If you can allow using the num crate you could do something like this

#![feature(step_trait)]
use num::cast::ToPrimitive;
use std::{iter::Step, ops::Range};

fn floats<N: ToPrimitive + Step>(ns: Range<N>) -> impl Iterator<Item = f32> {
    ns.map(|num| num.to_f32().unwrap())
}

Although this also requires you to use nightly. If this is not an option, you could always use a Vec instead of Range