mut無しとはこういうことか
https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=a5753729862ae878a1cf011314e90f3e

fn qsort<T: Copy+Ord>(list: &[T]) -> Vec<T> {
match list {
[pivot, ..] => {
let (smaller, rest): (Vec<T>, Vec<T>) =
list[1..].iter().partition(|&i| i < pivot);
[qsort(&smaller), vec![*pivot], qsort(&rest)].concat()
},
_ => vec![]
}
}

fn main() {
let list = [9, 0, 7, 3, 6, 1, 2, 4, 8, 5];
println!("{:?}", list);
println!("{:?}", qsort(&list));
}