perf: Pack the state and future of unfolds in the same memory

futures-util/src/lib.rs

@@ -4,20 +4,21 @@
 #![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
 #![cfg_attr(feature = "read-initializer", feature(read_initializer))]
 #![cfg_attr(feature = "write-all-vectored", feature(io_slice_advance))]
-
 #![cfg_attr(not(feature = "std"), no_std)]
-#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    unreachable_pub
+)]
 // It cannot be included in the published code because this lints have false positives in the minimum required version.
 #![cfg_attr(test, warn(single_use_lifetimes))]
 #![warn(clippy::all)]
-
 // mem::take requires Rust 1.40, matches! requires Rust 1.42
 // Can be removed if the minimum supported version increased or if https://github.com/rust-lang/rust-clippy/issues/3941
 // get's implemented.
 #![allow(clippy::mem_replace_with_default, clippy::match_like_matches_macro)]
-
 #![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]
-
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
 #[cfg(all(feature = "cfg-target-has-atomic", not(feature = "unstable")))]
@@ -49,7 +50,7 @@ pub use self::async_await::*;
 pub mod __private {
     pub use crate::*;
     pub use core::{
-        option::Option::{self, Some, None},
+        option::Option::{self, None, Some},
         pin::Pin,
         result::Result::{Err, Ok},
     };
@@ -76,10 +77,7 @@ macro_rules! delegate_sink {
             self.project().$field.poll_ready(cx)
         }
 
-        fn start_send(
-            self: core::pin::Pin<&mut Self>,
-            item: $item,
-        ) -> Result<(), Self::Error> {
+        fn start_send(self: core::pin::Pin<&mut Self>, item: $item) -> Result<(), Self::Error> {
             self.project().$field.start_send(item)
         }
 
@@ -96,7 +94,7 @@ macro_rules! delegate_sink {
         ) -> core::task::Poll<Result<(), Self::Error>> {
             self.project().$field.poll_close(cx)
         }
-    }
+    };
 }
 
 macro_rules! delegate_future {
@@ -107,7 +105,7 @@ macro_rules! delegate_future {
         ) -> core::task::Poll<Self::Output> {
             self.project().$field.poll(cx)
         }
-    }
+    };
 }
 
 macro_rules! delegate_stream {
@@ -121,34 +119,40 @@ macro_rules! delegate_stream {
         fn size_hint(&self) -> (usize, Option<usize>) {
             self.$field.size_hint()
         }
-    }
+    };
 }
 
 #[cfg(feature = "io")]
 #[cfg(feature = "std")]
 macro_rules! delegate_async_write {
     ($field:ident) => {
-        fn poll_write(self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>, buf: &[u8])
-            -> core::task::Poll<std::io::Result<usize>>
-        {
+        fn poll_write(
+            self: core::pin::Pin<&mut Self>,
+            cx: &mut core::task::Context<'_>,
+            buf: &[u8],
+        ) -> core::task::Poll<std::io::Result<usize>> {
             self.project().$field.poll_write(cx, buf)
         }
-        fn poll_write_vectored(self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>, bufs: &[std::io::IoSlice<'_>])
-            -> core::task::Poll<std::io::Result<usize>>
-        {
+        fn poll_write_vectored(
+            self: core::pin::Pin<&mut Self>,
+            cx: &mut core::task::Context<'_>,
+            bufs: &[std::io::IoSlice<'_>],
+        ) -> core::task::Poll<std::io::Result<usize>> {
             self.project().$field.poll_write_vectored(cx, bufs)
         }
-        fn poll_flush(self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>)
-            -> core::task::Poll<std::io::Result<()>>
-        {
+        fn poll_flush(
+            self: core::pin::Pin<&mut Self>,
+            cx: &mut core::task::Context<'_>,
+        ) -> core::task::Poll<std::io::Result<()>> {
             self.project().$field.poll_flush(cx)
         }
-        fn poll_close(self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>)
-            -> core::task::Poll<std::io::Result<()>>
-        {
+        fn poll_close(
+            self: core::pin::Pin<&mut Self>,
+            cx: &mut core::task::Context<'_>,
+        ) -> core::task::Poll<std::io::Result<()>> {
             self.project().$field.poll_close(cx)
         }
-    }
+    };
 }
 
 #[cfg(feature = "io")]
@@ -160,18 +164,22 @@ macro_rules! delegate_async_read {
             self.$field.initializer()
         }
 
