TAIT: use hack in ->HIR to avoid more changes

Author: Centril

Diff for: src/librustc/hir/lowering.rs

@@ -452,7 +452,6 @@ impl<'a> LoweringContext<'a> {
                     | ItemKind::Union(_, ref generics)
                     | ItemKind::Enum(_, ref generics)
                     | ItemKind::TyAlias(_, ref generics)
-                    | ItemKind::OpaqueTy(_, ref generics)
                     | ItemKind::Trait(_, _, ref generics, ..) => {
                         let def_id = self.lctx.resolver.definitions().local_def_id(item.id);
                         let count = generics

Diff for: src/librustc/hir/lowering/item.rs

@@ -335,20 +335,22 @@ impl LoweringContext<'_> {
             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
-            ItemKind::TyAlias(ref t, ref generics) => hir::ItemKind::TyAlias(
-                self.lower_ty(t, ImplTraitContext::disallowed()),
-                self.lower_generics(generics, ImplTraitContext::disallowed()),
-            ),
-            ItemKind::OpaqueTy(ref b, ref generics) => hir::ItemKind::OpaqueTy(
-                hir::OpaqueTy {
-                    generics: self.lower_generics(generics,
-                        ImplTraitContext::OpaqueTy(None)),
-                    bounds: self.lower_param_bounds(b,
-                        ImplTraitContext::OpaqueTy(None)),
-                    impl_trait_fn: None,
-                    origin: hir::OpaqueTyOrigin::TypeAlias,
+            ItemKind::TyAlias(ref ty, ref generics) => match ty.kind.opaque_top_hack() {
+                None => {
+                    let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
+                    let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
+                    hir::ItemKind::TyAlias(ty, generics)
                 },
-            ),
+                Some(bounds) => {
+                    let ty = hir::OpaqueTy {
+                        generics: self.lower_generics(generics, ImplTraitContext::OpaqueTy(None)),
+                        bounds: self.lower_param_bounds(bounds, ImplTraitContext::OpaqueTy(None)),
+                        impl_trait_fn: None,
+                        origin: hir::OpaqueTyOrigin::TypeAlias,
+                    };
+                    hir::ItemKind::OpaqueTy(ty)
+                }
+            }
             ItemKind::Enum(ref enum_definition, ref generics) => {
                 hir::ItemKind::Enum(
                     hir::EnumDef {
@@ -914,16 +916,20 @@ impl LoweringContext<'_> {
 
                 (generics, hir::ImplItemKind::Method(sig, body_id))
             }
-            ImplItemKind::TyAlias(ref ty) => (
-                self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
-                hir::ImplItemKind::TyAlias(self.lower_ty(ty, ImplTraitContext::disallowed())),
-            ),
-            ImplItemKind::OpaqueTy(ref bounds) => (
-                self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
-                hir::ImplItemKind::OpaqueTy(
-                    self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
-                ),
-            ),
+            ImplItemKind::TyAlias(ref ty) => {
+                let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
+                let kind = match ty.kind.opaque_top_hack() {
+                    None => {
+                        let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
+                        hir::ImplItemKind::TyAlias(ty)
+                    }
+                    Some(bs) => {
+                        let bounds = self.lower_param_bounds(bs, ImplTraitContext::disallowed());
+                        hir::ImplItemKind::OpaqueTy(bounds)
+                    }
+                };
+                (generics, kind)
+            },
             ImplItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
         };
 
@@ -948,11 +954,13 @@ impl LoweringContext<'_> {
             span: i.span,
             vis: self.lower_visibility(&i.vis, Some(i.id)),
             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
-            kind: match i.kind {
+            kind: match &i.kind {
                 ImplItemKind::Const(..) => hir::AssocItemKind::Const,
-                ImplItemKind::TyAlias(..) => hir::AssocItemKind::Type,
-                ImplItemKind::OpaqueTy(..) => hir::AssocItemKind::OpaqueTy,
-                ImplItemKind::Method(ref sig, _) => hir::AssocItemKind::Method {
+                ImplItemKind::TyAlias(ty) => match ty.kind.opaque_top_hack() {
+                    None => hir::AssocItemKind::Type,
+                    Some(_) => hir::AssocItemKind::OpaqueTy,
+                },
+                ImplItemKind::Method(sig, _) => hir::AssocItemKind::Method {
                     has_self: sig.decl.has_self(),
                 },
                 ImplItemKind::Macro(..) => unimplemented!(),

Diff for: src/librustc/hir/map/def_collector.rs

@@ -107,7 +107,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
             }
             ItemKind::Mod(..) | ItemKind::Trait(..) | ItemKind::TraitAlias(..) |
             ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) |
-            ItemKind::OpaqueTy(..) | ItemKind::ExternCrate(..) | ItemKind::ForeignMod(..) |
+            ItemKind::ExternCrate(..) | ItemKind::ForeignMod(..) |
             ItemKind::TyAlias(..) => DefPathData::TypeNs(i.ident.name),
             ItemKind::Fn(sig, generics, body) if sig.header.asyncness.node.is_async() => {
                 return self.visit_async_fn(
@@ -239,8 +239,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
             }
             ImplItemKind::Method(..) |
             ImplItemKind::Const(..) => DefPathData::ValueNs(ii.ident.name),
-            ImplItemKind::TyAlias(..) |
-            ImplItemKind::OpaqueTy(..) => DefPathData::TypeNs(ii.ident.name),
+            ImplItemKind::TyAlias(..) => DefPathData::TypeNs(ii.ident.name),
             ImplItemKind::Macro(..) => return self.visit_macro_invoc(ii.id),
         };
 

Diff for: src/libsyntax/ast.rs

@@ -1815,6 +1815,15 @@ impl TyKind {
             false
         }
     }
+
+    /// HACK(type_alias_impl_trait, Centril): A temporary crutch used
+    /// in lowering to avoid making larger changes there and beyond.
+    pub fn opaque_top_hack(&self) -> Option<&GenericBounds> {
+        match self {
+            Self::ImplTrait(_, bounds) => Some(bounds),
+            _ => None,
+        }
+    }
 }
 
 /// Syntax used to declare a trait object.