Method Receiver

The &self above indicates that the method borrows the object immutably. There are other possible receivers for a method:

• &self: borrows the object from the caller using a shared and immutable reference. The object can be used again afterwards.
• &mut self: borrows the object from the caller using a unique and mutable reference. The object can be used again afterwards.
• self: takes ownership of the object and moves it away from the caller. The method becomes the owner of the object. The object will be dropped (deallocated) when the method returns, unless its ownership is explicitly transmitted.
• mut self: same as above, but while the method owns the object, it can mutate it too. Complete ownership does not automatically mean mutability.
• No receiver: this becomes a static method on the struct. Typically used to create constructors which are called new by convention.

Beyond variants on self, there are also special wrapper types allowed to be receiver types, such as Box<Self>.

Consider emphasizing “shared and immutable” and “unique and mutable”. These constraints always come together in Rust due to borrow checker rules, and self is no exception. It isn’t possible to reference a struct from multiple locations and call a mutating (&mut self) method on it.