syntax: Add #[deriving(FromPrimitive)] syntax extension

Right now this only works for c-style enums.

src/test/run-pass/deriving-primitive.rs

@@ -0,0 +1,37 @@
+// Copyright 2013 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.
+
+use std::num::FromPrimitive;
+use std::int;
+
+#[deriving(Eq, FromPrimitive)]
+enum A {
+    Foo = int::max_value,
+    Bar = 1,
+    Baz = 3,
+    Qux,
+}
+
+fn main() {
+    let x: Option<A> = FromPrimitive::from_int(int::max_value);
+    assert_eq!(x, Some(Foo));
+
+    let x: Option<A> = FromPrimitive::from_int(1);
+    assert_eq!(x, Some(Bar));
+
+    let x: Option<A> = FromPrimitive::from_int(3);
+    assert_eq!(x, Some(Baz));
+
+    let x: Option<A> = FromPrimitive::from_int(4);
+    assert_eq!(x, Some(Qux));
+
+    let x: Option<A> = FromPrimitive::from_int(5);
+    assert_eq!(x, None);
+}