2 files changed, 24 insertions, 0 deletions
diff --git a/src/test/ui/union/union-deref.rs b/src/test/ui/union/union-deref.rs
new file mode 100644
index 00000000000..61bbe73354f
--- /dev/null
+++ b/src/test/ui/union/union-deref.rs
@@ -0,0 +1,13 @@
+//! Test the part of RFC 2514 that is about not applying `DerefMut` coercions
+//! of union fields.
+#![feature(untagged_unions)]
+
+use std::mem::ManuallyDrop;
+
+union U<T> { x:(), f: ManuallyDrop<(T,)> }
+
+fn main() {
+    let mut u : U<Vec<i32>> = U { x: () };
+    unsafe { (*u.f).0 = Vec::new() }; // explicit deref, this compiles
+    unsafe { u.f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on union field
+}
diff --git a/src/test/ui/union/union-deref.stderr b/src/test/ui/union/union-deref.stderr
new file mode 100644
index 00000000000..66cc90cbd3d
--- /dev/null
+++ b/src/test/ui/union/union-deref.stderr
@@ -0,0 +1,11 @@
+error: not automatically applying `DerefMut` on union field
+  --> $DIR/union-deref.rs:12:14
+   |
+LL |     unsafe { u.f.0 = Vec::new() };
+   |              ^^^
+   |
+   = help: writing to this field calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: aborting due to previous error
+