feat: Add spooky undefer implementation

examples/undefer.rs

@@ -0,0 +1,40 @@
+// Go's defer in Rust, with a little twist!
+
+struct Defer<F: Fn()> {
+    f: F
+}
+
+impl <F: Fn()> Drop for Defer<F> {
+    fn drop(&mut self) {
+        (self.f)()
+    }
+}
+
+// Only added this for Go-syntax familiarity ;-)
+fn defer<F: Fn()>(f: F) -> Defer<F> {
+    Defer { f }
+}
+
+// Changed your mind about the defer?
+// (Note: This leaks the closure! Don't actually do this!)
+fn undefer<F: Fn()>(token: Defer<F>) {
+    use std::mem;
+    mem::forget(token);
+}
+
+fn main() {
+    let mut i = 1;
+
+    // Calling it "token" ... could be something else. The lifetime of this
+    // controls when the action is run.
+    let token = defer(move || println!("Value is: {}", i));
+
+    i += 1;
+    println!("Value is: {}", i);
+
+    // Oh, now I changed my mind about the previous defer:
+    undefer(token);
+}
+
+// Prints:
+// Value is: 2