Why is std::rc::Rc<> not Copy?

Question

Can someone explain to me why Rc<> is not Copy?

I\'m writing code that uses a lot of shared pointers, and having to type .clone

Answer 1

A type cannot implement Copy if it implements Drop (source). Since Rc does implement it to decrement its reference count, it is not possible.

In addition, Rc is not just a pointer. It consists of a Shared:

pub struct Rc {
    ptr: Shared>,
}

Which, in turn, is not only a pointer:

pub struct Shared {
    pointer: NonZero<*const T>,
    _marker: PhantomData,
}

PhantomData is needed to express the ownership of T:

this marker has no consequences for variance, but is necessary for dropck to understand that we logically own a T.

For details, see: https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data