Drop the previous value when copying one unique box local to another

Issue #409

src/comp/middle/trans.rs

@@ -2031,8 +2031,7 @@ fn copy_val_no_check(cx: @block_ctxt, action: copy_action, dst: ValueRef,
     }
     if ty::type_is_unique_box(ccx.tcx, t) {
         let bcx = cx;
-        // FIXME (409): Write a test and uncomment
-        //if action == DROP_EXISTING { bcx = drop_ty(cx, dst, t); }
+        if action == DROP_EXISTING { bcx = drop_ty(cx, dst, t); }
         check trans_uniq::type_is_unique_box(bcx, t);
         ret trans_uniq::copy_val(bcx, dst, src, t);
     }

src/test/run-pass/unique-assign-drop.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let i = ~1;
+    let j = ~2;
+    // Should drop the previous value of j
+    j = i;
+    assert *j == 1;
+}