Address nits

compiler/rustc_hir/src/def.rs

@@ -193,7 +193,6 @@ impl DefKind {
             | DefKind::Variant
             | DefKind::Trait
             | DefKind::OpaqueTy
-            | DefKind::ImplTraitPlaceholder
             | DefKind::TyAlias
             | DefKind::ForeignTy
             | DefKind::TraitAlias
@@ -221,7 +220,8 @@ impl DefKind {
             | DefKind::Use
             | DefKind::ForeignMod
             | DefKind::GlobalAsm
-            | DefKind::Impl => None,
+            | DefKind::Impl
+            | DefKind::ImplTraitPlaceholder => None,
         }
     }
 

compiler/rustc_hir/src/hir.rs

@@ -2551,7 +2551,10 @@ 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
+    /// A type that represents an `impl Trait` in a trait function. This is
+    /// not an opaque type, since it acts more like an associated type than
+    /// an opaque, and since it needs no generics since it inherits those
+    /// from the item's parent.
     ImplTraitInTrait(ItemId),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.