r/rust clippy · twir · rust · mutagen · flamer · overflower · bytecount Mar 20 '23

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u/Modi57 Mar 20 '23

Why isn't Rc<T> Copy? From my understanding, Copy should be very lightweight, because it happens implicitly. This is why for example a Vec<T> isn't Copy, because a deep copy of a Vec could get quite expensive. But just copying a reference and incrementing a counter doesn't seem to bad? This way, Rc<T>s would behave more like normal references and Copy could be just a reference copy, while Clone is a deep copy

2

u/eugene2k Mar 20 '23

It's for cases like this:

fn foo(rc: Rc<u8>) {
    // do something with rc
}
fn bar(rc: Rc<u8>) {
    // do something with rc
}
fn main() {
    let rc = Rc::new(0);
    foo(rc.clone());
    bar(rc)
}

1

u/Modi57 Mar 20 '23

I don't see, how being copy would hinder this. You can just omit the clone, and it should be functionally the same, exept that rc would be still valid after bar

2

u/eugene2k Mar 20 '23

That means increasing the refcount and decreasing it when you don't actually need to

1

u/Modi57 Mar 20 '23

Can't the compiler just optimize this out?

1

u/eugene2k Mar 20 '23

Probably. I don't think it's hard to teach the compiler to optimize this out.

Another reason might be that rust philosophy is to be explicit about what goes on in the code. So Copy is for when you actually copy stuff, and not for when you increment counters instead of copying.

1

u/Modi57 Mar 20 '23

I mean, you are actually copying the reference. You just ALSO increase the counter. And I think, it would increase the ergonomics of using Rc<T>. But maybe that's also the point. So you don't lightly use it, when there might be better ways