cite reservation impls tracking issue

Author: nikomatsakis

src/librustc/ty/mod.rs

@@ -174,6 +174,9 @@ pub enum ImplPolarity {
     /// `impl !Trait for Type`
     Negative,
     /// `#[rustc_reservation_impl] impl Trait for Type`
+    ///
+    /// This is a "stability hack", not a real Rust feature.
+    /// See #64631 for details.
     Reservation,
 }