tokio: replace some futures with poll_fn (#7895)

tokio-util/src/io/read_buf.rs

@@ -1,8 +1,7 @@
 use bytes::BufMut;
-use std::future::Future;
+use std::future::poll_fn;
 use std::io;
 use std::pin::Pin;
-use std::task::{Context, Poll};
 use tokio::io::AsyncRead;
 
 /// Read data from an `AsyncRead` into an implementer of the [`BufMut`] trait.
@@ -46,20 +45,5 @@ where
     R: AsyncRead + Unpin,
     B: BufMut,
 {
-    return ReadBufFn(read, buf).await;
-
-    struct ReadBufFn<'a, R, B>(&'a mut R, &'a mut B);
-
-    impl<'a, R, B> Future for ReadBufFn<'a, R, B>
-    where
-        R: AsyncRead + Unpin,
-        B: BufMut,
-    {
-        type Output = io::Result<usize>;
-
-        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-            let this = &mut *self;
-            crate::util::poll_read_buf(Pin::new(this.0), cx, this.1)
-        }
-    }
+    poll_fn(|cx| crate::util::poll_read_buf(Pin::new(read), cx, buf)).await
 }

tokio/src/lib.rs

@@ -559,10 +559,6 @@ cfg_time! {
 }
 
 mod trace {
-    use std::future::Future;
-    use std::pin::Pin;
-    use std::task::{Context, Poll};
-
     cfg_taskdump! {
         pub(crate) use crate::runtime::task::trace::trace_leaf;
     }
@@ -576,19 +572,8 @@ mod trace {
     }
 
     #[cfg_attr(not(feature = "sync"), allow(dead_code))]
-    pub(crate) fn async_trace_leaf() -> impl Future<Output = ()> {
-        struct Trace;
-
-        impl Future for Trace {
-            type Output = ();
-
-            #[inline(always)]
-            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
-                trace_leaf(cx)
-            }
-        }
-
-        Trace
+    pub(crate) async fn async_trace_leaf() {
+        std::future::poll_fn(trace_leaf).await
     }
 }
 

tokio/src/task/yield_now.rs

@@ -1,8 +1,7 @@
 use crate::runtime::context;
 
-use std::future::Future;
-use std::pin::Pin;
-use std::task::{ready, Context, Poll};
+use std::future::poll_fn;
+use std::task::{ready, Poll};
 
 /// Yields execution back to the Tokio runtime.
 ///
@@ -37,28 +36,19 @@ use std::task::{ready, Context, Poll};
 /// [`tokio::select!`]: macro@crate::select
 #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
 pub async fn yield_now() {
-    /// Yield implementation
-    struct YieldNow {
-        yielded: bool,
-    }
+    let mut yielded = false;
+    poll_fn(|cx| {
+        ready!(crate::trace::trace_leaf(cx));
 
-    impl Future for YieldNow {
-        type Output = ();
-
-        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
-            ready!(crate::trace::trace_leaf(cx));
-
-            if self.yielded {
-                return Poll::Ready(());
-            }
-
-            self.yielded = true;
-
-            context::defer(cx.waker());
-
-            Poll::Pending
+        if yielded {
+            return Poll::Ready(());
         }
-    }
 
-    YieldNow { yielded: false }.await;
+        yielded = true;
+
+        context::defer(cx.waker());
+
+        Poll::Pending
+    })
+    .await
 }