn make_assoc(self) -> Self::Assoc;
+    }
+
+    /// `T::Assoc` can't be normalized any further here.
+    fn foo<T: Trait>(x: T) -> impl FooLike<Output = T::Assoc> {
+        Foo(x.make_assoc())
+    }
+
+    impl<'a> Trait for &'a () {
+        type Assoc = &'a ();
+
+        fn make_assoc(self) -> &'a () { &() }
+    }
+
+    fn usage() {
+        let x = {
+            let y = ();
+            foo(&y)
+            //~^ ERROR `y` does not live long enough
+        };
+        drop(x);
+    }
+}
+
+// Same with lifetimes in the trait
+
+mod lifetimes {
+    use super::*;
+
+    trait Trait<'a> {
+        type Assoc;
+
+        fn make_assoc(self) -> Self::Assoc;
+    }
+
+    /// Missing bound constraining `Assoc`, `T::Assoc` can't be normalized further.
+    fn foo<'a, T: Trait<'a>>(x: T) -> impl FooLike<Output = T::Assoc> {
+        Foo(x.make_assoc())
+    }
+
+    impl<'a> Trait<'a> for &'a () {
+        type Assoc = &'a ();
+
+        fn make_assoc(self) -> &'a () { &() }
+    }
+
+    fn usage() {
+        let x = {
+            let y = ();
+            foo(&y)
+            //~^ ERROR `y` does not live long enough
+        };
+        drop(x);
+    }
+}
+
+fn main() { }
diff --git a/src/test/ui/impl-trait/feature-self-return-type.stderr b/src/test/ui/impl-trait/feature-self-return-type.stderr
new file mode 100644
index 00000000000..601e53b7694
--- /dev/null
+++ b/src/test/ui/impl-trait/feature-self-return-type.stderr
@@ -0,0 +1,39 @@
+error[E0597]: `bar` does not live long enough
+  --> $DIR/feature-self-return-type.rs:23:22
+   |
+LL |         let x = {
+   |             - borrow later stored here
+LL |             let bar = 22;
+LL |             Foo::new(&bar).into()
+   |                      ^^^^ borrowed value does not live long enough
+LL |
+LL |         };
+   |         - `bar` dropped here while still borrowed
+
+error[E0597]: `y` does not live long enough
+  --> $DIR/feature-self-return-type.rs:63:17
+   |
+LL |         let x = {
+   |             - borrow later stored here
+LL |             let y = ();
+LL |             foo(&y)
+   |                 ^^ borrowed value does not live long enough
+LL |
+LL |         };
+   |         - `y` dropped here while still borrowed
+
+error[E0597]: `y` does not live long enough
+  --> $DIR/feature-self-return-type.rs:95:17
+   |
+LL |         let x = {
+   |             - borrow later stored here
+LL |             let y = ();
+LL |             foo(&y)
+   |                 ^^ borrowed value does not live long enough
+LL |
+LL |         };
+   |         - `y` dropped here while still borrowed
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/src/test/ui/impl-trait/impl-fn-predefined-lifetimes.stderr b/src/test/ui/impl-trait/impl-fn-predefined-lifetimes.stderr
index 7747319c153..c19420bbb0c 100644
--- a/src/test/ui/impl-trait/impl-fn-predefined-lifetimes.stderr
+++ b/src/test/ui/impl-trait/impl-fn-predefined-lifetimes.stderr
@@ -14,10 +14,7 @@ error[E0720]: cannot resolve opaque type
   --> $DIR/impl-fn-predefined-lifetimes.rs:4:35
    |
 LL | fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
-   |                                   ^^^^^^^^^^^^^^^ recursive opaque type
-...
-LL |     |x| x
-   |     ----- returning here with type `[closure@$DIR/impl-fn-predefined-lifetimes.rs:7:5: 7:8]`
+   |                                   ^^^^^^^^^^^^^^^ cannot resolve opaque type
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/nll/ty-outlives/impl-trait-captures.stderr b/src/test/ui/nll/ty-outlives/impl-trait-captures.stderr
index 737cb35841c..7b9ed171d2d 100644
--- a/src/test/ui/nll/ty-outlives/impl-trait-captures.stderr
+++ b/src/test/ui/nll/ty-outlives/impl-trait-captures.stderr
@@ -1,4 +1,4 @@
-error[E0700]: hidden type for `Opaque(DefId(0:13 ~ impl_trait_captures[1afc]::foo::{opaque#0}), [ReStatic, T, ReEarlyBound(0, 'a)])` captures lifetime that does not appear in bounds
+error[E0700]: hidden type for `Opaque(DefId(0:13 ~ impl_trait_captures[1afc]::foo::{opaque#0}), [ReEarlyBound(0, 'a), T, ReEarlyBound(0, 'a)])` captures lifetime that does not appear in bounds
   --> $DIR/impl-trait-captures.rs:11:5
    |
 LL | fn foo<'a, T>(x: &T) -> impl Foo<'a> {
diff --git a/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.rs b/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.rs
index 428194058c3..01d1f5db132 100644
--- a/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.rs
+++ b/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.rs
@@ -2,6 +2,6 @@
 
 type Opaque<'a, T> = impl Sized;
 fn defining<'a, T>(x: &'a i32) -> Opaque<T> { x }
-//~^ ERROR: non-defining opaque type use in defining scope
+//~^ ERROR: hidden type for `Opaque<'a, T>` captures lifetime that does not appear in bounds
 
 fn main() {}
diff --git a/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.stderr b/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.stderr
index df2b3ed1911..65a0af0d22f 100644
--- a/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.stderr
+++ b/src/test/ui/type-alias-impl-trait/missing_lifetime_bound.stderr
@@ -1,8 +1,11 @@
-error: non-defining opaque type use in defining scope
+error[E0700]: hidden type for `Opaque<'a, T>` captures lifetime that does not appear in bounds
   --> $DIR/missing_lifetime_bound.rs:4:47
    |
 LL | fn defining<'a, T>(x: &'a i32) -> Opaque<T> { x }
-   |                                               ^ lifetime `'a` is part of concrete type but not used in parameter list of the `impl Trait` type alias
+   |             --                                ^
+   |             |
+   |             hidden type `&'a i32` captures the lifetime `'a` as defined here
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0700`.