rt: add a method to determine the flavor of the runtime.

[agayev]

Closes: #5088

[Noah-Kennedy]

I'd recommend adding test cases for new APIs like this.

[agayev]

@Noah-Kennedy added doc tests.

[Noah-Kennedy]

Hmm. It has occurred to me that bikeshedding may regrettably be needed here.

I'm not actually sure that this is the right API for this. It may be better to have a get_runtime_flavor function which returns an enum. It isn't guaranteed that we will always have just two flavors. I wouldn't be surprised if future runtimes got added which were similar to multi_thread, but with differences in IO driver implementation, for example.

@Darksonn @carllerche thoughts?

[Darksonn]

Yes, I agree. We are likely add additional flavors in the future, so it would be better with a method that returns a flavor enum.

[agayev]

Added a new enum. To avoid repetition, I called it just Flavor and the function get_flavor, but I can rename them to RuntimeFlavor and get_runtime_flavor, respectively, if you think that's better @Noah-Kennedy @Darksonn.

[Darksonn]

There's a compilation failure when compiling without the rt-multi-thread feature flag.

info: running `cargo test --no-default-features --features rt` on tests-integration (4/10)
   Compiling tokio v1.21.2 (/Users/runner/work/tokio/tokio/tokio)
error[E0599]: no variant or associated item named `MultiThread` found for enum `scheduler::Handle` in the current scope
   --> tokio/src/runtime/handle.rs:304:32
    |
304 |             scheduler::Handle::MultiThread(_) => RuntimeFlavor::MultiThread,
    |                                ^^^^^^^^^^^ variant or associated item not found in `scheduler::Handle`
    |
   ::: tokio/src/runtime/scheduler/mod.rs:14:1
    |
14  | pub(crate) enum Handle {
    | ---------------------- variant or associated item `MultiThread` not found for this enum

For more information about this error, try `rustc --explain E0599`.

[agayev]

There's a compilation failure when compiling without the rt-multi-thread feature flag.

info: running `cargo test --no-default-features --features rt` on tests-integration (4/10)
   Compiling tokio v1.21.2 (/Users/runner/work/tokio/tokio/tokio)
error[E0599]: no variant or associated item named `MultiThread` found for enum `scheduler::Handle` in the current scope
   --> tokio/src/runtime/handle.rs:304:32
    |
304 |             scheduler::Handle::MultiThread(_) => RuntimeFlavor::MultiThread,
    |                                ^^^^^^^^^^^ variant or associated item not found in `scheduler::Handle`
    |
   ::: tokio/src/runtime/scheduler/mod.rs:14:1
    |
14  | pub(crate) enum Handle {
    | ---------------------- variant or associated item `MultiThread` not found for this enum

For more information about this error, try `rustc --explain E0599`.

Fixed it. I'm not sure if not(tokio_wasi) is necessary, but since all other places had it, I also added it.

[Darksonn]

Looks good to me.

[agayev]

Looks good to me.

Thank you!

[wimax-grapl]

Thank you for this :)