UnwindSafe and RefUnwindSafe should be in libcore

[matklad]

I think that would be useful to allow pure no_std libraries to implement those traits without needing the std cargo feature.

cc @gnzlbg, you've asked about this #40628.

[gnzlbg]

FWIW I also needed AssertUnwindSafe in #[no_std] back then, but I don't remember why.

[ohadravid]

I run into this when changing an std::sync::Mutex to a parking_lot one:

use std::panic::RefUnwindSafe;
use std::sync::Mutex as StdMutex;
use parking_lot::Mutex as ParkingLotMutex;

fn assert_ref_unwind_safe<T: RefUnwindSafe>(t: T) {

}

fn main() {
    assert_ref_unwind_safe(StdMutex::new(0));

    assert_ref_unwind_safe(ParkingLotMutex::new(0));
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the type `std::cell::UnsafeCell<{integer}>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
  --> src/main.rs:12:5
   |
5  | fn assert_ref_unwind_safe<T: RefUnwindSafe>(t: T) {
   |    ----------------------    ------------- required by this bound in `assert_ref_unwind_safe`
...
12 |     assert_ref_unwind_safe(ParkingLotMutex::new(0));
   |     ^^^^^^^^^^^^^^^^^^^^^^ `std::cell::UnsafeCell<{integer}>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   |
   = help: within `lock_api::mutex::Mutex<parking_lot::raw_mutex::RawMutex, {integer}>`, the trait `std::panic::RefUnwindSafe` is not implemented for `std::cell::UnsafeCell<{integer}>`
   = note: required because it appears within the type `lock_api::mutex::Mutex<parking_lot::raw_mutex::RawMutex, {integer}>`
...

But parking_lot is no_std, so it can't implement it!

[steveklabnik]

Triage: no change

[nico-abram]

I believe this was implemented in #84662 and shipped with rustc 1.56, so this can be closed now