Sharing closure-fed library functions across async and sync

lib.rs

/// Scenario #1:
mod not_fut{
    /// A lib function designed to take a closure that returns a `String`
    fn lib_function<F>(get_account_data_fn: F) -> String
    where
        F: Fn(u8) -> String,
    {
        let account_data = get_account_data_fn(42);
        println!("Account data: {}", account_data);
        account_data
    }

    /// A sample async function, such as an RPC client call
    async fn some_async_fn(num: u8) -> String {
        num.to_string()
    }
    
    /// This is the async context, where we want to call the lib function
    async fn async_context() {
        // [Error]
        // `await` is only allowed inside `async` functions and blocks
        let _: String = lib_function(|num| { some_async_fn(num).await });
        // [Error]
        // async closures are unstable
        // see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
        let _: String = lib_function(async |num| { some_async_fn(num).await });
    }

    /// This is the sync context, where we want to call the lib function
    fn sync_context<F, Fut>() {
        // [Ok]
        let _: String = lib_function(|num| { num.to_string() });
    }
}

/// Scenario #2:
mod with_fut{
    use std::future::Future;

    /// A lib function designed to take a closure that returns a `Fut<String>`
    fn lib_function<F, Fut>(get_account_data_fn: F) -> Fut
    where
        F: Fn(u8) -> Fut,
        Fut: Future<Output = String>,
    {
        let account_data = get_account_data_fn(42);
        // [Error]
        // `Fut` doesn't implement `std::fmt::Display`
        println!("Account data: {}", account_data);
        account_data
    }

    /// A sample async function, such as an RPC client call
    async fn some_async_fn(num: u8) -> String {
        num.to_string()
    }
    
    /// This is the async context, where we want to call the lib function
    async fn async_context() {
        // [Ok]
        let _: String = lib_function(some_async_fn).await;
    }

    /// This is the sync context, where we want to call the lib function
    fn sync_context<F, Fut>() {
        // [Error]
        // `String` is not a future
        let _: String = lib_function(|num| { num.to_string() });
    }
}