Add a model for how conditional trait impls might be used to implement

the Fn-FnMut-FnOnce hierarchy.

src/test/run-pass/traits-conditional-model-fn.rs

@@ -0,0 +1,101 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// A model for how the `Fn` traits could work. You can implement at
+// most one of `Go`, `GoMut`, or `GoOnce`, and then the others follow
+// automatically.
+
+use std::rc::Rc;
+use std::cell::Cell;
+
+trait Go {
+    fn go(&self, arg: int);
+}
+
+fn go<G:Go>(this: &G, arg: int) {
+    this.go(arg)
+}
+
+trait GoMut {
+    fn go_mut(&mut self, arg: int);
+}
+
+fn go_mut<G:GoMut>(this: &mut G, arg: int) {
+    this.go_mut(arg)
+}
+
+trait GoOnce {
+    fn go_once(self, arg: int);
+}
+
+fn go_once<G:GoOnce>(this: G, arg: int) {
+    this.go_once(arg)
+}
+
+impl<G> GoMut for G
+    where G : Go
+{
+    fn go_mut(&mut self, arg: int) {
+        go(&*self, arg)
+    }
+}
+
+impl<G> GoOnce for G
+    where G : GoMut
+{
+    fn go_once(mut self, arg: int) {
+        go_mut(&mut self, arg)
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+struct SomeGoableThing {
+    counter: Rc<Cell<int>>
+}
+
+impl Go for SomeGoableThing {
+    fn go(&self, arg: int) {
+        self.counter.set(self.counter.get() + arg);
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+struct SomeGoOnceableThing {
+    counter: Rc<Cell<int>>
+}
+
+impl GoOnce for SomeGoOnceableThing {
+    fn go_once(self, arg: int) {
+        self.counter.set(self.counter.get() + arg);
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+fn main() {
+    let counter = Rc::new(Cell::new(0));
+    let mut x = SomeGoableThing { counter: counter.clone() };
+
+    go(&x, 10);
+    assert_eq!(counter.get(), 10);
+
+    go_mut(&mut x, 100);
+    assert_eq!(counter.get(), 110);
+
+    go_once(x, 1_000);
+    assert_eq!(counter.get(), 1_110);
+
+    let x = SomeGoOnceableThing { counter: counter.clone() };
+
+    go_once(x, 10_000);
+    assert_eq!(counter.get(), 11_110);
+}