From d8951ee8a5984f1d7b2abedb5b03d6e18751fcf1 Mon Sep 17 00:00:00 2001
From: James Munns <james.munns@ferrous-systems.com>
Date: Fri, 19 Oct 2018 18:12:43 +0200
Subject: [PATCH 1/3] Implement Copy and Clone for UnsafeCell<T>

Only implemented if T implements Copy and/or Clone
---
 src/libcore/cell.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index ec7d366c3f5ce..421b741a3a330 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -1512,6 +1512,18 @@ impl<T: ?Sized> UnsafeCell<T> {
     }
 }
 
+// TODO: correct stable attribute
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Clone> Clone for UnsafeCell<T> {
+    fn clone(&self) -> UnsafeCell<T> {
+        UnsafeCell::new(self.value.clone())
+    }
+}
+
+// TODO: correct stable attribute
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Copy> Copy for UnsafeCell<T> {}
+
 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
 impl<T: Default> Default for UnsafeCell<T> {
     /// Creates an `UnsafeCell`, with the `Default` value for T.

From 4676b10670caa926d9eab2586060068095d255ce Mon Sep 17 00:00:00 2001
From: James Munns <james.munns@ferrous-systems.com>
Date: Thu, 1 Nov 2018 13:03:11 +0100
Subject: [PATCH 2/3] Modify stability attributes so CI can build, only impl
 Clone if T is Copy

---
 src/libcore/cell.rs | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index 421b741a3a330..d88d8d1fc6cd2 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -1512,16 +1512,14 @@ impl<T: ?Sized> UnsafeCell<T> {
     }
 }
 
-// TODO: correct stable attribute
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Clone> Clone for UnsafeCell<T> {
+#[stable(feature = "unsafe_cell_copy_clone_conservative", since = "1.31.0")]
+impl<T: Clone + Copy> Clone for UnsafeCell<T> {
     fn clone(&self) -> UnsafeCell<T> {
         UnsafeCell::new(self.value.clone())
     }
 }
 
-// TODO: correct stable attribute
-#[stable(feature = "rust1", since = "1.0.0")]
+#[stable(feature = "unsafe_cell_copy_clone_conservative", since = "1.31.0")]
 impl<T: Copy> Copy for UnsafeCell<T> {}
 
 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]

From 67bcac6665d21c686f500b79fe3efecb09913198 Mon Sep 17 00:00:00 2001
From: James Munns <james.munns@ferrous-systems.com>
Date: Thu, 1 Nov 2018 13:18:14 +0100
Subject: [PATCH 3/3] Remove call to clone, and explain why

---
 src/libcore/cell.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index d88d8d1fc6cd2..f7f58a02437c5 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -1515,7 +1515,10 @@ impl<T: ?Sized> UnsafeCell<T> {
 #[stable(feature = "unsafe_cell_copy_clone_conservative", since = "1.31.0")]
 impl<T: Clone + Copy> Clone for UnsafeCell<T> {
     fn clone(&self) -> UnsafeCell<T> {
-        UnsafeCell::new(self.value.clone())
+        // NOTE: we are explicitly not calling `.clone()` here, to enforce the
+        // use of Copy, rather than relying on what could be a custom impl
+        // of Clone
+        UnsafeCell::new(self.value)
     }
 }