Support using Self or projections inside an RPIT/async fn

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_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> {

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.

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2767,35 +2767,11 @@ 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);
@@ -2972,6 +2948,7 @@ 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>,

compiler/rustc_hir_analysis/src/check/check.rs

@@ -231,7 +231,9 @@ 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;
     }
@@ -240,6 +242,7 @@ 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))]
@@ -251,15 +254,19 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
     let item = tcx.hir().expect_item(def_id);
     debug!(?item, ?span);
 
+    #[derive(Debug)]
     struct FoundParentLifetime;
-    struct FindParentLifetimeVisitor<'tcx>(&'tcx ty::Generics);
+    struct FindParentLifetimeVisitor<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        parent_count: u32,
+    }
     impl<'tcx> ty::visit::TypeVisitor<'tcx> for FindParentLifetimeVisitor<'tcx> {
         type BreakTy = FoundParentLifetime;
 
+        #[instrument(level = "trace", skip(self), ret)]
         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 {
+                if index < self.parent_count {
                     return ControlFlow::Break(FoundParentLifetime);
                 } else {
                     return ControlFlow::CONTINUE;
@@ -269,6 +276,63 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
             r.super_visit_with(self)
         }
 
+        #[instrument(level = "trace", skip(self), ret)]
+        fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+            // We're only interested in types involving regions
+            if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
+                return ControlFlow::CONTINUE;
+            }
+
+            match ty.kind() {
+                ty::Closure(_, ref substs) => {
+                    // Skip lifetime parameters of the enclosing item(s)
+
+                    substs.as_closure().tupled_upvars_ty().visit_with(self)?;
+                    substs.as_closure().sig_as_fn_ptr_ty().visit_with(self)?;
+                }
+
+                ty::Generator(_, ref substs, _) => {
+                    // Skip lifetime parameters of the enclosing item(s)
+                    // Also skip the witness type, because that has no free regions.
+
+                    substs.as_generator().tupled_upvars_ty().visit_with(self)?;
+                    substs.as_generator().return_ty().visit_with(self)?;
+                    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)?;
+                }
+            }
+
+            ControlFlow::CONTINUE
+        }
+
         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
@@ -291,12 +355,15 @@ 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 FindParentLifetimeVisitor {
+                    tcx: self.tcx,
+                    parent_count: self.generics.parent_count as u32,
+                })
+                .map_break(|FoundParentLifetime| t)
             }
         }
     }
@@ -329,14 +396,18 @@ 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()),
-            ),
+            opaque_identity_ty,
             generics: tcx.generics_of(def_id),
             tcx,
             selftys: vec![],
@@ -345,10 +416,6 @@ 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);
@@ -359,15 +426,16 @@ 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,

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);
             }
         }
     }