Hard to find out whether a type is Send or Sync

[tirr-c]

Currently, only the explicit unsafe impls of Send and Sync (and their ! counterparts) are shown in the docs. This makes me write a test code that actually sends some value across thread boundaries. For example, to find out whether Core from tokio-core is Send, I should test it:

extern crate tokio_core;
extern crate futures;

fn main() {
    let core = tokio_core::reactor::Core::new().unwrap();
    std::thread::spawn(move || {
        let mut xcore = core;
        xcore.run(futures::future::ok::<usize, usize>(0)).unwrap();
    }).join().unwrap();
}

Then it turns out that Core does not implement Send. (And it's hard to get the point quickly.)

error[E0277]: the trait bound `std::rc::Rc<std::cell::RefCell<tokio_core::reactor::Inner>>: std::marker::Send` is not satisfied in `[closure@src/main.rs:6:24: 9:6 core:tokio_core::reactor::Core]`
 --> src/main.rs:6:5
  |
6 |     std::thread::spawn(move || {
  |     ^^^^^^^^^^^^^^^^^^ `std::rc::Rc<std::cell::RefCell<tokio_core::reactor::Inner>>` cannot be sent between threads safely
  |
  = help: within `[closure@src/main.rs:6:24: 9:6 core:tokio_core::reactor::Core]`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::cell::RefCell<tokio_core::reactor::Inner>>`
  = note: required because it appears within the type `tokio_core::reactor::Core`
  = note: required because it appears within the type `[closure@src/main.rs:6:24: 9:6 core:tokio_core::reactor::Core]`
  = note: required by `std::thread::spawn`

Of course, we normally don't send Core across thread boundaries, but this is just an example. I often encounter this kind of question when using a crate for the first time. I want rustdoc to automatically generate whether a type implements Send or Sync. Maybe the information is in the section other than "Trait Implementations."

[kennytm]

Duplicate of #17606

[tirr-c]

Oh, I forgot to search first.