Auto merge of #15 - notriddle:master, r=Manishearth

Add support for `Cow`.

Hyper uses it.

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Author: bors-servo

src/lib.rs

@@ -9,6 +9,7 @@
 extern crate libc;
 
 use libc::{c_void, size_t};
+use std::borrow::Cow;
 use std::cell::{Cell, RefCell};
 use std::collections::{HashMap, LinkedList};
 #[cfg(feature = "unstable")]
@@ -89,6 +90,15 @@ impl<T: HeapSizeOf> HeapSizeOf for Option<T> {
     }
 }
 
+impl<'a, B: ?Sized + ToOwned> HeapSizeOf for Cow<'a, B> where B::Owned: HeapSizeOf {
+    fn heap_size_of_children(&self) -> usize {
+        match *self {
+            Cow::Borrowed(_) => 0,
+            Cow::Owned(ref b) => b.heap_size_of_children(),
+        }
+    }
+}
+
 impl<T: HeapSizeOf, U: HeapSizeOf> HeapSizeOf for (T, U) {
     fn heap_size_of_children(&self) -> usize {
         self.0.heap_size_of_children() + self.1.heap_size_of_children()