AFAIK you can't do that on stable rust yet, you need `#![feature(generic_const_exprs)]`. If you need stable, you can make another const param and compute it ahead of time, e.g.
```rust
struct FFTThing<const N: usize, const M: usize> {
    time_buf: [f32; N],
    fft_buf: [Complex<f32>; M],
}

const fn m_from_n(n: usize) -> usize {
    n / 2 + 1
}

const MY_N: usize = 42;

const THING: FFTThing<MY_N, {m_from_n(MY_N)}> = FFTThing::const_new();
```