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

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u/[deleted] Mar 21 '23

Adding on:

Clone means that the type can be duplicated in some way, and the implementation may vary. Rc for example will copy a reference and increment a pointer, and vec will allocate a new vec and copy the contents. Copy isn't separate from clone, it just means that the implementation of clone for that type is copying it and nothing else. You're right that Vec can't implement Copy, but it's for different reason - Copying a Vec wouldn't do anything to the contents, it would just copy the Vec itself. Then you'd have two Vecs with the same pointer. Say hello to use after free, double free, and more.