maybe-async-cfg2

Don't repeat yourself when writing blocking and async code.

When implementing both sync and async variants of an API in a crate, the APIs of the two variants are almost the same except for async/await keywords.

maybe-async-cfg2 helps unify async and sync implementations using a procedural macro.

Write async code with normal async and await keywords, and let maybe_async_cfg2 handle removing them when blocking code is needed.
Add maybe attributes and specify feature conditions in the macro parameters to determine which variant of code should be generated.
Use only_if (or remove_if) to keep code in a specific variant when necessary.

The maybe procedural macro can be applied to the following code:

use declarations
trait declarations
trait implementations
function definitions
struct and enum definitions
modules

RECOMMENDATION: Use resolver version 2 in Cargo.toml, which was introduced in Rust 1.51. Without it, two crates in a dependency with conflicting versions (one async and another blocking) can fail compilation.

[package]
resolver = "2"
# or when using workspaces
[workspace]
resolver = "2"

Motivation

The async/await language feature transformed the async world of Rust. Compared with the map/and_then style, async code now more closely resembles sync code.

In many crates, the async and sync variants share the same API, but the minor difference that all async code must be awaited prevents the unification of async and sync code. In other words, it is necessary to write an async and a sync implementation respectively.

Macros in Detail

To use maybe-async-cfg2, it is necessary to distinguish which code is used exclusively in the sync vs. async variants. These two variants of the implementation should share the same function signatures except for async/await keywords.

Use the maybe macro for code that is the same in both async and sync variants. Specify in the macro parameters the conditions (based on features) under which async and/or sync variants of the code should appear.

attribute macro maybe

Offers a unified way to provide sync and async conversion on demand depending on enabled feature flags, with an async first policy.
```
[dependencies]
maybe_async_cfg2 = "0.3"

[features]
use_sync = []
use_async = []
```
In this and all the following examples, two features are used. Any conditions can be used, for example, replacing feature="use_sync" with not(feature="use_async") everywhere. maybe-async-cfg2 does not analyze the conditions in any way, just substituting them as is.

Add the maybe attribute before all items that must be different in sync vs. async code.

To keep async code, specify the async parameter with the condition (based on features) for when the code should be async.

To convert async code to sync, specify the sync parameter with the condition when sync code should be generated.
```
#[maybe_async_cfg2::maybe(
    idents(Foo),
    sync(feature="use_sync"),
    async(feature="use_async")
)]
struct Struct {
    f: Foo,
}
```
After conversion:
```
#[cfg(feature="use_sync")]
struct StructSync {
    f: FooSync,
}
#[cfg(feature="use_async")]
struct StructAsync {
    f: FooAsync,
}
```

procedural macro content

The content macro allows specifying common parameters for many maybe macros. Use the internal default attribute with the required parameters inside the content macro.

maybe_async_cfg2::content!{
#![maybe_async_cfg2::default(
    idents(Foo, Bar),
)]

#[maybe_async_cfg2::maybe(
    sync(feature="use_sync"), 
    async(feature="use_async")
)]
struct Struct {
    f: Foo,
}

#[maybe_async_cfg2::maybe(
    sync(feature="use_sync"), 
    async(feature="use_async")
)]
async fn func(b: Bar) {
    todo!()
}
} // content!

After conversion:

#[cfg(feature="use_sync")]
struct StructSync {
    f: FooSync,
}
#[cfg(feature="use_async")]
struct StructAsync {
    f: FooAsync,
}

#[cfg(feature="use_sync")]
fn func_sync(b: BarSync) {
    todo!()
}
#[cfg(feature="use_async")]
async fn func_async(b: BarAsync) {
    todo!()
}

Doctests

When writing doctests, they can be marked as applicable only in the corresponding code variant. To do this, specify only_if(VARIANT_KEY) in the doctest attributes. Then in all other variants, this doctest will be replaced with an empty string.

#[maybe_async_cfg2::maybe(
    idents(Foo),
    sync(feature="use_sync"),
    async(feature="use_async")
)]
/// This is a structure. 
/// ```rust, only_if(sync)
/// let s = StructSync{ f: FooSync::new() };
/// ```
/// ```rust, only_if(async)
/// let s = StructAsync{ f: FooAsync::new().await };
/// ```
struct Struct {
    f: Foo,
}

After conversion:

#[cfg(feature="use_sync")]
/// This is a structure. 
/// ```rust, only_if(sync)
/// let s = StructSync{ f: FooSync::new() };
/// ```
///
struct StructSync {
f: FooSync,
}
#[cfg(feature="use_async")]
/// This is a structure. 
///
/// ```rust, only_if(async)
/// let s = StructAsync{ f: FooAsync::new().await };
/// ```
struct StructAsync {
    f: FooAsync,
}

Examples

Rust client for services

When implementing a Rust client for any service, like AWS S3, the higher-level API of async and sync variants is almost the same, such as creating or deleting a bucket, retrieving an object, etc.

The example service_client is a proof of concept that maybe_async_cfg2 can eliminate the need to write duplicate code for sync and async variants. The is_sync feature gate allows toggling between sync and async AWZ3 client implementations.

Acknowledgements

This crate is a maintained fork of:

nvksv/maybe-async-cfg

which is a redesigned fork of these wonderful crates:

Thanks!

License

MIT

maybe-async-cfg2 0.3.0