From 30b27f350ccca8f2b08b54b256f7e1fdb8c8ddb0 Mon Sep 17 00:00:00 2001
From: Andrew Champion <andrew.champion@gmail.com>
Date: Sat, 8 Jun 2019 16:26:34 +0100
Subject: [PATCH] core: check for pointer equality when comparing Eq slices

Because Eq types must be reflexively equal, an equal-length slice to the
same memory location must be equal.
---
 src/libcore/slice/mod.rs | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 0e782bef39dd..f972d13f7c39 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -5304,6 +5304,29 @@ impl<A, B> SlicePartialEq<B> for [A]
     }
 }
 
+// Use an equal-pointer optimization when types are `Eq`
+impl<A> SlicePartialEq<A> for [A]
+    where A: PartialEq<A> + Eq
+{
+    default fn equal(&self, other: &[A]) -> bool {
+        if self.len() != other.len() {
+            return false;
+        }
+
+        if self.as_ptr() == other.as_ptr() {
+            return true;
+        }
+
+        for i in 0..self.len() {
+            if !self[i].eq(&other[i]) {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
 // Use memcmp for bytewise equality when the types allow
 impl<A> SlicePartialEq<A> for [A]
     where A: PartialEq<A> + BytewiseEquality
@@ -5409,7 +5432,7 @@ impl SliceOrd<u8> for [u8] {
 #[doc(hidden)]
 /// Trait implemented for types that can be compared for equality using
 /// their bytewise representation
-trait BytewiseEquality { }
+trait BytewiseEquality: Eq + Copy { }
 
 macro_rules! impl_marker_for {
     ($traitname:ident, $($ty:ty)*) => {