test for troublesome case where preserve was freeing uninitialized memory

Author: nikomatsakis

src/test/run-pass/borrowck-preserve-box-sometimes-needed.rs

@@ -0,0 +1,16 @@
+// xfail-fast   (compile-flags unsupported on windows)
+// compile-flags:--borrowck=err
+// exec-env:RUST_POISON_ON_FREE=1
+
+fn switcher(x: option<@int>) {
+    let mut x = x;
+    alt x {
+      some(@y) { copy y; x = none; }
+      none { }
+    }
+}
+
+fn main() {
+    switcher(none);
+    switcher(some(@3));
+}