Rollup merge of #45095 - bluss:discriminant-send-sync, r=alexcrichton

Ensure std::mem::Discriminant is Send + Sync

`PhantomData<*const T>` has the implication of Send / Syncness following
the *const T type, but the discriminant should always be Send and Sync.

Use `PhantomData<fn() -> T>` which has the same variance in T, but is Send + Sync

1 files changed, 16 insertions, 0 deletions

diff --git a/src/libcore/tests/mem.rs b/src/libcore/tests/mem.rs
index 86e59c736ba..f55a1c81463 100644
--- a/src/libcore/tests/mem.rs
+++ b/src/libcore/tests/mem.rs
@@ -121,3 +121,19 @@ fn test_transmute() {
     }
 }
 
+#[test]
+#[allow(dead_code)]
+fn test_discriminant_send_sync() {
+    enum Regular {
+        A,
+        B(i32)
+    }
+    enum NotSendSync {
+        A(*const i32)
+    }
+
+    fn is_send_sync<T: Send + Sync>() { }
+
+    is_send_sync::<Discriminant<Regular>>();
+    is_send_sync::<Discriminant<NotSendSync>>();
+}