Add regression test

Author: oli-obk

tests/ui/statics/read_before_init.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+
+use std::mem::MaybeUninit;
+
+pub static X: (i32, MaybeUninit<i32>) = (1, foo(&X.0));
+
+const fn foo(x: &i32) -> MaybeUninit<i32> {
+    let mut temp = MaybeUninit::<i32>::uninit();
+    unsafe {
+        std::ptr::copy(x, temp.as_mut_ptr(), 1);
+    }
+    temp
+}
+
+fn main() {}