Lower RPITIT to ImplTraitPlaceholder item

compiler/rustc_ast_lowering/src/lib.rs

@@ -255,6 +255,8 @@ enum ImplTraitContext {
     },
     /// Impl trait in type aliases.
     TypeAliasesOpaqueTy,
+    /// Return-position `impl Trait` in trait definition
+    InTrait,
     /// `impl Trait` is not accepted in this position.
     Disallowed(ImplTraitPosition),
 }
@@ -323,9 +325,17 @@ enum FnDeclKind {
 }
 
 impl FnDeclKind {
-    fn impl_trait_return_allowed(&self) -> bool {
+    fn impl_trait_return_allowed(&self, tcx: TyCtxt<'_>) -> bool {
         match self {
             FnDeclKind::Fn | FnDeclKind::Inherent => true,
+            FnDeclKind::Impl if tcx.features().return_position_impl_trait_in_trait => true,
+            _ => false,
+        }
+    }
+
+    fn impl_trait_in_trait_allowed(&self, tcx: TyCtxt<'_>) -> bool {
+        match self {
+            FnDeclKind::Trait if tcx.features().return_position_impl_trait_in_trait => true,
             _ => false,
         }
     }
@@ -1346,6 +1356,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             &mut nested_itctx,
                         )
                     }
+                    ImplTraitContext::InTrait => {
+                        // FIXME(RPITIT): Should we use def_node_id here?
+                        self.lower_impl_trait_in_trait(span, def_node_id, bounds)
+                    }
                     ImplTraitContext::Universal => {
                         let span = t.span;
                         let ident = Ident::from_str_and_span(&pprust::ty_to_string(t), span);
@@ -1532,6 +1546,32 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         hir::TyKind::OpaqueDef(hir::ItemId { def_id: opaque_ty_def_id }, lifetimes)
     }
 
+    #[tracing::instrument(level = "debug", skip(self))]
+    fn lower_impl_trait_in_trait(
+        &mut self,
+        span: Span,
+        opaque_ty_node_id: NodeId,
+        bounds: &GenericBounds,
+    ) -> hir::TyKind<'hir> {
+        let opaque_ty_def_id = self.local_def_id(opaque_ty_node_id);
+        self.with_hir_id_owner(opaque_ty_node_id, |lctx| {
+            // FIXME(RPITIT): This should be a more descriptive ImplTraitPosition, i.e. nested RPITIT
+            // FIXME(RPITIT): We _also_ should support this eventually
+            let hir_bounds = lctx
+                .lower_param_bounds(bounds, ImplTraitContext::Disallowed(ImplTraitPosition::Trait));
+            let rpitit_placeholder = hir::ImplTraitPlaceholder { bounds: hir_bounds };
+            let rpitit_item = hir::Item {
+                def_id: opaque_ty_def_id,
+                ident: Ident::empty(),
+                kind: hir::ItemKind::ImplTraitPlaceholder(rpitit_placeholder),
+                span: lctx.lower_span(span),
+                vis_span: lctx.lower_span(span.shrink_to_lo()),
+            };
+            hir::OwnerNode::Item(lctx.arena.alloc(rpitit_item))
+        });
+        hir::TyKind::ImplTraitInTrait(hir::ItemId { def_id: opaque_ty_def_id })
+    }
+
     /// Registers a new opaque type with the proper `NodeId`s and
     /// returns the lowered node-ID for the opaque type.
     fn generate_opaque_type(
@@ -1690,12 +1730,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             match decl.output {
                 FnRetTy::Ty(ref ty) => {
                     let mut context = match fn_node_id {
-                        Some(fn_node_id) if kind.impl_trait_return_allowed() => {
+                        Some(fn_node_id) if kind.impl_trait_return_allowed(self.tcx) => {
                             let fn_def_id = self.local_def_id(fn_node_id);
                             ImplTraitContext::ReturnPositionOpaqueTy {
                                 origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
                             }
                         }
+                        Some(_) if kind.impl_trait_in_trait_allowed(self.tcx) => {
+                            ImplTraitContext::InTrait
+                        }
                         _ => ImplTraitContext::Disallowed(match kind {
                             FnDeclKind::Fn | FnDeclKind::Inherent => {
                                 unreachable!("fn should allow in-band lifetimes")

