I am still unsure when we should better save function or method results in local variables to avoid multiple evaluations. Consider the trivial example

fn remove_zeros(v: &mut Vec<i32>) {
    let l = v.len();
    let mut d = 0;
    let mut s = 0;
    while s < l {
        v[d] = v[s];
        if v[d] != 0 {
            d += 1;
        }
        s += 1;
    }
    v.truncate(d);
}

fn main() {
    let mut v: Vec<i32> = vec![0, 1, 0, 2, 0, 0, 3, 0];
    remove_zeros(&mut v);
    println!("{:?}", v);
}

Do we need the variable l, or would it be guaranteed that

while s < v.len() {

compile to identical code? I think some programming language have the concept of pure functions, which are guaranteed to return always the same result for the same arguments and do not modify global state. But for the concrete example above, the vector is mut, so the concept of pure functions might not help.

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