-        fn poll_read(self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>, buf: &mut [u8])
-            -> core::task::Poll<std::io::Result<usize>>
-        {
+        fn poll_read(
+            self: core::pin::Pin<&mut Self>,
+            cx: &mut core::task::Context<'_>,
+            buf: &mut [u8],
+        ) -> core::task::Poll<std::io::Result<usize>> {
             self.project().$field.poll_read(cx, buf)
         }
 
-        fn poll_read_vectored(self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>, bufs: &mut [std::io::IoSliceMut<'_>])
-            -> core::task::Poll<std::io::Result<usize>>
-        {
+        fn poll_read_vectored(
+            self: core::pin::Pin<&mut Self>,
+            cx: &mut core::task::Context<'_>,
+            bufs: &mut [std::io::IoSliceMut<'_>],
+        ) -> core::task::Poll<std::io::Result<usize>> {
             self.project().$field.poll_read_vectored(cx, bufs)
         }
-    }
+    };
 }
 
 #[cfg(feature = "io")]
@@ -188,7 +196,7 @@ macro_rules! delegate_async_buf_read {
         fn consume(self: core::pin::Pin<&mut Self>, amt: usize) {
             self.project().$field.consume(amt)
         }
-    }
+    };
 }
 
 macro_rules! delegate_access_inner {
@@ -304,16 +312,19 @@ macro_rules! delegate_all {
 }
 
 pub mod future;
-#[doc(hidden)] pub use crate::future::{FutureExt, TryFutureExt};
+#[doc(hidden)]
+pub use crate::future::{FutureExt, TryFutureExt};
 
 pub mod stream;
-#[doc(hidden)] pub use crate::stream::{StreamExt, TryStreamExt};
+#[doc(hidden)]
+pub use crate::stream::{StreamExt, TryStreamExt};
 
 #[cfg(feature = "sink")]
 #[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
 pub mod sink;
 #[cfg(feature = "sink")]
-#[doc(hidden)] pub use crate::sink::SinkExt;
+#[doc(hidden)]
+pub use crate::sink::SinkExt;
 
 pub mod task;
 
@@ -329,10 +340,11 @@ pub mod compat;
 pub mod io;
 #[cfg(feature = "io")]
 #[cfg(feature = "std")]
-#[doc(hidden)] pub use crate::io::{AsyncReadExt, AsyncWriteExt, AsyncSeekExt, AsyncBufReadExt};
+#[doc(hidden)]
+pub use crate::io::{AsyncBufReadExt, AsyncReadExt, AsyncSeekExt, AsyncWriteExt};
 
 mod fns;
-
+mod unfold_state;
 
 cfg_target_has_atomic! {
     #[cfg(feature = "alloc")]

futures-util/src/sink/unfold.rs

@@ -1,3 +1,4 @@
+use crate::unfold_state::UnfoldState;
 use core::{future::Future, pin::Pin};
 use futures_core::ready;
 use futures_core::task::{Context, Poll};
@@ -9,10 +10,9 @@ pin_project! {
     #[derive(Debug)]
     #[must_use = "sinks do nothing unless polled"]
     pub struct Unfold<T, F, R> {
-        state: Option<T>,
         function: F,
         #[pin]
-        future: Option<R>,
+        state: UnfoldState<T, R>,
     }
 }
 
@@ -37,9 +37,8 @@ pin_project! {
 /// ```
 pub fn unfold<T, F, R>(init: T, function: F) -> Unfold<T, F, R> {
     Unfold {
-        state: Some(init),
         function,
-        future: None,
+        state: UnfoldState::Value(init),
     }
 }
 
@@ -56,24 +55,24 @@ where
 
     fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error> {
         let mut this = self.project();
-        debug_assert!(this.future.is_none());
-        let future = (this.function)(this.state.take().unwrap(), item);
-        this.future.set(Some(future));
+        let future = match this.state.as_mut().take_value() {
+            Some(value) => (this.function)(value, item),
+            None => panic!("start_send called without poll_ready being called first"),
+        };
+        this.state.set(UnfoldState::Future(future));
         Ok(())
     }
 
     fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         let mut this = self.project();
-        Poll::Ready(if let Some(future) = this.future.as_mut().as_pin_mut() {
-            let result = match ready!(future.poll(cx)) {
+        Poll::Ready(if let Some(future) = this.state.as_mut().project_future() {
+            match ready!(future.poll(cx)) {
                 Ok(state) => {
-                    *this.state = Some(state);
+                    this.state.set(UnfoldState::Value(state));
                     Ok(())
                 }
                 Err(err) => Err(err),
-            };
-            this.future.set(None);
-            result
+            }
         } else {
             Ok(())
         })

