Macro vertex_struct

macro_rules! vertex_struct {
    { @len } => { ... };
    { @len $last:tt } => { ... };
    { @len $first:tt $( $rest:tt )+ } => { ... };
    { $struct:ident [
        $( $num:tt $field:ident => $attr:ident ),* $(,)?
    ] } => { ... };
    { $struct:ident step $step:ident [
        $( $num:tt $field:ident => $attr:ident ),* $(,)?
    ] } => { ... };
}