Rust macro with optional attribute statements over fields

November 7, 2022

I am trying to make a macro that modifies the declaration of a struct, for now I have this:

macro_rules! Reflect
{
(pub struct $name:ident {$($field_name:ident : $field_type:ty,) *}) =>
{
    #[derive(Debug, Clone, serde::Serialize)]
    #[serde(rename_all = "camelCase")]
    pub struct $name
    {
        $(pub $field_name : $field_type,) *
    }
}

Reflect!(
pub struct Node
{
    name : String,
    #[serde(skip_serializing_if = "<[_]>::is_empty")] // causes compilation error because macro does not expect it
    children : Vec::<usize>,
    rotation : na::UnitQuaternion::<f32>,
    translation : na::Vector3::<f32>,
    scale : na::Vector3::<f32>,
    skin : usize,
    mesh : usize,
}
);

In this case I would like the serde attribbute declaration to be left intact on top of the field if defined.

With the above I get a compilation error because the macro does not have the right pattern to handle it, but I don’t know how to do it.

>Solution :

You can capture attributes with the :meta fragment:

macro_rules! Reflect
{
    (
        pub struct $name:ident {
            $(
                $( #[$attrs:meta] )*
                $field_name:ident : $field_type:ty,
            )*
        }
    ) =>
    {
        #[derive(Debug, Clone, serde::Serialize)]
        #[serde(rename_all = "camelCase")]
        pub struct $name
        {
            $(
                $( #[$attrs] )*
                pub $field_name : $field_type,
            )*
        }
    };
}