make mem::Discriminant invariant

Author: lcnr

src/libcore/mem/mod.rs

@@ -929,8 +929,9 @@ pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
 /// See the [`discriminant`] function in this module for more information.
 ///
 /// [`discriminant`]: fn.discriminant.html
+// We use `PhantomData<fn(T) -> T>` because we want `Discriminant` to be invariant.
 #[stable(feature = "discriminant_value", since = "1.21.0")]
-pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
+pub struct Discriminant<T>(u64, PhantomData<fn(T) -> T>);
 
 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.