fallback

This is a helper library to implement fallback mechaism. It contains two Option, and if the "desired" one is None, the "base" one will be chosen.

A trait called FallbackSpec is used to implement field fallback for a struct.

``` rust use fallback::*;

[derive(FallbackSpec)]

struct Foo { data1: i32, data2: String, }

let data = Foo { data1: 123, data2: "Hello".to_string(), };

let data = Fallback::new(None, Some(data)); let data = data.spec();

asserteq!(data.data1.unzip(), (None, Some(123))); asserteq!(data.data2.unzip(), (None, Some("Hello".to_string()))); ```