Function trace_with 

pub fn trace_with<FN, FT, R>(f: FN, trace_leaf: FT) -> R
where
    FN: FnOnce() -> R,
    FT: FnMut(&TraceMeta),

Available on tokio_unstable and crate feature taskdump and crate feature rt and Linux and (AArch64 or x86 or x86-64) only.

Runs f. If f hits a Tokio yield point trace_leaf will be invoked.

This allows taking a task dump with caller-provided task dump machinery. If f is the poll function of a future and that future returns Poll::Pending, then trace_leaf will be invoked. trace_leaf can then take a backtrace to determine exactly where the yield occurred.

Example

use std::future::Future;
use std::task::Poll;
use tokio::runtime::dump::{trace_with, Trace, TraceMeta};

fn my_trace_leaf(_meta: &TraceMeta, count: &mut u32) {
    *count += 1;
}

let mut fut = std::pin::pin!(async {
    tokio::task::yield_now().await;
});

let mut leaf_count = 0;

Trace::root(std::future::poll_fn(|cx| {
    trace_with(
        || { let _ = fut.as_mut().poll(cx); },
        |meta| my_trace_leaf(meta, &mut leaf_count),
    );
    Poll::Ready(())
})).await;

assert!(leaf_count > 0);