Ditch async_trait in favor of the official async_fn_in_trait feature

[wvwwvwwv]

Hello @drmingdrmer,

I guess you may already be aware of it, but the async_fn_in_trait feature has recently been stabilized in the nightly Rust channel and will be generally available in Rust 1.75.0. Currently, openraft heavily relies on the use of async_trait in order to provide asynchronous methods in traits, but obviously, async_trait has a negative effect on overall performance in many cases due to heap memory allocation.

So, I suggest that we try to get rid of async_trait incrementally.

• I didn't try it yet, but it would be possible since the current toolchain (nightly-10-17) already includes Stabilize async fn and return-position impl Trait in trait rust-lang/rust#115822
• Unfortunately, a complete removal will not be easy due to many known issues with async_fn_in_trait and related language features, e.g., Send bound errors.
• Due to the notorious Send bound error, I think we should firstly focus on the !OptionalSend case.

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[wvwwvwwv]

Update: just tried to remove async_trait by slightly modifying macros/lib.rs to not emit async_trait, and it works pretty well.

[drmingdrmer]

Make sense.

Since you've already provided an entry in macros/lib.rs thus it would be easy to switch off async_trait, when rust 1.75.0 becomes stable :)

Before that, I think I should add a single threaded application example as a test for singlethreaded feature 🤔

[wvwwvwwv]

Great! Then I'll try removing async_trait in my forked repository, and then make a pull request once 1.75 is out.

[drmingdrmer]

@wvwwvwwv
Is this issue fixed by?:

• Change: #959 ditch async_trait for the singlethreade feature #982

Is there any additional action required?

[wvwwvwwv]

Nope, we can close this issue for now since we cannot currently get rid of every async_trait usage due to some limitations in the Rust compiler.