1 files changed, 44 insertions, 0 deletions
diff --git a/src/test/ui/consts/invalid-union.rs b/src/test/ui/consts/invalid-union.rs
new file mode 100644
index 000000000..efeddf75c
--- /dev/null
+++ b/src/test/ui/consts/invalid-union.rs
@@ -0,0 +1,44 @@
+// Check that constants with interior mutability inside unions are rejected
+// during validation.
+//
+// Note that this test case relies on undefined behaviour to construct a
+// constant with interior mutability that is "invisible" to the static checks.
+// If for some reason this approach no longer works, it is should be fine to
+// remove the test case.
+//
+// build-fail
+// stderr-per-bitwidth
+#![feature(const_mut_refs)]
+
+use std::cell::Cell;
+use std::mem::ManuallyDrop;
+
+#[repr(C)]
+struct S {
+    x: u32,
+    y: E,
+}
+
+#[repr(u32)]
+enum E {
+    A,
+    B(U)
+}
+
+union U {
+    cell: ManuallyDrop<Cell<u32>>,
+}
+
+const C: S = {
+    let s = S { x: 0, y: E::A };
+    // Go through an &u32 reference which is definitely not allowed to mutate anything.
+    let p = &s.x as *const u32 as *mut u32;
+    // Change enum tag to E::B.
+    unsafe { *p.add(1) = 1 };
+    s
+};
+
+fn main() { //~ ERROR it is undefined behavior to use this value
+    let _: &'static _ = &C; //~ ERROR erroneous constant used
+    //~^ WARN this was previously accepted
+}