YEAH! It compiles!

test.rs

pub fn par_trans2(
    pos: V2<f32>,
    f: Box<Fn(&[V2<f32>], &mut [bool]) + Sync>
) -> Box<Fn(&[V2<f32>], &mut [bool]) + Sync>{
    const SIZE: usize = 1024;
    Box::new(move |input, output| {
        input.par_chunks(SIZE)
             .zip(output.par_chunks_mut(SIZE))
             .for_each(|(i, o): (&[V2<f32>], &mut [bool])| {
                 /// Use stack memory to avoid allocations.
                 let mut i2: [V2<f32>; SIZE] = [V2 {x: 0.0, y: 0.0}; SIZE];
                 for p in i2.iter_mut().take(i.len()) {
                     *p = *p - pos;
                 }
                 f(&i2[..i.len()], o)
             })
    })
}

This is a breakthrough because it might allow composing such functions together to perform parallel stream processing.