RPITIT placeholder items

21 files changed, 70 insertions, 2 deletions

diff --git a/compiler/rustc_hir/src/def.rs b/compiler/rustc_hir/src/def.rs
index b44ee02cfe3..8563b588585 100644
--- a/compiler/rustc_hir/src/def.rs
+++ b/compiler/rustc_hir/src/def.rs
@@ -109,6 +109,8 @@ pub enum DefKind {
     InlineConst,
     /// Opaque type, aka `impl Trait`.
     OpaqueTy,
+    /// A return-position `impl Trait` in a trait definition
+    ImplTraitPlaceholder,
     Field,
     /// Lifetime parameter: the `'a` in `struct Foo<'a> { ... }`
     LifetimeParam,
@@ -138,6 +140,7 @@ impl DefKind {
                 panic!("impossible struct constructor")
             }
             DefKind::OpaqueTy => "opaque type",
+            DefKind::ImplTraitPlaceholder => "opaque type in trait",
             DefKind::TyAlias => "type alias",
             DefKind::TraitAlias => "trait alias",
             DefKind::AssocTy => "associated type",
@@ -190,6 +193,7 @@ impl DefKind {
             | DefKind::Variant
             | DefKind::Trait
             | DefKind::OpaqueTy
+            | DefKind::ImplTraitPlaceholder
             | DefKind::TyAlias
             | DefKind::ForeignTy
             | DefKind::TraitAlias
@@ -254,6 +258,7 @@ impl DefKind {
             | DefKind::Use
             | DefKind::ForeignMod
             | DefKind::OpaqueTy
+            | DefKind::ImplTraitPlaceholder
             | DefKind::Impl
             | DefKind::Field
             | DefKind::TyParam
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index cc5aed6cb54..334d4fd0b04 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -2518,6 +2518,12 @@ pub enum OpaqueTyOrigin {
     TyAlias,
 }
 
+/// Placeholder representation of an `impl Trait` in a trait. Since this never gets lowered into a `ty::Opaque` of its own, we just keep this as
+#[derive(Debug, HashStable_Generic)]
+pub struct ImplTraitPlaceholder<'hir> {
+    pub bounds: GenericBounds<'hir>,
+}
+
 /// The various kinds of types recognized by the compiler.
 #[derive(Debug, HashStable_Generic)]
 pub enum TyKind<'hir> {
@@ -2545,6 +2551,8 @@ pub enum TyKind<'hir> {
     /// The generic argument list contains the lifetimes (and in the future
     /// possibly parameters) that are actually bound on the `impl Trait`.
     OpaqueDef(ItemId, &'hir [GenericArg<'hir>]),
+    /// The placeholder
+    ImplTraitInTrait(ItemId),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.
     TraitObject(&'hir [PolyTraitRef<'hir>], &'hir Lifetime, TraitObjectSyntax),
@@ -3000,6 +3008,8 @@ pub enum ItemKind<'hir> {
     TyAlias(&'hir Ty<'hir>, &'hir Generics<'hir>),
     /// An opaque `impl Trait` type alias, e.g., `type Foo = impl Bar;`.
     OpaqueTy(OpaqueTy<'hir>),
+    /// An `impl Trait` in a trait
+    ImplTraitPlaceholder(ImplTraitPlaceholder<'hir>),
     /// An enum definition, e.g., `enum Foo<A, B> {C<A>, D<B>}`.
     Enum(EnumDef<'hir>, &'hir Generics<'hir>),
     /// A struct definition, e.g., `struct Foo<A> {x: A}`.
@@ -3068,6 +3078,7 @@ impl ItemKind<'_> {
             ItemKind::Trait(..) => "trait",
             ItemKind::TraitAlias(..) => "trait alias",
             ItemKind::Impl(..) => "implementation",
+            ItemKind::ImplTraitPlaceholder(..) => "opaque type in trait",
         }
     }
 }
diff --git a/compiler/rustc_hir/src/intravisit.rs b/compiler/rustc_hir/src/intravisit.rs
index 20ec788463a..3e9ab10b1f7 100644
--- a/compiler/rustc_hir/src/intravisit.rs
+++ b/compiler/rustc_hir/src/intravisit.rs
@@ -562,6 +562,10 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item<'v>) {
             walk_generics(visitor, generics);
             walk_list!(visitor, visit_param_bound, bounds);
         }
+        ItemKind::ImplTraitPlaceholder(ImplTraitPlaceholder { bounds }) => {
+            visitor.visit_id(item.hir_id());
+            walk_list!(visitor, visit_param_bound, bounds);
+        }
         ItemKind::Enum(ref enum_definition, ref generics) => {
             visitor.visit_generics(generics);
             // `visit_enum_def()` takes care of visiting the `Item`'s `HirId`.
