warn against implementing Freeze

Author: RalfJung

library/core/src/marker.rs

@@ -817,6 +817,11 @@ pub trait DiscriminantKind {
 /// This can be used to declare that a constant with a generic type
 /// will not contain interior mutability, and subsequently allow
 /// placing the constant behind references.
+///
+/// This trait is a core part of the language, it just just expressed as a trait in libcore for
+/// convenience. Do *not* implement it for other types.
+// FIXME: Eventually this trait should become `#[rustc_deny_explicit_impl]`.
+// That requires porting the impls below to native internal impls.
 #[lang = "freeze"]
 #[unstable(feature = "freeze", issue = "121675")]
 pub unsafe auto trait Freeze {}