Rollup merge of #60727 - chpio:master, r=sfackler

add comment to `Rc`/`Arc`'s `Eq` specialization

in addition to https://github.com/rust-lang/rust/pull/56550

https://github.com/rust-lang/rust/pull/42965#issuecomment-312702479

2 files changed, 10 insertions, 0 deletions

diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 68eecd97ea1..0dffb19476f 100644
--- a/src/liballoc/rc.rs
+++ b/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]
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 466e806663c..90c7859b3db 100644
--- a/src/liballoc/sync.rs
+++ b/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]