auto merge of #7142 : alexcrichton/rust/deriving-zero, r=pcwalton

This allows mass-initialization of large structs without having to specify all the fields.

I'm a bit hesitant, but I wanted to get this out there. I don't really like using the `Zero` trait, because it doesn't really make sense for a type like `HashMap` to use `Zero` as the 'blank allocation' trait. In theory there'd be a new trait, but then that's adding cruft to the language which may not necessarily need to be there.

I do think that this can be useful, but I only implemented `Zero` on the basic types where I thought it made sense, so it may not be all that usable yet. (opinions?)

src/test/run-pass/deriving-zero.rs

@@ -0,0 +1,40 @@
+// Copyright 2012-2013 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::util;
+use std::num::Zero;
+
+#[deriving(Zero)]
+struct A;
+#[deriving(Zero)]
+struct B(int);
+#[deriving(Zero)]
+struct C(int, int);
+#[deriving(Zero)]
+struct D { a: int }
+#[deriving(Zero)]
+struct E { a: int, b: int }
+
+#[deriving(Zero)]
+struct Lots {
+    a: ~str,
+    b: @str,
+    c: Option<util::NonCopyable>,
+    d: u8,
+    e: char,
+    f: float,
+    g: (f32, char),
+    h: ~[util::NonCopyable],
+    i: @mut (int, int),
+}
+
+fn main() {
+    assert!(Zero::zero::<Lots>().is_zero());
+}