`Clone` documentation can be confusing to beginners around "duplication" language

[Eveheeero]

Hi, I'm mainly using rust for developing application

I wonder is it safe to impl Clone for Arc<Mutex>

Arc is a frequently used pattern for modifying data in multiple places.

However, if you end up cloning a parent struct that contains an Arc, it's hard to tell that the data inside is the same data without knowing more about the internal structure

For example

#[derive(Clone)]
struct Context {
  things: Object,
  id_to_object: Arc<Mutex<usize, BigObject>>,
}
impl ThreadContext {
  fn add_data(id: usize, obj: BigObject) -> ...;
}
// I cloned ctx to reuse it, but when I call `add_data`, we see that the original ctx also modified
let ctx = old_ctx.clone();

This becomes more likely to happen whenever you wrap it in a parent that implements #[derive(Clone)].

So I thought that struct with Arc<Mutex> shouldn't be able to implement clone as simply as writing #[derive(Clone)].

Is this unsafe? or is it something that users should be aware of?

[cuviper]

Arc<T> implements Clone for any T, without restriction. We don't have any mechanism that could exclude Arc<Mutex>, and even if we did, that would be a breaking change.

Cloning Arc is definitely safe in the usual Rust meaning of the term, but you can reasonably argue that this could be surprising in some cases. In fact, it's often recommended style to call Arc::clone(&x) rather than x.clone() even though they mean the same thing, just to make it clear that the underlying type is not cloned. There's no workaround like that for derive(Clone) though.

[fee1-dead]

The docs for Clone currently says "A common trait for the ability to explicitly duplicate an object."

What would it mean to people new to the language, or have different notions of "duplicating an object" coming from a different programming language?

This is probably a good opportunity to make the documentation clearer and more helpful to describe what exactly Clone is, to prevent people from getting surprised by behaviors like this.