The "rc_buffer" lint is probably wrong for inner Vec types which implement Clone or Copy

I previously landed a clarification that this lint is not always correct, depending on the use-case: https://github.com/rust-lang/rust-clippy/pull/6090

Today I discovered that Rc::make_mut() and Arc::make_mut() exist for Rc<T: Clone> and Arc<T: Clone> respectively.

The rc_buffer lint says that you can only get a mutable reference if there are no mutable references or else panic, which is not true due to these functions. Copy-on-write is perfectly reasonable for types which are Clone. See also https://github.com/http-rs/tide/pull/747.

I'm willing to make a change to only have this lint apply when T is not Clone if someone can point me in the direction of how to do that. It looks like there are 4 lint sites, two of which are for slices. The lint should also be updated to mention make_mut, which could be done either in the same PR or separately.

Start by locating the four rc_buffer lint sites mentioned in the issue, including the two handling slices, and inspect how they determine whether the inner type is Clone. Check the existing lint tests and documentation around rc_buffer. Done means the lint no longer recommends the change for Clone types and its message explains Rc::make_mut() and Arc::make_mut().