[Issue 23998] @mustuse should require opCast(T:bool) is checked

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23998

Paul Backus <snarwin+bugzilla at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |snarwin+bugzilla at gmail.com

--- Comment #5 from Paul Backus <snarwin+bugzilla at gmail.com> ---
Dennis is correct. This is completely outside the purview of @mustuse. What you
are asking for is a guarantee not just that the object has been used, but that
it has been used *correctly* (according to some programmer-defined criteria).

For this specific example, I think the best solution is to redesign your Result
type so that it is impossible to access the wrapped value without performing a
null check. Here's one possible way to do so:

---
import core.attribute, std.stdio;

@mustuse struct Result {
    private int* value;
    bool isNull() => value is null;
    int opApply(scope int delegate(int) dg)
    {
        if (value is null) return 0;
        return dg(*value);
    }
}

Result getResult() => Result(new int(42));

void main()
{
    auto r = getResult();
    foreach (int n; r)
        writeln("got ", n);
}
---

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