We /lightweight dependent types/ now, again, maybe

const_generic_arithmetic.rs

#![feature(const_generics, const_evaluatable_checked)]
#![allow(incomplete_features)]

// tested with rustc 1.49.0-nightly (91a79fb29 2020-10-07)

fn append<T: Copy + Default, const N: usize, const M: usize>(
  xs: &[T; N], ys: &[T; M]) -> [T; N + M] {
    let mut result = [T::default(); N + M];
    result[..N].copy_from_slice(&xs[..]);
    result[N..].copy_from_slice(&ys[..]);
    result
}

fn main() {
    let arr = append(&[1i32; 3], &[2i32; 4]);
    dbg!(arr);
}

now_with_const_fn.rs

#![feature(const_generics, const_evaluatable_checked)]
#![allow(incomplete_features)]

// tested with rustc 1.49.0-nightly (91a79fb29 2020-10-07)

#[inline]
const fn add_em(x: usize, y: usize) -> usize {
    x + y
}

#[inline]
fn append<T: Copy + Default, const N: usize, const M: usize>(
  xs: &[T; N], ys: &[T; M]) -> [T; add_em(N, M)] {
    let mut result = [T::default(); add_em(N, M)];
    result[..N].copy_from_slice(&xs[..]);
    result[N..].copy_from_slice(&ys[..]);
    result
}

fn main() {
    let arr = append(&[1i32; 3], &[2i32; 4]);
    dbg!(arr);
}