Add a Default trait.

src/libstd/default.rs

@@ -0,0 +1,17 @@
+// Copyright 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.
+
+//! The Default trait
+
+/// A trait that types which have a useful default value should implement.
+pub trait Default {
+    /// Return the "default value" for a type.
+    fn default() -> Self;
+}

src/libstd/prelude.rs

@@ -81,6 +81,7 @@ pub use vec::{Vector, VectorVector, CopyableVector, ImmutableVector};
 pub use vec::{ImmutableEqVector, ImmutableTotalOrdVector, ImmutableCopyableVector};
 pub use vec::{OwnedVector, OwnedCopyableVector,OwnedEqVector, MutableVector};
 pub use io::{Reader, ReaderUtil, Writer, WriterUtil};
+pub use default::Default;
 
 // Reexported runtime types
 pub use comm::{stream, Port, Chan, GenericChan, GenericSmartChan, GenericPort, Peekable};

src/libstd/std.rs

@@ -148,7 +148,7 @@ pub mod clone;
 pub mod io;
 pub mod hash;
 pub mod container;
-
+pub mod default;
 
 /* Common data structures */
 

src/libstd/str.rs

@@ -26,7 +26,7 @@ use iterator::{Iterator, FromIterator, Extendable};
 use iterator::{Filter, AdditiveIterator, Map};
 use iterator::{Invert, DoubleEndedIterator};
 use libc;
-use num::{Saturating, Zero};
+use num::{Saturating};
 use option::{None, Option, Some};
 use ptr;
 use ptr::RawPtr;
@@ -35,6 +35,7 @@ use uint;
 use unstable::raw::{Repr, Slice};
 use vec;
 use vec::{OwnedVector, OwnedCopyableVector, ImmutableVector, MutableVector};
+use default::Default;
 
 /*
 Section: Conditions
@@ -2467,19 +2468,16 @@ impl Extendable<char> for ~str {
 }
 
 // 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<'self> Default for &'self str {
+    fn default() -> &'self str { "" }
 }
 
-impl Zero for ~str {
-    fn zero() -> ~str { ~"" }
-    fn is_zero(&self) -> bool { self.len() == 0 }
+impl Default for ~str {
+    fn default() -> ~str { ~"" }
 }
 
-impl Zero for @str {
-    fn zero() -> @str { @"" }
-    fn is_zero(&self) -> bool { self.len() == 0 }
+impl Default for @str {
+    fn default() -> @str { @"" }
 }
 
 #[cfg(test)]
@@ -3660,12 +3658,11 @@ mod tests {
     }
 
     #[test]
-    fn test_str_zero() {
-        use num::Zero;
-        fn t<S: Zero + Str>() {
-            let s: S = Zero::zero();
+    fn test_str_default() {
+        use default::Default;
+        fn t<S: Default + Str>() {
+            let s: S = Default::default();
             assert_eq!(s.as_slice(), "");
-            assert!(s.is_zero());
         }
 
         t::<&str>();

src/libstd/unit.rs

@@ -52,3 +52,8 @@ impl Zero for () {
     #[inline]
     fn is_zero(&self) -> bool { true }
 }
+
+#[cfg(not(test))]
+impl Default for () {
+    fn default() -> () { () }
+}

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

@@ -24,8 +24,6 @@ struct E { a: int, b: int }
 
 #[deriving(Zero)]
 struct Lots {
-    a: ~str,
-    b: @str,
     c: Option<util::NonCopyable>,
     d: u8,
     e: char,