*a == *b moves when dereferencing &Rc<RefCell<dyn Trait>>

[cuviper]

This bug report comes from a question in the users forum:
https://users.rust-lang.org/t/attempting-to-contain-an-rc-refcell-sometrait-in-an-enum-field-with-clone/46913

They had a problem with #[derive(PartialEq)] on an enum where one variant contains Rc<RefCell<dyn Trait>>, and there is a manual PartialEq for that dyn Trait. I looked at the macro expansion to see that it matches ref patterns and then uses (*__self_0) == (*__arg_1_0), so I reduced the error to this code:

use std::cell::RefCell;
use std::rc::Rc;

pub trait Trait {}

impl PartialEq<dyn Trait> for dyn Trait {
    fn eq(&self, _other: &Self) -> bool {
        todo!();
    }
}

pub fn eq(a: &Rc<RefCell<dyn Trait>>, b: &Rc<RefCell<dyn Trait>>) -> bool {
    *a == *b
}

I expected to see this happen: successful compilation

Instead, this happened: (playground)

error[E0507]: cannot move out of `*b` which is behind a shared reference
  --> src/lib.rs:13:11
   |
13 |     *a == *b
   |           ^^ move occurs because `*b` has type `std::rc::Rc<std::cell::RefCell<dyn Trait>>`, which does not implement the `Copy` trait

Meta

The error is the same with all compilers currently on the playground, 1.45.2, 1.46.0-beta.2, and 1.47.0-nightly (2020-08-05 22ee68dc586440f96b76).

Reduction

Changing to a == b works, but that's not helpful when we want the derived code to work.

Reducing the parameters to &RefCell<dyn Trait> does build successfully.

Reducing the parameters to &dyn Trait changes to a lifetime error:

error[E0621]: explicit lifetime required in the type of `a`
  --> src/lib.rs:13:8
   |
12 | pub fn eq(a: &dyn Trait, b: &dyn Trait) -> bool {
   |              ---------- help: add explicit lifetime `'static` to the type of `a`: `&'static (dyn Trait + 'static)`
13 |     *a == *b
   |        ^^ lifetime `'static` required

error[E0621]: explicit lifetime required in the type of `b`
  --> src/lib.rs:13:11
   |
12 | pub fn eq(a: &dyn Trait, b: &dyn Trait) -> bool {
   |                             ---------- help: add explicit lifetime `'static` to the type of `b`: `&'static (dyn Trait + 'static)`
13 |     *a == *b
   |           ^^ lifetime `'static` required

This one is solved by changing to 'static as suggested, or by changing the PartialEq to:

impl<'a> PartialEq<dyn Trait + 'a> for dyn Trait + 'a { ... }
// or even
impl<'a, 'b> PartialEq<dyn Trait + 'b> for dyn Trait + 'a { ... }

... but that change does nothing for the original issue.

[lcnr]

Is this a duplicate of #31740?

[cuviper]

I suppose it is a dupe, or at least close enough that it's probably the same root cause. The context is a little different here, since it came up as part of a derived implementation, which makes it even more troublesome.

BTW, changing the parameters to &Rc<dyn Trait> also fails the same way, so I guess that's a further reduction. Same for &Box<dyn Trait>, more like #31740, but it may be important to remember that this doesn't only affect Box.