Ok, going with thsis:
/// Square a number.
fn sq(val: f32) -> f32 {
val * val
}
/// Wrapper for `CMSIS-DSP`'s real-number square root fn. Doesn't check for negative inputs.
fn sqrt(val: f32) -> f32 {
let mut result = 0.;
arm_sqrt_f32(val, &mut result);
result
}