inspect.rs

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};

struct Inspect<F>(F);

impl<F: Future> Future for Inspect<F> {
    type Output = F::Output;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let t = Instant::now();
        let res = unsafe { self.map_unchecked_mut(|f| &mut f.0) }.poll(cx);
        let t = t.elapsed();
        if t > Duration::from_millis(10) {
            warn!("future polled for {:?}", t);
        }
        res
    }
}