executor: move into tokio crate

[carllerche]

A step towards collapsing Tokio sub crates into a single tokio
crate (#1318).

The executor implementation is now provided by the main tokio crate.
Functionality can be opted out of by using the various net related
feature flags.

[hawkw]

Looks like this needs to be rebased, since #1625 made a bunch of conflicting changes.

[najamelan]

I was using the tokio-executor crate directly, to avoid bloating dependencies. It looks like currently the only way to get the functionality back is to enable the "rt-full" feature.

There are many reasons not to want to pull all of this in. Maybe a library just wants to implement traits Executor and TypedExecutor without even pulling in an impl, maybe one writes an application which does FS access, but no networking, maybe one wants to use the Actor model as a software design model and do async even without IO, etc...

Would it be possible to design the features so as to be able to enable things bottom up, eg. not pulling anything not needed at each step?

Slightly off topic: Other recent changes have made tokio harder to use flexibly. eg. Networking types will no longer work when not within a runtime (before it was possible to use the networking primitives and spawn their futures on a non-tokio executor).

It's unfortunate that the reasoning is package-deal like. I am very reluctant to use frameworks like this, even though I feel tokio has an excellent executor implementation. It's all good when writing a small app, but when writing libraries that shall be used by others, there's no way to justify forcing the package deal on downstream users.

[taiki-e]

@najamelan FYI: Once #1695 is merged, it should be able to use traits without depending on the runtime: https://github.com/tokio-rs/tokio/pull/1695/files#diff-66fb4a030b180360b997fa738670656fR124-R125

[najamelan]

@taiki-e Thanks, so I see there will be executor-core. Will it be possible to use the 2 executor impls that existed in tokio-executor without pulling reactor/timer/network?

[carllerche]

@najamelan thanks for the heads up. The state if things is not final. The plan will be to enable the ability to get fine grained access to the various components using feature flags.

Collapsing crates is just the first step.

Could you open a new issue so we are sure to cover your use case?

[najamelan]

@carllerche Ok, thanks will open an issue.