Implement `Default`

[tgross35]

Discussed at #t-libs > Questions and polls about `libc` 1.0 @ 💬, we would like to have a Default impl for pretty much everything. This should be part of s!/s_no_extra_traits! but won't work out of the box with a derive.

In particular, we need a workaround for [T; N] where N > 32, and unions somehow.

[tgross35]

I think ideally we would accept a #[skip_derive_default] attribute in s! which forwards to a impl_default! macro. That macro could then look for a custom_default attribute use its contents rather than Default::default().

For example, this:

s! {
    #[skip_derive_default]
    struct foo {
        pub a: i32,
        #[custom_default([0u8; 1024])]
        pub b: [u8; 1024],
        #[custom_default(bar { a: 0 })]
        pub bar: bar,
    }

    union bar {
        a: i32,
        b: f32,
    }
}

Expands to this:

#[derive(Clone, Copy, Debug, ....)]
strip_custom_attributes! {
    struct foo {
        pub a: i32,
        #[custom_default([0u8; 1024])]
        pub b: [u8; 1024],
        #[custom_default(bar { a: 0 })]
        pub bar: bar,
    }
}

custom_default! {
    struct foo {
        pub a: i32,
        #[custom_default([0u8; 1024])]
        pub b: [u8; 1024],
        #[custom_default(bar { a: 0 })]
        pub bar: bar,
    }
}

// ignoring `union bar`

Which then expands to:

#[derive(Clone, Copy, Debug, ....)]
struct foo {
    pub a: i32,
    pub b: [u8; 1024],
    pub bar: bar,
}

impl Default for foo {
    fn default() -> Self {
        Self {
            // For fields without `custom_default`, just use `Default::default()`
            a: Default::default(),
            // For `custom_default`, use that token tree
            b: [0u8; 1024],
            bar: bar { a: 0 },
        }
    }
}

// ignoring `union bar`

This isn't going to be the most fun macro to write, but it's much preferred to dealing with hundreds of custom derives.

I won't be working on this for a while but can provide guidance if anybody is interested.