CODEWITHSUNDEEP
rustpattern-matchingmove-semantics
George Shuklin
George Shuklin

Reputation: 7937

How to match enum with parameter without destructing in Rust?

I have enum with variants with value:

enum Foobar<T, G> {
    Foo,
    Bar(T),
    Baz(G),
}

I have a piece of code where I need to match enum value, but I don't want to destruct it.

fn foobar<T, G, F1, F2>(value: Foobar<T, G>, f1: F1, f2: F2) -> bool
where
    F1: Fn(T) -> bool,
    F2: Fn(G),
{
    let res = match value {
        Foobar::Foo => true,
        Foobar::Bar(v) => f1(v),
        Foobar::Baz => false,
    };
    if let Foobar::Baz(v2) = value {
        f2(v2);
    }
    res
}

It won't compile because expected unit struct, unit variant or constant, found tuple variant Foobar::Baz

I don't want to cange it to Foobar::Baz(_) because it will force Copy trait on G, and I don't want it.

My example is a bit artificial, but let's assume I need to keep separate call to f2.

Is there a way to match an enum variant without destructing it?

Upvotes: 1

Views: 3207

Answers (1)

Peter Hall
Peter Hall

Reputation: 58875

You can't.

But it wouldn't help anyway. The problem is that value is not available after the match because you (might have) moved data from it into f1.

You can fix it by reorganising the code a little, so that you only look at Baz when the value from Bar has not been moved:

fn foobar<T, G, F1, F2>(value: Foobar<T, G>, f1: F1, f2: F2) -> bool
where
    F1: Fn(T) -> bool,
    F2: Fn(G),
{
    let res = match value {
        Foobar::Foo => true,
        Foobar::Bar(v) => f1(v),
        Foobar::Baz(v2) => {
            f2(v2);
            false
        },
    };
    res
}

Upvotes: 2

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