refac(subscriber): remove futures dependency #460)

I noticed that `console-subscriber` was my only dependency pulling in
the `futures` top-level library so I decided to try my hand at removing
that dependency. This still relies on `futures-task` for an optimized
`NoopWaker`

console-subscriber/Cargo.toml

@@ -40,7 +40,7 @@ tonic = { version = "0.9", features = ["transport"] }
 tracing-core = "0.1.24"
 tracing = "0.1.26"
 tracing-subscriber = { version = "0.3.11", default-features = false, features = ["fmt", "registry"] }
-futures = { version = "0.3", default-features = false }
+futures-task = { version = "0.3", default-features = false }
 hdrhistogram = { version = "7.3.0", default-features = false, features = ["serialization"] }
 # The parking_lot dependency is renamed, because we want our `parking_lot`
 # feature to also enable `tracing-subscriber`'s parking_lot feature flag.

console-subscriber/src/aggregator/mod.rs

@@ -7,7 +7,6 @@ use console_api as proto;
 use proto::resources::resource;
 use tokio::sync::{mpsc, Notify};
 
-use futures::FutureExt;
 use std::{
     sync::{
         atomic::{AtomicBool, Ordering::*},
@@ -221,7 +220,7 @@ impl Aggregator {
             // to be woken when the flush interval has elapsed, or when the
             // channel is almost full.
             let mut drained = false;
-            while let Some(event) = self.events.recv().now_or_never() {
+            while let Some(event) = recv_now_or_never(&mut self.events) {
                 match event {
                     Some(event) => {
                         self.update_state(event);
@@ -500,6 +499,16 @@ impl Aggregator {
     }
 }
 
+fn recv_now_or_never<T>(receiver: &mut mpsc::Receiver<T>) -> Option<Option<T>> {
+    let waker = futures_task::noop_waker();
+    let mut cx = std::task::Context::from_waker(&waker);
+
+    match receiver.poll_recv(&mut cx) {
+        std::task::Poll::Ready(opt) => Some(opt),
+        std::task::Poll::Pending => None,
+    }
+}
+
 // ==== impl Flush ===
 
 impl Flush {