Add a test for rust-lang#10876

Author: varkor

src/test/ui/borrowck/issue-10876.rs

@@ -0,0 +1,19 @@
+// run-pass
+
+#![feature(nll)]
+
+enum Nat {
+    S(Box<Nat>),
+    Z
+}
+fn test(x: &mut Nat) {
+    let mut p = &mut *x;
+    loop {
+        match p {
+            &mut Nat::Z => break,
+            &mut Nat::S(ref mut n) => p = &mut *n
+        }
+    }
+}
+
+fn main() {}