std: add Zero impls for &[] and &str.

src/libstd/str.rs

@@ -2202,6 +2202,12 @@ impl<'self> Iterator<u8> for StrBytesRevIterator<'self> {
     }
 }
 
+// This works because every lifetime is a sub-lifetime of 'static
+impl<'self> Zero for &'self str {
+    fn zero() -> &'self str { "" }
+    fn is_zero(&self) -> bool { self.is_empty() }
+}
+
 impl Zero for ~str {
     fn zero() -> ~str { ~"" }
     fn is_zero(&self) -> bool { self.len() == 0 }
@@ -3317,4 +3323,18 @@ mod tests {
         t("zzz", "zz", ~["","z"]);
         t("zzzzz", "zz", ~["","","z"]);
     }
+
+    #[test]
+    fn test_str_zero() {
+        use num::Zero;
+        fn t<S: Zero + Str>() {
+            let s: S = Zero::zero();
+            assert_eq!(s.as_slice(), "");
+            assert!(s.is_zero());
+        }
+
+        t::<&str>();
+        t::<@str>();
+        t::<~str>();
+    }
 }

src/libstd/vec.rs

@@ -2629,6 +2629,12 @@ impl<A:Clone> Clone for ~[A] {
     }
 }
 
+// This works because every lifetime is a sub-lifetime of 'static
+impl<'self, A> Zero for &'self [A] {
+    fn zero() -> &'self [A] { &'self [] }
+    fn is_zero(&self) -> bool { self.is_empty() }
+}
+
 impl<A> Zero for ~[A] {
     fn zero() -> ~[A] { ~[] }
     fn is_zero(&self) -> bool { self.len() == 0 }
@@ -4293,4 +4299,20 @@ mod tests {
         }
         assert_eq!(v, ~[~[1,2,3],~[1,3,2],~[2,1,3],~[2,3,1],~[3,1,2],~[3,2,1]]);
     }
+
+    #[test]
+    fn test_vec_zero() {
+        use num::Zero;
+        macro_rules! t (
+            ($ty:ty) => {
+                let v: $ty = Zero::zero();
+                assert!(v.is_empty());
+                assert!(v.is_zero());
+            }
+        );
+
+        t!(&[int]);
+        t!(@[int]);
+        t!(~[int]);
+    }
 }