libstd: Make vec_from_set pure

Author: pcwalton

src/libstd/map.rs

@@ -382,7 +382,7 @@ pub fn set_add<K:Eq IterBytes Hash Const Copy>(set: Set<K>, key: K) -> bool {
 }
 
 /// Convert a set into a vector.
-pub fn vec_from_set<T:Eq IterBytes Hash Copy>(s: Set<T>) -> ~[T] {
+pub pure fn vec_from_set<T:Eq IterBytes Hash Copy>(s: Set<T>) -> ~[T] {
     do vec::build_sized(s.size()) |push| {
         for s.each_key() |k| {
             push(k);