Rework timeout implementation to panic when the timer is dropped.

[rjsberry]

This also changes the extension traits to extend Future/Stream instead of TryFuture/TryStream.

Follows up on review of #22 (sorry about the delay!)

For now I've left this as a draft because I've set the Output/Item in the extension traits to an Option. @yoshuawuyts I'd be interested on your thoughts as to whether we want this to be:

• Option as is.
• io::Error as in runtime.
• Another custom error.
• Something else.

I don't love Option here, especially with the nested option in the stream extension. I think something like

struct TimeoutError(());

might work quite well instead of io::Error.

---

Description

This reworks the timer implementation to panic on error instead of returning an io::Error.

Motivation and Context

This simplifies the extension traits and removes the From<io::Error> trait bound from #21.

How Has This Been Tested?

Tests reworked/pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[rjsberry]

@yoshuawuyts Have you had a chance to give this a once over?

[rjsberry]

I've updated Option to Result using a custom error type over io::Error.

[yoshuawuyts]

@rjsberry heya! -- sorry about the long wait, I was a bit busy over the past month. Um so there've been a few developments here:

• Timeout improvements #26 exists which adds a new timeout error, and I believe overlaps slightly with this PR.
• https://github.com/async-rs/async-std/blob/38a86766d3e3394e2f22cbba03cc7af448c67614/src/future/timeout.rs has a similar approach as Timeout improvements #26, but differentiates between "regular" timeouts, and io::timeout.

I'm not 100% sure how this relates back to this PR, but I have a hunch it might influence the design taken. I think at least finding a resolution with #26 would probably be good because it seems both PRs have a lot of overlap.

[rjsberry]

No problem!

Yes, this and #26 are quite similar but I think incompatible. This PR promotes extending Future and TryFuture, whereas #26 keeps the extension traits locked to TryFuture.

I'm not sure I agree with the direction of TimeoutError error in #26:

pub enum TimeoutError<E> {
    TimedOut,
    Inner(E),
}

If we parameterize the output of the extension traits with E, are we saying that futures with no error always need to be converted before they can be used with the extension traits? That is exactly what I would like to avoid.

If we agree the timer implementation should panic on error, I think the extension trait will work best as

pub trait FutureExt: Future {
    fn timeout(self, dur: Duration) -> Timeout { ... }
}

Instead of

pub trait TryFutureExt: TryFuture {
    fn timeout(self, dur: Duration) -> Timeout { ... }
}