Account for associated items when denying _

Author: estebank

src/librustc_typeck/collect.rs

@@ -715,13 +715,21 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::HirId) {
     tcx.generics_of(def_id);
 
     match trait_item.kind {
-        hir::TraitItemKind::Const(..)
-        | hir::TraitItemKind::Type(_, Some(_))
-        | hir::TraitItemKind::Method(..) => {
+        hir::TraitItemKind::Method(..) => {
             tcx.type_of(def_id);
-            if let hir::TraitItemKind::Method(..) = trait_item.kind {
-                tcx.fn_sig(def_id);
-            }
+            tcx.fn_sig(def_id);
+        }
+
+        hir::TraitItemKind::Const(.., Some(_)) => {
+            tcx.type_of(def_id);
+        }
+
+        hir::TraitItemKind::Const(..) | hir::TraitItemKind::Type(_, Some(_)) => {
+            tcx.type_of(def_id);
+            // Account for `const C: _;` and `type T = _;`.
+            let mut visitor = PlaceholderHirTyCollector::default();
+            visitor.visit_trait_item(trait_item);
+            placeholder_type_error(tcx, DUMMY_SP, &[], visitor.0, false);
         }
 
         hir::TraitItemKind::Type(_, None) => {}
@@ -735,8 +743,18 @@ fn convert_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::HirId) {
     tcx.generics_of(def_id);
     tcx.type_of(def_id);
     tcx.predicates_of(def_id);
-    if let hir::ImplItemKind::Method(..) = tcx.hir().expect_impl_item(impl_item_id).kind {
-        tcx.fn_sig(def_id);
+    let impl_item = tcx.hir().expect_impl_item(impl_item_id);
+    match impl_item.kind {
+        hir::ImplItemKind::Method(..) => {
+            tcx.fn_sig(def_id);
+        }
+        hir::ImplItemKind::TyAlias(_) | hir::ImplItemKind::OpaqueTy(_) => {
+            // Account for `type T = _;`
+            let mut visitor = PlaceholderHirTyCollector::default();
+            visitor.visit_impl_item(impl_item);
+            placeholder_type_error(tcx, DUMMY_SP, &[], visitor.0, false);
+        }
+        hir::ImplItemKind::Const(..) => {}
     }
 }
 

src/test/ui/typeck/typeck_type_placeholder_item.rs

@@ -1,4 +1,5 @@
-#![feature(type_alias_impl_trait)] // Needed for single test `type Y = impl Trait<_>`
+// Needed for `type Y = impl Trait<_>` and `type B = _;`
+#![feature(type_alias_impl_trait, associated_type_defaults)]
 // This test checks that it is not possible to enable global type
 // inference by using the `_` type placeholder.
 
@@ -188,3 +189,25 @@ type Y = impl Trait<_>;
 fn foo() -> Y {
     Struct
 }
+
+trait Qux {
+    type A;
+    type B = _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    const C: _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    const D: _ = 42;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    // type E: _; // FIXME: make the parser propagate the existence of `B`
+}
+impl Qux for Struct {
+    type A = _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    type B = _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    const C: _;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+    //~| ERROR associated constant in `impl` without body
+    const D: _ = 42;
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
+}