Mutable comptime globals

[jfecher]

Problem

comptime globals should also be mutable - unlike runtime globals currently.

Happy Case

comptime mut global COUNTER: Field = 0;

comptime fn get_unique_id() -> Field {
    let id = COUNTER;
    COUNTER += 1;
    id
}

fn id1() -> Field {
    comptime { get_unique_id() }
}

fn id2() -> Field {
    comptime { get_unique_id() }
}

fn main() {
    // Order of comptime evaluation between functions isn't guaranteed
    // so we don't know if (id1 == 0 && id2 == 1) or if (id1 == 1 && id2 == 0).
    // we only know they are not equal
    let id1 = id1();
    let id2 = id2();
    assert(id1 != id2);
}

Project Impact

None

Impact Context

No response

Workaround

None

Workaround Description

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

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Support Needs

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[jfecher]

Implemented by #4926