@@ -674,6 +678,9 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) {
             visitor.visit_nested_item(item_id);
             walk_list!(visitor, visit_generic_arg, lifetimes);
         }
+        TyKind::ImplTraitInTrait(item_id) => {
+            visitor.visit_nested_item(item_id);
+        }
         TyKind::Array(ref ty, ref length) => {
             visitor.visit_ty(ty);
             visitor.visit_array_length(length)
diff --git a/compiler/rustc_hir/src/target.rs b/compiler/rustc_hir/src/target.rs
index 78bfd7191db..ea23c48cc53 100644
--- a/compiler/rustc_hir/src/target.rs
+++ b/compiler/rustc_hir/src/target.rs
@@ -36,6 +36,7 @@ pub enum Target {
     GlobalAsm,
     TyAlias,
     OpaqueTy,
+    ImplTraitPlaceholder,
     Enum,
     Variant,
     Struct,
@@ -80,6 +81,7 @@ impl Target {
             ItemKind::GlobalAsm(..) => Target::GlobalAsm,
             ItemKind::TyAlias(..) => Target::TyAlias,
             ItemKind::OpaqueTy(..) => Target::OpaqueTy,
+            ItemKind::ImplTraitPlaceholder(..) => Target::ImplTraitPlaceholder,
             ItemKind::Enum(..) => Target::Enum,
             ItemKind::Struct(..) => Target::Struct,
             ItemKind::Union(..) => Target::Union,
@@ -157,6 +159,7 @@ impl Target {
             Target::GlobalAsm => "global asm",
             Target::TyAlias => "type alias",
             Target::OpaqueTy => "opaque type",
+            Target::ImplTraitPlaceholder => "opaque type in trait",
             Target::Enum => "enum",
             Target::Variant => "enum variant",
             Target::Struct => "struct",
diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index 35a58296e37..01f445c8a52 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/compiler/rustc_hir_pretty/src/lib.rs
@@ -327,6 +327,7 @@ impl<'a> State<'a> {
                 self.print_ty_fn(f.abi, f.unsafety, f.decl, None, f.generic_params, f.param_names);
             }
             hir::TyKind::OpaqueDef(..) => self.word("/*impl Trait*/"),
+            hir::TyKind::ImplTraitInTrait(..) => self.word("/*impl Trait*/"),
             hir::TyKind::Path(ref qpath) => self.print_qpath(qpath, false),
             hir::TyKind::TraitObject(bounds, ref lifetime, syntax) => {
                 if syntax == ast::TraitObjectSyntax::Dyn {
@@ -608,6 +609,9 @@ impl<'a> State<'a> {
                     state.print_bounds("= impl", real_bounds);
                 });
             }
+            hir::ItemKind::ImplTraitPlaceholder(..) => {
+                unreachable!("FIXME(RPITIT): I don't think this ever gets called here...");
+            }
             hir::ItemKind::Enum(ref enum_definition, params) => {
                 self.print_enum_def(enum_definition, params, item.ident.name, item.span);
             }
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index b807663b10f..02ca34181cd 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -847,6 +847,7 @@ fn should_encode_visibility(def_kind: DefKind) -> bool {
         | DefKind::Use
         | DefKind::ForeignMod
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Impl
         | DefKind::Field => true,
         DefKind::TyParam
@@ -879,6 +880,7 @@ fn should_encode_stability(def_kind: DefKind) -> bool {
         | DefKind::ForeignMod
         | DefKind::TyAlias
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Enum
         | DefKind::Union
         | DefKind::Impl
@@ -967,6 +969,7 @@ fn should_encode_variances(def_kind: DefKind) -> bool {
         | DefKind::ForeignMod
         | DefKind::TyAlias
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Impl
         | DefKind::Trait
         | DefKind::TraitAlias
@@ -1003,6 +1006,7 @@ fn should_encode_generics(def_kind: DefKind) -> bool {
