auto merge of #21162 : apasel422/rust/issue-16530, r=huonw

This fixes #16530 by hashing nullary structs [the same way as the empty tuple] (https://github.com/rust-lang/rust/blob/master/src/libcore/hash/mod.rs#L185). Other approaches are possible, but this was the simplest.

src/test/run-pass/issue-16530.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::hash::{SipHasher, hash};
+
+#[derive(Hash)]
+struct Empty;
+
+pub fn main() {
+    assert!(hash::<_, SipHasher>(&Empty) == hash::<_, SipHasher>(&Empty));
+}