Function project 

pub fn project<S, T, V>(
    src: S,
    label: &'static str,
    get: fn(&T) -> &V,
    get_mut: fn(&mut T) -> &mut V,
) -> Field<S, T, V>
where
    S: Source<T>,
    T: 'static,
    V: 'static,

What the derive calls. Named project rather than field so a struct may have a field called field without a collision.