impl Trait in return position is inconsistently RefUnwindSafe

[jakoschiko]

The following code compiles:

fn foo() -> impl std::fmt::Debug {
    42
}

fn main() {
    let x = foo();
    let _ = std::panic::catch_unwind(|| println!("{:?}", x));
}

I would expect that changing the implementation of foo is not a breaking change. But the following code does not compile:

fn foo() -> impl std::fmt::Debug {
    std::rc::Rc::new(42)
}

fn main() {
    let x = foo();
    let _ = std::panic::catch_unwind(|| println!("{:?}", x));
}

The compiler says:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the type `std::cell::UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
 --> src/main.rs:7:13
  |
7 |     let _ = std::panic::catch_unwind(|| println!("{:?}", x));
  |             ^^^^^^^^^^^^^^^^^^^^^^^^ `std::cell::UnsafeCell<usize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
  |
  = help: within `impl std::fmt::Debug`, the trait `std::panic::RefUnwindSafe` is not implemented for `std::cell::UnsafeCell<usize>`
  = note: required because it appears within the type `std::cell::Cell<usize>`
  = note: required because it appears within the type `std::rc::RcBox<i32>`
  = note: required because it appears within the type `*const std::rc::RcBox<i32>`
  = note: required because it appears within the type `std::ptr::NonNull<std::rc::RcBox<i32>>`
  = note: required because it appears within the type `std::rc::Rc<i32>`
  = note: required because it appears within the type `impl std::fmt::Debug`
  = note: required because of the requirements on the impl of `std::panic::UnwindSafe` for `&impl std::fmt::Debug`
  = note: required because it appears within the type `[closure@src/main.rs:7:38: 7:60 x:&impl std::fmt::Debug]`
  = note: required by `std::panic::catch_unwind`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: Could not compile `playground`.

Tested version: rustc 1.38.0 (625451e37 2019-09-23)

[jakoschiko]

Related to #45924?

[hanna-kruppe]

UnwindSafe and RefUnwindSafe are auto traits (like Send and Sync) and impl trait return types are defined to "leak" auto traits, i.e., code can rely on whether auto traits are implemented by the concrete underlying type even if the auto trait is not explicitly listed in the bounds. This has been (and probably still is) controversial, but the feature is working the way it's been decided it should work after a years-long design process. So I'm going to close this as "not a bug".