         | DefKind::AnonConst
         | DefKind::InlineConst
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Impl
         | DefKind::Field
         | DefKind::TyParam
@@ -1493,6 +1497,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             }
             hir::ItemKind::OpaqueTy(..) => {
                 self.encode_explicit_item_bounds(def_id);
+                EntryKind::OpaqueTy
             }
             hir::ItemKind::Enum(..) => {
                 let adt_def = self.tcx.adt_def(def_id);
diff --git a/compiler/rustc_metadata/src/rmeta/table.rs b/compiler/rustc_metadata/src/rmeta/table.rs
index d5f151f0ed8..e7c1abd126e 100644
--- a/compiler/rustc_metadata/src/rmeta/table.rs
+++ b/compiler/rustc_metadata/src/rmeta/table.rs
@@ -90,6 +90,7 @@ fixed_size_enum! {
         ( AnonConst                                )
         ( InlineConst                              )
         ( OpaqueTy                                 )
+        ( ImplTraitPlaceholder                     )
         ( Field                                    )
         ( LifetimeParam                            )
         ( GlobalAsm                                )
diff --git a/compiler/rustc_middle/src/hir/map/mod.rs b/compiler/rustc_middle/src/hir/map/mod.rs
index 6217bffb8f7..1a0bea68293 100644
--- a/compiler/rustc_middle/src/hir/map/mod.rs
+++ b/compiler/rustc_middle/src/hir/map/mod.rs
@@ -213,6 +213,7 @@ impl<'hir> Map<'hir> {
                 ItemKind::Macro(_, macro_kind) => DefKind::Macro(macro_kind),
                 ItemKind::Mod(..) => DefKind::Mod,
                 ItemKind::OpaqueTy(..) => DefKind::OpaqueTy,
+                ItemKind::ImplTraitPlaceholder(..) => DefKind::ImplTraitPlaceholder,
                 ItemKind::TyAlias(..) => DefKind::TyAlias,
                 ItemKind::Enum(..) => DefKind::Enum,
                 ItemKind::Struct(..) => DefKind::Struct,
@@ -1188,6 +1189,7 @@ fn hir_id_to_string(map: Map<'_>, id: HirId) -> String {
                 ItemKind::GlobalAsm(..) => "global asm",
                 ItemKind::TyAlias(..) => "ty",
                 ItemKind::OpaqueTy(..) => "opaque type",
+                ItemKind::ImplTraitPlaceholder(..) => "opaque type in trait",
                 ItemKind::Enum(..) => "enum",
                 ItemKind::Struct(..) => "struct",
                 ItemKind::Union(..) => "union",
diff --git a/compiler/rustc_monomorphize/src/polymorphize.rs b/compiler/rustc_monomorphize/src/polymorphize.rs
index af4b35db3ba..98156a94ec4 100644
--- a/compiler/rustc_monomorphize/src/polymorphize.rs
+++ b/compiler/rustc_monomorphize/src/polymorphize.rs
@@ -170,6 +170,7 @@ fn mark_used_by_default_parameters<'tcx>(
         | DefKind::AnonConst
         | DefKind::InlineConst
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Field
         | DefKind::LifetimeParam
         | DefKind::GlobalAsm
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index d0b46aa2c45..bb684e784c5 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -661,6 +661,7 @@ impl CheckAttrVisitor<'_> {
             | Target::GlobalAsm
             | Target::TyAlias
             | Target::OpaqueTy
+            | Target::ImplTraitPlaceholder
             | Target::Enum
             | Target::Variant
             | Target::Struct
diff --git a/compiler/rustc_passes/src/reachable.rs b/compiler/rustc_passes/src/reachable.rs
index f7e3fac6b2e..74c5ccf9cd2 100644
--- a/compiler/rustc_passes/src/reachable.rs
+++ b/compiler/rustc_passes/src/reachable.rs
@@ -235,6 +235,7 @@ impl<'tcx> ReachableContext<'tcx> {
                     hir::ItemKind::ExternCrate(_)
                     | hir::ItemKind::Use(..)
                     | hir::ItemKind::OpaqueTy(..)
