Implement RFC#28: Add PartialOrd::partial_cmp

I ended up altering the semantics of Json's PartialOrd implementation.
It used to be the case that Null < Null, but I can't think of any reason
for an ordering other than the default one so I just switched it over to
using the derived implementation.

This also fixes broken `PartialOrd` implementations for `Vec` and
`TreeMap`.

RFC: 0028-partial-cmp

src/libcollections/btree.rs

@@ -107,8 +107,8 @@ impl<K: Ord, V: Eq> PartialEq for BTree<K, V> {
 impl<K: Ord, V: Eq> Eq for BTree<K, V> {}
 
 impl<K: Ord, V: Eq> PartialOrd for BTree<K, V> {
-    fn lt(&self, other: &BTree<K, V>) -> bool {
-        self.cmp(other) == Less
+    fn partial_cmp(&self, other: &BTree<K, V>) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 
@@ -229,8 +229,8 @@ impl<K: Ord, V: Eq> PartialEq for Node<K, V> {
 impl<K: Ord, V: Eq> Eq for Node<K, V> {}
 
 impl<K: Ord, V: Eq> PartialOrd for Node<K, V> {
-    fn lt(&self, other: &Node<K, V>) -> bool {
-        self.cmp(other) == Less
+    fn partial_cmp(&self, other: &Node<K, V>) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 
@@ -408,8 +408,8 @@ impl<K: Ord, V: Eq> PartialEq for Leaf<K, V> {
 impl<K: Ord, V: Eq> Eq for Leaf<K, V> {}
 
 impl<K: Ord, V: Eq> PartialOrd for Leaf<K, V> {
-    fn lt(&self, other: &Leaf<K, V>) -> bool {
-        self.cmp(other) == Less
+    fn partial_cmp(&self, other: &Leaf<K, V>) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 
@@ -638,8 +638,8 @@ impl<K: Ord, V: Eq> PartialEq for Branch<K, V> {
 impl<K: Ord, V: Eq> Eq for Branch<K, V> {}
 
 impl<K: Ord, V: Eq> PartialOrd for Branch<K, V> {
-    fn lt(&self, other: &Branch<K, V>) -> bool {
-        self.cmp(other) == Less
+    fn partial_cmp(&self, other: &Branch<K, V>) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 
@@ -706,8 +706,8 @@ impl<K: Ord, V: Eq> PartialEq for LeafElt<K, V> {
 impl<K: Ord, V: Eq> Eq for LeafElt<K, V> {}
 
 impl<K: Ord, V: Eq> PartialOrd for LeafElt<K, V> {
-    fn lt(&self, other: &LeafElt<K, V>) -> bool {
-        self.cmp(other) == Less
+    fn partial_cmp(&self, other: &LeafElt<K, V>) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 
@@ -755,8 +755,8 @@ impl<K: Ord, V: Eq> PartialEq for BranchElt<K, V>{
 impl<K: Ord, V: Eq> Eq for BranchElt<K, V>{}
 
 impl<K: Ord, V: Eq> PartialOrd for BranchElt<K, V> {
-    fn lt(&self, other: &BranchElt<K, V>) -> bool {
-        self.cmp(other) == Less
+    fn partial_cmp(&self, other: &BranchElt<K, V>) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 

src/libcollections/dlist.rs

@@ -595,17 +595,8 @@ impl<A: PartialEq> PartialEq for DList<A> {
 }
 
 impl<A: PartialOrd> PartialOrd for DList<A> {
-    fn lt(&self, other: &DList<A>) -> bool {
-        iter::order::lt(self.iter(), other.iter())
-    }
-    fn le(&self, other: &DList<A>) -> bool {
-        iter::order::le(self.iter(), other.iter())
-    }
-    fn gt(&self, other: &DList<A>) -> bool {
-        iter::order::gt(self.iter(), other.iter())
-    }
-    fn ge(&self, other: &DList<A>) -> bool {
-        iter::order::ge(self.iter(), other.iter())
+    fn partial_cmp(&self, other: &DList<A>) -> Option<Ordering> {
+        iter::order::partial_cmp(self.iter(), other.iter())
     }
 }
 

src/libcollections/str.rs

@@ -572,8 +572,8 @@ impl<'a> Eq for MaybeOwned<'a> {}
 
 impl<'a> PartialOrd for MaybeOwned<'a> {
     #[inline]
-    fn lt(&self, other: &MaybeOwned) -> bool {
-        self.as_slice().lt(&other.as_slice())
+    fn partial_cmp(&self, other: &MaybeOwned) -> Option<Ordering> {
+        Some(self.cmp(other))
     }
 }
 

src/libcollections/treemap.rs

@@ -56,23 +56,11 @@ impl<K: PartialEq + Ord, V: PartialEq> PartialEq for TreeMap<K, V> {
     }
 }
 
-// Lexicographical comparison
-fn lt<K: PartialOrd + Ord, V: PartialOrd>(a: &TreeMap<K, V>,
-                                 b: &TreeMap<K, V>) -> bool {
-    // the Zip iterator is as long as the shortest of a and b.
-    for ((key_a, value_a), (key_b, value_b)) in a.iter().zip(b.iter()) {
-        if *key_a < *key_b { return true; }
-        if *key_a > *key_b { return false; }
-        if *value_a < *value_b { return true; }
-        if *value_a > *value_b { return false; }
-    }
-
-    a.len() < b.len()
-}
-
-impl<K: PartialOrd + Ord, V: PartialOrd> PartialOrd for TreeMap<K, V> {
+impl<K: Ord, V: PartialOrd> PartialOrd for TreeMap<K, V> {
     #[inline]
-    fn lt(&self, other: &TreeMap<K, V>) -> bool { lt(self, other) }
+    fn partial_cmp(&self, other: &TreeMap<K, V>) -> Option<Ordering> {
+        iter::order::partial_cmp(self.iter(), other.iter())
+    }
 }
 
 impl<K: Ord + Show, V: Show> Show for TreeMap<K, V> {
@@ -568,9 +556,11 @@ impl<T: PartialEq + Ord> PartialEq for TreeSet<T> {
     fn eq(&self, other: &TreeSet<T>) -> bool { self.map == other.map }
 }
 
-impl<T: PartialOrd + Ord> PartialOrd for TreeSet<T> {
+impl<T: Ord> PartialOrd for TreeSet<T> {
     #[inline]
-    fn lt(&self, other: &TreeSet<T>) -> bool { self.map < other.map }
+    fn partial_cmp(&self, other: &TreeSet<T>) -> Option<Ordering> {
+        self.map.partial_cmp(&other.map)
+    }
 }
 
 impl<T: Ord + Show> Show for TreeSet<T> {

src/libcollections/vec.rs

@@ -389,8 +389,8 @@ impl<T: PartialEq> PartialEq for Vec<T> {
 
 impl<T: PartialOrd> PartialOrd for Vec<T> {
     #[inline]
-    fn lt(&self, other: &Vec<T>) -> bool {
-        self.as_slice() < other.as_slice()
+    fn partial_cmp(&self, other: &Vec<T>) -> Option<Ordering> {
+        self.as_slice().partial_cmp(&other.as_slice())
     }
 }