tokio::net::UdpSocket is not marked as UnwindSafe

[heinrich5991]

Version

├── tokio v1.6.0
│   └── tokio-macros v1.2.0 (proc-macro)
    │   │   ├── tokio v1.6.0 (*)
    │   │   ├── tokio-util v0.6.7
    │   │   │   └── tokio v1.6.0 (*)
    │   ├── tokio v1.6.0 (*)
    ├── tokio v1.6.0 (*)
    ├── tokio-stream v0.1.6
    │   └── tokio v1.6.0 (*)
    ├── tokio-tungstenite v0.13.0
    │   ├── tokio v1.6.0 (*)
    ├── tokio-util v0.6.7 (*)

Platform
Should be irrelevant.

Description
Cannot use &tokio::net::UdpSocket across std::panic::catch_unwind because net::UdpSocket is not marked as UnwindSafe.

error[E0277]: the type `UnsafeCell<AtomicUsize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   --> src/main.rs:316:19
    |
316 |             match panic::catch_unwind(|| {
    |                   ^^^^^^^^^^^^^^^^^^^ `UnsafeCell<AtomicUsize>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    | 
   ::: toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:430:40
    |
430 | pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
    |                                        ---------- required by this bound in `catch_unwind`
    |
    = help: within `Arc<tokio::net::UdpSocket>`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<AtomicUsize>`
    = note: required because it appears within the type `tokio::loom::std::atomic_usize::AtomicUsize`
    = note: required because it appears within the type `tokio::io::driver::scheduled_io::ScheduledIo`
    = note: required because it appears within the type `tokio::util::slab::Value<tokio::io::driver::scheduled_io::ScheduledIo>`
    = note: required because it appears within the type `*const tokio::util::slab::Value<tokio::io::driver::scheduled_io::ScheduledIo>`
    = note: required because it appears within the type `tokio::util::slab::Ref<tokio::io::driver::scheduled_io::ScheduledIo>`
    = note: required because it appears within the type `tokio::io::driver::registration::Registration`
    = note: required because it appears within the type `tokio::io::poll_evented::PollEvented<mio::net::udp::UdpSocket>`
    = note: required because it appears within the type `tokio::net::UdpSocket`
    = note: required because it appears within the type `alloc::sync::ArcInner<tokio::net::UdpSocket>`
    = note: required because it appears within the type `PhantomData<alloc::sync::ArcInner<tokio::net::UdpSocket>>`
    = note: required because it appears within the type `Arc<tokio::net::UdpSocket>`
    = note: required because of the requirements on the impl of `UnwindSafe` for `&Arc<tokio::net::UdpSocket>`
    = note: required because it appears within the type `[closure@src/main.rs:316:39: 324:14]`

[Darksonn]

I'm fine with adding these impls. You can safely use AssertUnwindSafe to circumvent it.