compiler/rustc_feature/src/active.rs

@@ -487,6 +487,8 @@ declare_features! (
     (incomplete, repr128, "1.16.0", Some(56071), None),
     /// Allows `repr(simd)` and importing the various simd intrinsics.
     (active, repr_simd, "1.4.0", Some(27731), None),
+    /// Allows return-position `impl Trait` in traits.
+    (incomplete, return_position_impl_trait_in_trait, "CURRENT_RUSTC_VERSION", Some(91611), None),
     /// Allows `extern "rust-cold"`.
     (active, rust_cold_cc, "1.63.0", Some(97544), None),
     /// Allows the use of SIMD types in functions declared in `extern` blocks.

compiler/rustc_hir/src/target.rs

@@ -105,6 +105,7 @@ impl Target {
             DefKind::GlobalAsm => Target::GlobalAsm,
             DefKind::TyAlias => Target::TyAlias,
             DefKind::OpaqueTy => Target::OpaqueTy,
+            DefKind::ImplTraitPlaceholder => Target::ImplTraitPlaceholder,
             DefKind::Enum => Target::Enum,
             DefKind::Struct => Target::Struct,
             DefKind::Union => Target::Union,

compiler/rustc_middle/src/ty/error.rs

@@ -2,6 +2,7 @@ use crate::traits::{ObligationCause, ObligationCauseCode};
 use crate::ty::diagnostics::suggest_constraining_type_param;
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::{self, BoundRegionKind, Region, Ty, TyCtxt};
+use hir::def::DefKind;
 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
 use rustc_errors::{pluralize, Diagnostic, MultiSpan};
 use rustc_hir as hir;
@@ -538,7 +539,7 @@ impl<T> Trait<T> for X {
                             diag.span_label(p_span, "this type parameter");
                         }
                     }
-                    (ty::Projection(proj_ty), _) => {
+                    (ty::Projection(proj_ty), _) if self.def_kind(proj_ty.item_def_id) != DefKind::ImplTraitPlaceholder => {
                         self.expected_projection(
                             diag,
                             proj_ty,
@@ -547,7 +548,7 @@ impl<T> Trait<T> for X {
                             cause.code(),
                         );
                     }
-                    (_, ty::Projection(proj_ty)) => {
+                    (_, ty::Projection(proj_ty)) if self.def_kind(proj_ty.item_def_id) != DefKind::ImplTraitPlaceholder => {
                         let msg = format!(
                             "consider constraining the associated type `{}` to `{}`",
                             values.found, values.expected,

compiler/rustc_middle/src/ty/sty.rs

@@ -11,6 +11,7 @@ use crate::ty::{
     TypeVisitor,
 };
 use crate::ty::{List, ParamEnv};
+use hir::def::DefKind;
 use polonius_engine::Atom;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::intern::Interned;
@@ -1196,7 +1197,9 @@ pub struct ProjectionTy<'tcx> {
 
 impl<'tcx> ProjectionTy<'tcx> {
     pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
-        tcx.parent(self.item_def_id)
+        let parent = tcx.parent(self.item_def_id);
+        assert_eq!(tcx.def_kind(parent), DefKind::Trait);
+        parent
     }
 
     /// Extracts the underlying trait reference and own substs from this projection.

