Ownership 101

Now that we can construct a list, it'd be nice to be able to do something with it. We do that with "normal" (non-static) methods. Methods are a special case of function in Rust because of the self argument, which doesn't have a declared type:

fn main() { fn foo(self, arg2: Type2) -> ReturnType { // body } }

fn foo(self, arg2: Type2) -> ReturnType {
    // body
}

There's 3 primary forms that self can take: self, &mut self, and &self. These 3 forms represent the three primary forms of ownership in Rust:

• self - Value
• &mut self - mutable reference
• &self - shared reference

A value represents true ownership. You can do whatever you want with a value: move it, destroy it, mutate it, or loan it out via a reference. When you pass something by value, it's moved to the new location. The new location now owns the value, and the old location can no longer access it. For this reason most methods don't want self -- it would be pretty lame if trying to work with a list made it go away!

A mutable reference represents temporary exclusive access to a value that you don't own. You're allowed to do absolutely anything you want to a value you have a mutable reference to as long as when your loan expires, wherever you loaned it from still sees a valid value. This means you can actually completely overwrite the value. A really useful special case of this is swapping a value out for another, which we'll be using a lot. The only thing you can't do with an &mut is move the value out with no replacement. &mut self is great for methods that want to mutate self.

A shared reference represents temporary shared access to a value that you don't own. Because you have shared access, you're generally not allowed to mutate anything. Think of & as putting the value out on display in a museum. & is great for methods that only want to observe self.

Later we'll see that the rule about mutation can be bypassed in certain cases. This is why shared references aren't called immutable references. Really, mutable references could be called unique references, but we've found that relating ownership to mutability gives the right intuition 99% of the time.