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?)

6 files changed, 63 insertions, 2 deletions

diff --git a/src/libstd/char.rs b/src/libstd/char.rs
index 073ced8988a..7bfac2927c8 100644
--- a/src/libstd/char.rs
+++ b/src/libstd/char.rs
@@ -17,8 +17,8 @@ use u32;
 use uint;
 use unicode::{derived_property, general_category};
 
-#[cfg(not(test))]
-use cmp::{Eq, Ord};
+#[cfg(not(test))] use cmp::{Eq, Ord};
+#[cfg(not(test))] use num::Zero;
 
 /*
     Lu  Uppercase_Letter        an uppercase letter
@@ -328,6 +328,12 @@ impl Ord for char {
     fn ge(&self, other: &char) -> bool { *self >= *other }
 }
 
+#[cfg(not(test))]
+impl Zero for char {
+    fn zero() -> char { 0 as char }
+    fn is_zero(&self) -> bool { *self == 0 as char }
+}
+
 #[test]
 fn test_is_lowercase() {
     assert!('a'.is_lowercase());
diff --git a/src/libstd/num/num.rs b/src/libstd/num/num.rs
index a9893579721..4681e4f4f53 100644
--- a/src/libstd/num/num.rs
+++ b/src/libstd/num/num.rs
@@ -418,6 +418,21 @@ pub fn pow_with_uint<T:NumCast+One+Zero+Copy+Div<T,T>+Mul<T,T>>(radix: uint, pow
     total
 }
 
+impl<T: Zero> Zero for @mut T {
+    fn zero() -> @mut T { @mut Zero::zero() }
+    fn is_zero(&self) -> bool { (**self).is_zero() }
+}
+
+impl<T: Zero> Zero for @T {
+    fn zero() -> @T { @Zero::zero() }
+    fn is_zero(&self) -> bool { (**self).is_zero() }
+}
+
+impl<T: Zero> Zero for ~T {
+    fn zero() -> ~T { ~Zero::zero() }
+    fn is_zero(&self) -> bool { (**self).is_zero() }
+}
+
 /// Helper function for testing numeric operations
 #[cfg(test)]
 pub fn test_num<T:Num + NumCast>(ten: T, two: T) {
diff --git a/src/libstd/option.rs b/src/libstd/option.rs
index 80f4fb7643c..76272706445 100644
--- a/src/libstd/option.rs
+++ b/src/libstd/option.rs
@@ -350,6 +350,11 @@ impl<T:Copy + Zero> Option<T> {
     }
 }
 
+impl<T> Zero for Option<T> {
+    fn zero() -> Option<T> { None }
+    fn is_zero(&self) -> bool { self.is_none() }
+}
+
 /// Immutable iterator over an `Option<A>`
 pub struct OptionIterator<'self, A> {
     priv opt: Option<&'self A>
diff --git a/src/libstd/str.rs b/src/libstd/str.rs
index 21f747317f4..aa6b1470b26 100644
--- a/src/libstd/str.rs
+++ b/src/libstd/str.rs
@@ -27,6 +27,7 @@ use container::Container;
 use iter::Times;
 use iterator::{Iterator, IteratorUtil, FilterIterator, AdditiveIterator, MapIterator};
 use libc;
+use num::Zero;
 use option::{None, Option, Some};
 use old_iter::{BaseIter, EqIter};
 use ptr;
@@ -2201,6 +2202,16 @@ impl<'self> Iterator<u8> for StrBytesRevIterator<'self> {
     }
 }
 
+impl Zero for ~str {
+    fn zero() -> ~str { ~"" }
+    fn is_zero(&self) -> bool { self.len() == 0 }
+}
+
+impl Zero for @str {
+    fn zero() -> @str { @"" }
+    fn is_zero(&self) -> bool { self.len() == 0 }
+}
+
 #[cfg(test)]
 mod tests {
     use iterator::IteratorUtil;
diff --git a/src/libstd/tuple.rs b/src/libstd/tuple.rs
index 589c18de0ab..b80f152a5b1 100644
--- a/src/libstd/tuple.rs
+++ b/src/libstd/tuple.rs
@@ -135,6 +135,7 @@ macro_rules! tuple_impls {
         pub mod inner {
             use clone::Clone;
             #[cfg(not(test))] use cmp::*;
+            #[cfg(not(test))] use num::Zero;
 
             $(
                 pub trait $cloneable_trait<$($T),+> {
@@ -210,6 +211,18 @@ macro_rules! tuple_impls {
                         lexical_cmp!($(self.$get_ref_fn(), other.$get_ref_fn()),+)
                     }
                 }
+
+                #[cfg(not(test))]
+                impl<$($T:Zero),+> Zero for ($($T),+) {
+                    #[inline]
+                    fn zero() -> ($($T),+) {
+                        ($(Zero::zero::<$T>()),+)
+                    }
+                    #[inline]
+                    fn is_zero(&self) -> bool {
+                        $(self.$get_ref_fn().is_zero())&&+
+                    }
+                }
             )+
         }
     }
diff --git a/src/libstd/vec.rs b/src/libstd/vec.rs
index f330e13c5bd..1a236a49a32 100644
--- a/src/libstd/vec.rs
+++ b/src/libstd/vec.rs
@@ -23,6 +23,7 @@ use iterator::{Iterator, IteratorUtil};
 use iter::FromIter;
 use kinds::Copy;
 use libc;
+use num::Zero;
 use old_iter::CopyableIter;
 use option::{None, Option, Some};
 use ptr::to_unsafe_ptr;
@@ -2628,6 +2629,16 @@ impl<A:Clone> Clone for ~[A] {
     }
 }
 
+impl<A> Zero for ~[A] {
+    fn zero() -> ~[A] { ~[] }
+    fn is_zero(&self) -> bool { self.len() == 0 }
+}
+
+impl<A> Zero for @[A] {
+    fn zero() -> @[A] { @[] }
+    fn is_zero(&self) -> bool { self.len() == 0 }
+}
+
 macro_rules! iterator {
     /* FIXME: #4375 Cannot attach documentation/attributes to a macro generated struct.
     (struct $name:ident -> $ptr:ty, $elem:ty) => {