engine-macros/src/reflection/default_value.rs


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use quote::quote;

pub fn gen_get_default_value_fn(
    type_ident: &proc_macro2::Ident,
    generic_args: Option<&syn::AngleBracketedGenericArguments>,
) -> proc_macro2::TokenStream
{
    quote! {
        struct SpecializationTarget<T>(std::marker::PhantomData<T>);

        trait HasDefaultValue
        {
            fn default_value_fn(&self) -> Option<fn() -> Box<dyn std::any::Any>>;
        }

        trait NoDefaultValue
        {
            fn default_value_fn(&self) -> Option<fn() -> Box<dyn std::any::Any>>;
        }

        impl<T> NoDefaultValue for &SpecializationTarget<T>
        {
            fn default_value_fn(&self) -> Option<fn() -> Box<dyn std::any::Any>>
            {
                None
            }
        }

        impl<T> HasDefaultValue for SpecializationTarget<T>
        where
            T: std::default::Default + 'static
        {
            fn default_value_fn(&self) -> Option<fn() -> Box<dyn std::any::Any>>
            {
                Some(|| Box::new(T::default()))
            }
        }

        (&SpecializationTarget::<#type_ident #generic_args>(std::marker::PhantomData))
            .default_value_fn()
    }
}