Implement Zero for unit

src/libstd/nil.rs

@@ -16,6 +16,10 @@ Functions for the unit type.
 
 #[cfg(not(test))]
 use prelude::*;
+#[cfg(not(test))]
+use num::Zero;
+
+
 
 #[cfg(not(test))]
 impl Eq for () {
@@ -48,3 +52,10 @@ impl TotalEq for () {
     #[inline]
     fn equals(&self, _other: &()) -> bool { true }
 }
+
+#[cfg(not(test))]
+impl Zero for () {
+    fn zero() -> () { () }
+    fn is_zero(&self) -> bool { true }
+}
+

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

@@ -34,6 +34,7 @@ struct Lots {
     h: ~[util::NonCopyable],
     i: @mut (int, int),
     j: bool,
+    k: (),
 }
 
 fn main() {