Merge branch 'master' into add_future_delay

.github/workflows/ci.yml

@@ -11,6 +11,8 @@ jobs:
   build_and_test:
     name: Build and test
     runs-on: ${{ matrix.os }}
+    env:
+      RUSTFLAGS: -Dwarnings
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
@@ -29,7 +31,7 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: check
-        args: --all --benches --bins --examples --tests
+        args: --all --bins --examples
 
     - name: check unstable
       uses: actions-rs/cargo@v1
@@ -41,11 +43,13 @@ jobs:
       uses: actions-rs/cargo@v1
       with:
         command: test
-        args: --all --doc --features unstable
+        args: --all --features unstable
 
   check_fmt_and_docs:
     name: Checking fmt and docs
     runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: -Dwarnings
     steps:
     - uses: actions/checkout@master
 
@@ -77,6 +81,9 @@ jobs:
   clippy_check:
     name: Clippy check
     runs-on: ubuntu-latest
+    # TODO: There is a lot of warnings
+    # env:
+    #   RUSTFLAGS: -Dwarnings
     steps:
       - uses: actions/checkout@v1
       - id: component

CHANGELOG.md

@@ -7,6 +7,104 @@ and this project adheres to [Semantic Versioning](https://book.async.rs/overview
 
 ## [Unreleased]
 
+# [0.99.10] - 2019-10-16
+
+This patch stabilizes several core concurrency macros, introduces async versions
+of `Path` and `PathBuf`, and adds almost 100 other commits.
+
+## Examples
+
+__Asynchronously read directories from the filesystem__
+```rust
+use async_std::fs;
+use async_std::path::Path;
+use async_std::prelude::*;
+
+let path = Path::new("/laputa");
+let mut dir = fs::read_dir(&path).await.unwrap();
+while let Some(entry) = dir.next().await {
+    if let Ok(entry) = entry {
+        println!("{:?}", entry.path());
+    }
+}
+```
+
+__Cooperatively reschedule the current task on the executor__
+```rust
+use async_std::prelude::*;
+use async_std::task;
+
+task::spawn(async {
+    let x = fibonnacci(1000); // Do expensive work
+    task::yield_now().await;  // Allow other tasks to run
+    x + fibonnacci(100)       // Do more work
+})
+```
+
+__Create an interval stream__
+```rust
+use async_std::prelude::*;
+use async_std::stream;
+use std::time::Duration;
+
+let mut interval = stream::interval(Duration::from_secs(4));
+while let Some(_) = interval.next().await {
+    println!("prints every four seconds");
+}
+```
+
+## Added
+
+- Added `FutureExt` to the `prelude`, allowing us to extend `Future`
+- Added `Stream::cmp`
+- Added `Stream::ge`
+- Added `Stream::last`
+- Added `Stream::le`
+- Added `Stream::lt`
+- Added `Stream::merge` as "unstable", replacing `stream::join!`
+- Added `Stream::partial_cmp`
+- Added `Stream::take_while`
+- Added `Stream::try_fold`
+- Added `future::IntoFuture` as "unstable"
+- Added `io::BufRead::split`
+- Added `io::Write::write_fmt`
+- Added `print!`, `println!`, `eprint!`, `eprintln!` macros as "unstable"
+- Added `process` as "unstable", re-exporting std types only for now
+- Added `std::net` re-exports to the `net` submodule
+- Added `std::path::PathBuf` with all associated methods
+- Added `std::path::Path` with all associated methods
+- Added `stream::ExactSizeStream` as "unstable"
+- Added `stream::FusedStream` as "unstable"
+- Added `stream::Product`
+- Added `stream::Sum`
+- Added `stream::from_fn`
+- Added `stream::interval` as "unstable"
+- Added `stream::repeat_with`
+- Added `task::spawn_blocking` as "unstable", replacing `task::blocking`
+- Added `task::yield_now`
+- Added `write!` and `writeln!` macros as "unstable"
+- Stabilized `future::join!` and `future::try_join!`
+- Stabilized `future::timeout`
+- Stabilized `path`
+- Stabilized `task::ready!`
+
+## Changed
+
+- Fixed `BufWriter::into_inner` so it calls `flush` before yielding
+- Refactored `io::BufWriter` internals
+- Refactored `net::ToSocketAddrs` internals
+- Removed Travis CI entirely
+- Rewrote the README.md
+- Stabilized `io::Cursor`
+- Switched bors over to use GitHub actions
+- Updated the `io` documentation to match std's `io` docs
+- Updated the `task` documentation to match std's `thread` docs
+
+## Removed
+
+- Removed the "unstable" `stream::join!` in favor of `Stream::merge`
+- Removed the "unstable" `task::blocking` in favor of `task::spawn_blocking`
+
 # [0.99.9] - 2019-10-08
 
