Add LocalSet::enter

[gftea]

Motivation

add LocalSet::enter #4736

Solution

LocalSet::enter(&self) -> EnterGuard will set LocalSet's context into TLS CURRENT, and return EnterGuard with a referene to LocalSet itself, the LocalSet reference should outlive EnterGuard.

[Darksonn]

As far as I can tell, we can use Cell rather than RefCell here. When that's possible, then I prefer Cell because it doesn't risk panics.

Besides that, this looks good to me.

[gftea]

OK, just to confirm your thoughts, we have to take value out of Cell everytime and set it back, because the Option<Rc<Context>> is not Copy, cannot use Cell::get

should we set context before panic?

    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
        CURRENT.with(|maybe_cx| match maybe_cx.take() {
            None => panic!("scheduler context missing"),
            Some(cx) => {
                assert!(cx.shared.ptr_eq(self));
                maybe_cx.set(Some(cx.clone()));
                cx.owned.remove(task)
            }
        })
    }

draft solution : maybe set back context before match

    fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
        CURRENT.with(|maybe_cx| {
            let ctx = maybe_cx.take();
            maybe_cx.set(ctx.clone());
            match ctx {
                None => panic!("scheduler context missing"),
                Some(cx) => {
                    assert!(cx.shared.ptr_eq(self));
                    cx.owned.remove(task)
                }
            }
        })
    }

[gftea]

The two failure in CI seems due to server environment issue

[Darksonn]

Please see #4794 for those.

[Darksonn]

If you merge master, then CI should pass.

[gftea]

how to trigger rerun of the failed job in CI?

[Darksonn]

LGTM