No, I mean single-letter vars are standard in physics and math, but reusing vars is not acceptable. Obviously they're not good practice except in the scenarios you describe, but mathies gonna math.

Length might have mattered in the 80s and 90s when IDEs were crap but we got autocomplete in pretty much all our text editors (even TUI ones like vim).

As for readability there is an argument to be had in specific contexts, but 9 out of 10 times it makes more sense to use a proper word.

Example:

let list = [1, 2, 3];
for i in list {
    println!("{}", i);
}

In this case using item in the place of i would be more fitting.