do not premote non-ZST mutable references ever

src/test/ui/consts/promotion-mutable-ref.rs

@@ -0,0 +1,17 @@
+// run-pass
+#![feature(const_mut_refs)]
+
+static mut TEST: i32 = {
+    // We cannot promote this, as CTFE needs to be able to mutate it later.
+    let x = &mut [1,2,3];
+    x[0] += 1;
+    x[0]
+};
+
+// This still works -- it's not done via promotion.
+#[allow(unused)]
+static mut TEST2: &'static mut [i32] = &mut [0,1,2];
+
+fn main() {
+    assert_eq!(unsafe { TEST }, 2);
+}