Structural configuration approaches

// AVOID: Using cfg directly on function parameters
fn spawn(
    &mut self,
    task: T,
    scheduler: S,
    id: super::Id,
    #[cfg(tokio_unstable)] spawned_at: &'static Location<'static>,
) { /* ... */ }

// PREFER: Wrapping conditional data in a struct
struct SpawnLocation {
    #[cfg(tokio_unstable)]
    location: Option<&'static Location<'static>>,
    // The struct is effectively zero-sized when the feature is disabled
}

fn spawn(
    &mut self,
    task: T,
    scheduler: S,
    id: super::Id,
    location: SpawnLocation,
) { /* ... */ }

// AVOID: Scattered cfg blocks within functions
fn now() {
    #[cfg(target_family = "wasm")]
    { return None; }
    #[cfg(not(target_family = "wasm"))]
    { return Some(Instant::now()); }
}

// PREFER: Abstraction functions that handle conditional logic
fn now() -> Option<Instant> {
    if cfg!(target_family = "wasm") { None } else { Some(Instant::now()) }
}

// AVOID: Mixed conditional blocks within function bodies
fn submit(&mut self, worker: &WorkerMetrics, mean_poll_time: u64) {
    #[cfg(tokio_unstable)]
    {
        // unstable metrics code...
    }
    // stable metrics code...
}

// PREFER: Separate implementations with a common pattern
cfg_unstable_metrics! {
    fn submit_unstable(&mut self, worker: &WorkerMetrics, mean_poll_time: u64) {
        // unstable metrics code...
    }
}

fn submit(&mut self, worker: &WorkerMetrics, mean_poll_time: u64) {
    // stable metrics code...
    #[cfg(tokio_unstable)]
    self.submit_unstable(worker, mean_poll_time);
}

// For metrics that have both stable and unstable implementations
macro_rules! cfg_metrics_impl {
    ($name:ident, stable: $stable_impl:block, unstable: $unstable_impl:block) => {
        cfg_unstable_metrics! {
            fn $name(&mut self) $unstable_impl
        }
        cfg_not_unstable_metrics! {
            fn $name(&mut self) $stable_impl
        }
    }
}