Rust Ownership Smart Pointers

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I've recently started learning Rust and just learned about the Smart Pointers (Box, Rc and RefCell).

In the guide they talked about Rc implementing "shared ownership". But if I understood it correctly, the whole point of the ownership system is that there can only be one owner.

And to me (still a Rust newbie) it seems as if Rc and RefCell take ownership of they value they contain and just "expose" different types of references to the contained value?

Am I wrong and if yes: why is Rust allowed to "cheat" the ownership system like that and would I be theoretically able to implement my own "cheating" types?

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1 Answers

if I understood it correctly, the whole point of the ownership system is that there can only be one owner.

No. Rust guarantees that there can be no more than a single mutable borrow and there cannot be mutable and non-mutable borrows at the same time. It doesn't say anything about owners.

why is Rust allowed to "cheat" the ownership system

It doesn't.

would I be theoretically able to implement my own "cheating" types

Yes. Those types are all implemented in Rust¹. Those types are battle-tested and perfectly safe under Rust's safety rules, but they require the use of unsafe at a lower level.

Note that unsafe doesn't permit going around the rule that you can have one mutable borrow XOR any number of non-mutable borrows, but using unsafe, you could do it anyway. This, of course, would actually be unsafe (and trigger undefined behavior).

1: Although some of those types are implemented using features that are still private to the compiler so you wouldn't be able to do everything as efficiently as the standard library, and Box and UnsafeCell are special to the language and cannot be reproduced by a normal library. There are for example many crates providing Rc or Arc alternatives which are better that the standard ones in some cases.

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