Create bidirectional bounds between original and duplicated parameters.

compiler/rustc_ast_lowering/src/lib.rs

@@ -61,8 +61,8 @@ 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;
@@ -1457,17 +1457,11 @@ 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).

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1538,7 +1538,6 @@ fn check_fn_or_method<'tcx>(
 
     check_return_position_impl_trait_in_trait_bounds(
         tcx,
-        wfcx,
         def_id,
         sig.output(),
         hir_decl.output.span(),
@@ -1574,9 +1573,9 @@ 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,
@@ -1590,18 +1589,22 @@ 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);
             }
         }
     }

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -84,40 +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
-                }
-                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
+        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::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,
@@ -161,6 +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>`).
@@ -279,6 +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),

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);
 

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