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -597,7 +597,8 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                 }
             }
             hir::ItemKind::ImplTraitPlaceholder(..) => {
-                // FIXME(RPITIT): We don't need to do anything here, right?
+                // FIXME(RPITIT): We don't need to do anything special here, right?
+                intravisit::walk_item(self, item);
             }
             hir::ItemKind::TyAlias(_, ref generics)
             | hir::ItemKind::Enum(_, ref generics)

compiler/rustc_span/src/symbol.rs

@@ -1183,6 +1183,7 @@ symbols! {
         require,
         residual,
         result,
+        return_position_impl_trait_in_trait,
         rhs,
         rintf32,
         rintf64,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1465,6 +1465,11 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
     obligation: &ProjectionTyObligation<'tcx>,
     candidate_set: &mut ProjectionCandidateSet<'tcx>,
 ) {
+    // Can't assemble candidate from impl for RPITIT
+    if selcx.tcx().def_kind(obligation.predicate.item_def_id) == DefKind::ImplTraitPlaceholder {
+        return;
+    }
+
     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
     // start out by selecting the predicate `T as TraitRef<...>`:
     let poly_trait_ref = ty::Binder::dummy(obligation.predicate.trait_ref(selcx.tcx()));

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2638,8 +2638,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
                 }
             }
-            hir::TyKind::ImplTraitInTrait(..) => {
+            hir::TyKind::ImplTraitInTrait(item_id) => {
-                span_bug!(ast_ty.span, "not yet implemented")
+                let def_id = item_id.def_id.to_def_id();
+                tcx.mk_projection(def_id, InternalSubsts::identity_for_item(tcx, def_id))
             }
             hir::TyKind::Path(hir::QPath::TypeRelative(ref qself, ref segment)) => {
                 debug!(?qself, ?segment);

compiler/rustc_typeck/src/collect.rs

@@ -1600,6 +1600,11 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::Generics {
                 // inherit the generics of the item.
                 Some(parent_id.to_def_id())
             }
+            ItemKind::ImplTraitPlaceholder(_) => {
+                let parent_id = tcx.hir().get_parent_item(hir_id).to_def_id();
+                assert_eq!(tcx.def_kind(parent_id), DefKind::AssocFn);
+                Some(parent_id)
+            }
             _ => None,
         },
         _ => None,

compiler/rustc_typeck/src/collect/item_bounds.rs

@@ -74,6 +74,29 @@ fn opaque_type_bounds<'tcx>(
     })
 }
 
+/// Opaque types don't inherit bounds from their parent: for return position
+/// impl trait it isn't possible to write a suitable predicate on the
+/// containing function and for type-alias impl trait we don't have a backwards
+/// compatibility issue.
+fn impl_trait_in_trait_item_bounds<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    opaque_def_id: DefId,
+    ast_bounds: &'tcx [hir::GenericBound<'tcx>],
+    span: Span,
+) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
+    ty::print::with_no_queries!({
+        // FIXME(RPITIT): DRY-er code please
+        let item_ty =
+            tcx.mk_projection(opaque_def_id, InternalSubsts::identity_for_item(tcx, opaque_def_id));
+
+        let icx = ItemCtxt::new(tcx, opaque_def_id);
+        let mut bounds = <dyn AstConv<'_>>::compute_bounds(&icx, item_ty, ast_bounds);
+        // RPITITs are implicitly sized unless a `?Sized` bound is found
+        <dyn AstConv<'_>>::add_implicitly_sized(&icx, &mut bounds, ast_bounds, None, span);
+        tcx.arena.alloc_from_iter(bounds.predicates(tcx, item_ty))
+    })
+}
+
 pub(super) fn explicit_item_bounds(
     tcx: TyCtxt<'_>,
     def_id: DefId,
@@ -90,6 +113,11 @@ pub(super) fn explicit_item_bounds(
             span,
             ..
         }) => opaque_type_bounds(tcx, def_id, bounds, *span),
+        hir::Node::Item(hir::Item {
+            kind: hir::ItemKind::ImplTraitPlaceholder(hir::ImplTraitPlaceholder { bounds }),
+            span,
+            ..
+        }) => impl_trait_in_trait_item_bounds(tcx, def_id, bounds, *span),
         _ => bug!("item_bounds called on {:?}", def_id),
     }
 }