might_be_async

A proc macro that gen both async & sync fn, toggle via feature flag.

Installation

Add the dependency in Cargo.toml:

[dependencies]
might_be_async = "0.1.0"

Or use the cargo add command:

cargo add might_be_async

Usage

First, choose a feature name for your async toggle. By default might_be_async uses the feature name "async", but you can use any name you like.

# Cargo.toml

[package.metadata.might_be_async]
default_feature_name = "async" # Default

[dependencies]
might_be_async = "0.1.0"

Now you can annotate your functions with #[func]:

#[might_be_async::func]
fn greet(name: &str) -> String {
    format!("Hello, {name}!")
}

When the async feature is off, greet remains a plain synchronous fn. When the async feature is on, greet becomes an async fn — its signature changes to async fn greet(name: &str) -> String.

Calling annotated functions

If one #[func] function calls another, use the invoke! macro to handle the .await automatically:

#[might_be_async::func]
fn double(x: i32) -> i32 {
    x * 2
}

#[might_be_async::func]
fn compute(a: i32, b: i32) -> i32 {
    let sum = a + b;
    might_be_async::invoke!(double(sum))   // adds .await in async mode, does nothing in sync mode
}

Feature-specific logic with select!

Use select! to write code that differs between sync and async modes:

#[might_be_async::func]
fn load_data() -> Vec<u8> {
    might_be_async::select!("async" => fetch_async().await else ! => fetch_sync())
}

async fn fetch_async() -> Vec<u8> {
    // ...
    vec![]
}

fn fetch_sync() -> Vec<u8> {
    // ...
    vec![]
}

When the feature "async" is active, the first branch runs. When it's not, the second branch (marked with !) runs.

Switching modes

Run in sync mode (default):

cargo build
cargo run

Run in async mode:

cargo build --features async
cargo run --features async

License

MIT OR Apache-2.0"