Add #[derive(serde::Default)] that is aware of serde default annotations

[BrynCooke]

When using serde defaulting users currently have to manually create Default implementations that match the serde annotations.

For example:

    #[test]
    fn test() {
        #[derive(Deserialize)]
        struct Container {
            a: A,
        }
        #[derive(Deserialize)]
        struct A {
            #[serde(default)]
            b: B,
        }

        #[derive(Deserialize, Default)]
        struct B {
            #[serde(default = "true_fn")]
            c: bool,
        }

        fn true_fn() -> bool {
            true
        }

        let container1 = serde_yaml::from_str::<Container>("a: {}").unwrap();
        let container2 = serde_yaml::from_str::<Container>("a: {b: {}}").unwrap();
        println!("{}", container1.a.b.c);
        println!("{}", container2.a.b.c);
    }

The output is:

true
false

This is because the implementation of Default disagrees with the serde defaults.

The current workaround for this is to manually create the default implementation of B such that it agrees exactly with serde. On large and complex structs this is almost guaranteed to lead to errors.

It'd be great if serde provided derive support for Default that is aware of the serde default annotations.

[dtolnay]

I would prefer not to build this into this crate. But this derive can be implemented in a separate crate.

[BrynCooke]

Looks like such a crate already exists: https://docs.rs/serde_default/0.1.0/serde_default/

[BrynCooke]

The crate listed above didn't work for me, so I created a new one: https://crates.io/crates/serde_derive_default