+                    | hir::ItemKind::ImplTraitPlaceholder(..)
                     | hir::ItemKind::TyAlias(..)
                     | hir::ItemKind::Macro(..)
                     | hir::ItemKind::Mod(..)
diff --git a/compiler/rustc_privacy/src/lib.rs b/compiler/rustc_privacy/src/lib.rs
index b2966f0d218..309eb753553 100644
--- a/compiler/rustc_privacy/src/lib.rs
+++ b/compiler/rustc_privacy/src/lib.rs
@@ -596,6 +596,7 @@ impl<'tcx> EmbargoVisitor<'tcx> {
             | DefKind::ForeignTy
             | DefKind::Fn
             | DefKind::OpaqueTy
+            | DefKind::ImplTraitPlaceholder
             | DefKind::AssocFn
             | DefKind::Trait
             | DefKind::TyParam
@@ -685,6 +686,7 @@ impl<'tcx> Visitor<'tcx> for EmbargoVisitor<'tcx> {
             }
 
             hir::ItemKind::OpaqueTy(..)
+            | hir::ItemKind::ImplTraitPlaceholder(..)
             | hir::ItemKind::Use(..)
             | hir::ItemKind::Static(..)
             | hir::ItemKind::Const(..)
@@ -720,6 +722,9 @@ impl<'tcx> Visitor<'tcx> for EmbargoVisitor<'tcx> {
                     self.reach(item.def_id, exist_level).generics().predicates().ty();
                 }
             }
+            hir::ItemKind::ImplTraitPlaceholder(..) => {
+                // FIXME(RPITIT): Do we need to do anything here?
+            }
             // Visit everything.
             hir::ItemKind::Const(..)
             | hir::ItemKind::Static(..)
diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 9cb735b3685..81b67b758f7 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -972,6 +972,7 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                 | DefKind::TyAlias
                 | DefKind::ForeignTy
                 | DefKind::OpaqueTy
+                | DefKind::ImplTraitPlaceholder
                 | DefKind::TraitAlias
                 | DefKind::AssocTy,
                 _,
diff --git a/compiler/rustc_resolve/src/late/lifetimes.rs b/compiler/rustc_resolve/src/late/lifetimes.rs
index 101679aa6dc..b72b033534b 100644
--- a/compiler/rustc_resolve/src/late/lifetimes.rs
+++ b/compiler/rustc_resolve/src/late/lifetimes.rs
@@ -596,6 +596,9 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     }
                 }
             }
+            hir::ItemKind::ImplTraitPlaceholder(..) => {
+                // FIXME(RPITIT): We don't need to do anything here, right?
+            }
             hir::ItemKind::TyAlias(_, ref generics)
             | hir::ItemKind::Enum(_, ref generics)
             | hir::ItemKind::Struct(_, ref generics)
diff --git a/compiler/rustc_save_analysis/src/lib.rs b/compiler/rustc_save_analysis/src/lib.rs
index 89bca39512f..ebe44a56449 100644
--- a/compiler/rustc_save_analysis/src/lib.rs
+++ b/compiler/rustc_save_analysis/src/lib.rs
@@ -685,6 +685,7 @@ impl<'tcx> SaveContext<'tcx> {
                 | HirDefKind::AssocTy
                 | HirDefKind::Trait
                 | HirDefKind::OpaqueTy
+                | HirDefKind::ImplTraitPlaceholder
                 | HirDefKind::TyParam,
                 def_id,
             ) => Some(Ref { kind: RefKind::Type, span, ref_id: id_from_def_id(def_id) }),
diff --git a/compiler/rustc_save_analysis/src/sig.rs b/compiler/rustc_save_analysis/src/sig.rs
index d1286c9b8b0..ea9c82b3597 100644
--- a/compiler/rustc_save_analysis/src/sig.rs
+++ b/compiler/rustc_save_analysis/src/sig.rs
@@ -320,6 +320,10 @@ impl<'hir> Sig for hir::Ty<'hir> {
                 let item = scx.tcx.hir().item(item_id);
                 item.make(offset, Some(item_id.hir_id()), scx)
             }
+            hir::TyKind::ImplTraitInTrait(item_id) => {
+                let item = scx.tcx.hir().item(item_id);
+                item.make(offset, Some(item_id.hir_id()), scx)
+            }
             hir::TyKind::Typeof(_) | hir::TyKind::Infer | hir::TyKind::Err => Err("Ty"),
         }
     }
@@ -562,6 +566,7 @@ impl<'hir> Sig for hir::Item<'hir> {
             hir::ItemKind::GlobalAsm(_) => Err("global asm"),
             hir::ItemKind::ExternCrate(_) => Err("extern crate"),
             hir::ItemKind::OpaqueTy(..) => Err("opaque type"),
+            hir::ItemKind::ImplTraitPlaceholder(..) => Err("opaque type in trait"),
             // FIXME should implement this (e.g., pub use).
