Rust - Mix default macro and personal default implementation

Question

I want to know if it's possible to let Rust define the default implementation for my structure with the macro #[derive(Default)] but define some default values for some fields.

I want to do this to avoid to define the default values of all fields when Rust could do it.

Thanks for your time

Answer 1

As far as I know derive(Default) is all-or-nothing.

However, you can initialize Defaultable fields using field: Default::default() (which may already be simple enough). You could use this fact to write a macro that takes all Defaultable fields and initializes them, e.g.:

macro_rules! defaultable{
    (
        $t: ty,
        @default($($default_field:ident,)*)
        @custom($($custom_field:ident = $custom_value:expr,)*)
    )
    => {
    impl Default for $t {
        fn default() -> Self {
            Self {
                $($default_field: Default::default(),)*
                $($custom_field: $custom_value,)*
            }
        }
    }
}}

struct S {
    n1: usize,
    n2: usize,
    b: bool,
    non_default: usize,
}

defaultable!(S, @default(n1, n2, b,) @custom(non_default=42,));

fn main() {
    let s : S = Default::default();
    println!("{}", s.non_default);
}