futures-util/src/stream/unfold.rs

@@ -1,3 +1,4 @@
+use crate::unfold_state::UnfoldState;
 use core::fmt;
 use core::pin::Pin;
 use futures_core::future::Future;
@@ -46,13 +47,13 @@ use pin_project_lite::pin_project;
 /// # });
 /// ```
 pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut>
-    where F: FnMut(T) -> Fut,
-          Fut: Future<Output = Option<(Item, T)>>,
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = Option<(Item, T)>>,
 {
     Unfold {
         f,
-        state: Some(init),
-        fut: None,
+        state: UnfoldState::Value(init),
     }
 }
 
@@ -61,9 +62,8 @@ pin_project! {
     #[must_use = "streams do nothing unless polled"]
     pub struct Unfold<T, F, Fut> {
         f: F,
-        state: Option<T>,
         #[pin]
-        fut: Option<Fut>,
+        state: UnfoldState<T, Fut>,
     }
 }
 
@@ -75,44 +75,48 @@ where
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Unfold")
             .field("state", &self.state)
-            .field("fut", &self.fut)
             .finish()
     }
 }
 
 impl<T, F, Fut, Item> FusedStream for Unfold<T, F, Fut>
-    where F: FnMut(T) -> Fut,
-          Fut: Future<Output = Option<(Item, T)>>,
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = Option<(Item, T)>>,
 {
     fn is_terminated(&self) -> bool {
-        self.state.is_none() && self.fut.is_none()
+        if let UnfoldState::Empty = self.state {
+            true
+        } else {
+            false
+        }
     }
 }
 
 impl<T, F, Fut, Item> Stream for Unfold<T, F, Fut>
-    where F: FnMut(T) -> Fut,
-          Fut: Future<Output = Option<(Item, T)>>,
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = Option<(Item, T)>>,
 {
     type Item = Item;
 
-    fn poll_next(
-        self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-    ) -> Poll<Option<Self::Item>> {
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         let mut this = self.project();
 
-        if let Some(state) = this.state.take() {
-            this.fut.set(Some((this.f)(state)));
+        if let Some(state) = this.state.as_mut().take_value() {
+            this.state.set(UnfoldState::Future((this.f)(state)));
         }
 
-        let step = ready!(this.fut.as_mut().as_pin_mut()
-            .expect("Unfold must not be polled after it returned `Poll::Ready(None)`").poll(cx));
-        this.fut.set(None);
+        let step = match this.state.as_mut().project_future() {
+            Some(fut) => ready!(fut.poll(cx)),
+            None => panic!("Unfold must not be polled after it returned `Poll::Ready(None)`"),
+        };
 
         if let Some((item, next_state)) = step {
-            *this.state = Some(next_state);
+            this.state.set(UnfoldState::Value(next_state));
             Poll::Ready(Some(item))
         } else {
+            this.state.set(UnfoldState::Empty);
             Poll::Ready(None)
         }
     }

futures-util/src/unfold_state.rs

@@ -0,0 +1,34 @@
+use core::pin::Pin;
+
+/// UnfoldState used for stream and sink unfolds
+#[derive(Debug)]
+pub(crate) enum UnfoldState<T, R> {
+    Value(T),
+    Future(/* #[pin] */ R),
+    Empty,
+}
+
+impl<T, R> UnfoldState<T, R> {
+    pub(crate) fn project_future(self: Pin<&mut Self>) -> Option<Pin<&mut R>> {
+        // SAFETY Normal pin projection on the `Future` variant
+        unsafe {
+            match self.get_unchecked_mut() {
+                Self::Future(f) => Some(Pin::new_unchecked(f)),
+                _ => None,
+            }
+        }
+    }
+
+    pub(crate) fn take_value(self: Pin<&mut Self>) -> Option<T> {
+        // SAFETY We only move out of the `Value` variant which is not pinned
+        match *self {
+            Self::Value(_) => unsafe {
+                match core::mem::replace(self.get_unchecked_mut(), UnfoldState::Empty) {
+                    UnfoldState::Value(v) => Some(v),
+                    _ => core::hint::unreachable_unchecked(),
+                }
+            },
+            _ => None,
+        }
+    }
+}