Mutable int parameter

[tmssngr]

I'm no Rust expert, so bare with me if my assumption is wrong:

fn main() {
    ...
    let rnd = rand.IntN(max);
    let chr = to_char(rnd);
    ...
}

fn to_char(mut n: int) -> int {
    ...
    n = n - 10;
    ...
}

I defined the parameter n as mutable in to_char, because I want to modify it in to_char only. However, lis complains for main because rnd is not defined as mut.

  🔴 Immutable argument passed to mut parameter
    ╭─[main.lis:11:21]
 10 │         let rnd = rand.IntN(max);
 11 │         let chr = to_char(rnd) as byte;
    ·                           ─┬─
    ·                            ╰── expected mutable, found immutable
 12 │         buffer.WriteByte(chr);
    ╰────
  help: Bindings in Lisette are immutable by default. Use let mut rnd = ... to allow mutation, or make the parameter immutable by removing mut from the function signature · code: [infer.immutable_arg_to_mut_param]

Here comes my limited Rust knowledge into the room: I assume that modifying n in to_char does not influence rnd in main (it is no reference, but a primitive which I expect to be copied), why lis wants me to make rnd mut?

[ivov]

Your understanding is correct, the rule is misfiring in this case. On params it should apply only to types where caller-side mutation is possible. Tracking at #369

[ivov]

Addressed in lisette-v0.2.2