Add core::util, with swap, replace, and noncopyable

src/libcore/core.rc

@@ -179,6 +179,10 @@ mod option_iter {
 mod result;
 mod to_str;
 mod to_bytes;
+mod util;
+
+// Data structure modules
+
 mod dvec;
 #[path="iter-trait"]
 mod dvec_iter {

src/libcore/util.rs

@@ -0,0 +1,46 @@
+/**
+ * Miscellaneous helpers for common patterns.
+ */
+
+/**
+ * Swap the values at two mutable locations of the same type, without
+ * deinitialising or copying either one.
+ */
+fn swap<T>(x: &mut T, y: &mut T) {
+    *x <-> *y;
+}
+
+/**
+ * Replace the value at a mutable location with a new one, returning the old
+ * value, without deinitialising or copying either one.
+ */
+fn replace<T>(dest: &mut T, +src: T) -> T {
+    let mut tmp = src;
+    swap(dest, &mut tmp);
+    tmp
+}
+
+/// A non-copyable dummy type.
+class noncopyable {
+    i: ();
+    new() { self.i = (); }
+    drop { }
+}
+
+mod tests {
+    #[test]
+    fn test_swap() {
+        let mut x = 31337;
+        let mut y = 42;
+        swap(&mut x, &mut y);
+        assert x == 42;
+        assert y == 31337;
+    }
+    #[test]
+    fn test_replace() {
+        let mut x = some(noncopyable());
+        let y = replace(&mut x, none);
+        assert x.is_none();
+        assert y.is_some();
+    }
+}