task: fix task module feature flags

[mattiapitossi]

Motivation

Currently, if you look at the Tokio docs the task module it's not under the rt feature flag:

But every public module and struct inside task is under rt feature flag (except for coop which is only available for the crate).

This also leads to confusion because items that have multiple feature flags don't display the rt flag:

Solution

The solution is to gate the task module under rt feature flag:

Docs

rendered docs

[mattiapitossi]

Inside the task module only crate coop is available under not_rt

[Darksonn]

I believe tokio::task::coop is public and available when rt is not enabled, so this change seems wrong.

[mattiapitossi]

I believe tokio::task::coop is public and available when rt is not enabled, so this change seems wrong.

Do you mean before this change? coop should be only visible inside the crate when the rt is not enabled:

inside task/mod is defined as:

cfg_not_rt! {
    pub(crate) mod coop;
}

So for example when importing tokio without rt it's private:

[Darksonn]

Even if the module is empty this is a breaking change because some consumers may write:

use tokio::task;

in their codebase even if rt is not enabled.

Can we make this a doc-only change?

#[doc(cfg(feature = "rt"))]
pub mod task;

[mattiapitossi]

Make sense to me if we don't want to introduce such breaking change. I added the ff with a comment to explain, let me know if it makes sense to you to have it.