add comment to Rc/Arc's Eq specialization

src/liballoc/rc.rs

@@ -932,6 +932,11 @@ impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
     }
 }
 
+/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
+/// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
+/// store large values, that are slow to clone, but also heavy to check for equality, causing this
+/// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
+/// the same value, than two `&T`s.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
     #[inline]

src/liballoc/sync.rs

@@ -1377,6 +1377,11 @@ impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
     }
 }
 
+/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
+/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
+/// store large values, that are slow to clone, but also heavy to check for equality, causing this
+/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
+/// the same value, than two `&T`s.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
     #[inline]