             hir::ItemKind::Use(..) => Err("import"),
         }
diff --git a/compiler/rustc_ty_utils/src/implied_bounds.rs b/compiler/rustc_ty_utils/src/implied_bounds.rs
index a77ea440aaa..f0d8c240ea5 100644
--- a/compiler/rustc_ty_utils/src/implied_bounds.rs
+++ b/compiler/rustc_ty_utils/src/implied_bounds.rs
@@ -51,6 +51,7 @@ fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx ty::List<Ty
         | DefKind::AnonConst
         | DefKind::InlineConst
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Field
         | DefKind::LifetimeParam
         | DefKind::GlobalAsm
diff --git a/compiler/rustc_typeck/src/astconv/mod.rs b/compiler/rustc_typeck/src/astconv/mod.rs
index d66cf6d099a..1f43221a420 100644
--- a/compiler/rustc_typeck/src/astconv/mod.rs
+++ b/compiler/rustc_typeck/src/astconv/mod.rs
@@ -2638,6 +2638,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
                 }
             }
+            hir::TyKind::ImplTraitInTrait(..) => {
+                span_bug!(ast_ty.span, "not yet implemented")
+            }
             hir::TyKind::Path(hir::QPath::TypeRelative(ref qself, ref segment)) => {
                 debug!(?qself, ?segment);
                 let ty = self.ast_ty_to_ty_inner(qself, false, true);
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index e70f728d7dc..293a32051b9 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -815,6 +815,11 @@ fn convert_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
             tcx.ensure().predicates_of(def_id);
             tcx.ensure().explicit_item_bounds(def_id);
         }
+        hir::ItemKind::ImplTraitPlaceholder(..) => {
+            tcx.ensure().generics_of(def_id);
+            tcx.ensure().predicates_of(def_id);
+            tcx.ensure().explicit_item_bounds(def_id);
+        }
         hir::ItemKind::TyAlias(..)
         | hir::ItemKind::Static(..)
         | hir::ItemKind::Const(..)
diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index a0280ddca4b..ded2ac1308e 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -336,6 +336,9 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                 ItemKind::OpaqueTy(OpaqueTy { origin: hir::OpaqueTyOrigin::FnReturn(owner) | hir::OpaqueTyOrigin::AsyncFn(owner), .. }) => {
                     find_opaque_ty_constraints_for_rpit(tcx, def_id, owner)
                 }
+                ItemKind::ImplTraitPlaceholder(..) => {
+                    span_bug!(item.span, "not yet implemented")
+                }
                 ItemKind::Trait(..)
                 | ItemKind::TraitAlias(..)
                 | ItemKind::Macro(..)
diff --git a/src/librustdoc/clean/utils.rs b/src/librustdoc/clean/utils.rs
index a9d511ae11e..395f213ca87 100644
--- a/src/librustdoc/clean/utils.rs
+++ b/src/librustdoc/clean/utils.rs
@@ -511,8 +511,8 @@ pub(crate) fn register_res(cx: &mut DocContext<'_>, res: Res) -> DefId {
         | Res::Err => return res.def_id(),
         Res::Def(
             TyParam | ConstParam | Ctor(..) | ExternCrate | Use | ForeignMod | AnonConst
-            | InlineConst | OpaqueTy | Field | LifetimeParam | GlobalAsm | Impl | Closure
-            | Generator,
+            | InlineConst | OpaqueTy | ImplTraitPlaceholder | Field | LifetimeParam | GlobalAsm
+            | Impl | Closure | Generator,
             id,
         ) => return id,
     };