 This patch upgrades our `futures-rs` version, allowing us to build on the 1.39
@@ -183,7 +281,8 @@ task::blocking(async {
 
 - Initial beta release
 
-[Unreleased]: https://github.com/async-rs/async-std/compare/v0.99.9...HEAD
+[Unreleased]: https://github.com/async-rs/async-std/compare/v0.99.10...HEAD
+[0.99.10]: https://github.com/async-rs/async-std/compare/v0.99.9...v0.99.10
 [0.99.9]: https://github.com/async-rs/async-std/compare/v0.99.8...v0.99.9
 [0.99.8]: https://github.com/async-rs/async-std/compare/v0.99.7...v0.99.8
 [0.99.7]: https://github.com/async-rs/async-std/compare/v0.99.6...v0.99.7

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "async-std"
-version = "0.99.9"
+version = "0.99.10"
 authors = [
   "Stjepan Glavina <stjepang@gmail.com>",
   "Yoshua Wuyts <yoshuawuyts@gmail.com>",
@@ -21,15 +21,15 @@ features = ["docs"]
 rustdoc-args = ["--cfg", "feature=\"docs\""]
 
 [features]
-docs = ["broadcaster"]
+docs = ["unstable"]
 unstable = ["broadcaster"]
 
 [dependencies]
 async-macros = "1.0.0"
 async-task = "1.0.0"
-cfg-if = "0.1.9"
 crossbeam-channel = "0.3.9"
 crossbeam-deque = "0.7.1"
+crossbeam-utils = "0.6.6"
 futures-core-preview = "=0.3.0-alpha.19"
 futures-io-preview = "=0.3.0-alpha.19"
 futures-timer = "1.0.2"
@@ -43,9 +43,11 @@ pin-utils = "0.1.0-alpha.4"
 slab = "0.4.2"
 kv-log-macro = "1.0.4"
 broadcaster = { version = "0.2.6", optional = true, default-features = false, features = ["default-channels"] }
+pin-project-lite = "0.1"
 
 [dev-dependencies]
 femme = "1.2.0"
+rand = "0.7.2"
 # surf = "1.0.2"
 tempdir = "0.3.7"
 futures-preview = { version = "=0.3.0-alpha.19", features = ["async-await"] }

src/fs/dir_builder.rs

@@ -1,7 +1,5 @@
 use std::future::Future;
 
-use cfg_if::cfg_if;
-
 use crate::io;
 use crate::path::Path;
 use crate::task::blocking;
@@ -113,17 +111,9 @@ impl DirBuilder {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::fs::DirBuilderExt;
-    } else if #[cfg(unix)] {
-        use std::os::unix::fs::DirBuilderExt;
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl DirBuilderExt for DirBuilder {
         fn mode(&mut self, mode: u32) -> &mut Self {
             self.mode = Some(mode);
@@ -131,4 +121,3 @@ cfg_if! {
         }
     }
 }
-}

src/fs/dir_entry.rs

@@ -2,8 +2,6 @@ use std::ffi::OsString;
 use std::fmt;
 use std::sync::Arc;
 
-use cfg_if::cfg_if;
-
 use crate::fs::{FileType, Metadata};
 use crate::io;
 use crate::path::PathBuf;
@@ -160,21 +158,12 @@ impl fmt::Debug for DirEntry {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::fs::DirEntryExt;
-    } else if #[cfg(unix)] {
-        use std::os::unix::fs::DirEntryExt;
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl DirEntryExt for DirEntry {
         fn ino(&self) -> u64 {
             self.0.ino()
         }
     }
 }
-}

src/fs/file.rs

@@ -7,8 +7,6 @@ use std::pin::Pin;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 
-use cfg_if::cfg_if;
-
 use crate::fs::{Metadata, Permissions};
 use crate::future;
 use crate::io::{self, Read, Seek, SeekFrom, Write};
@@ -401,20 +399,9 @@ impl From<std::fs::File> for File {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-        use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        use std::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for File {
         fn as_raw_fd(&self) -> RawFd {
             self.file.as_raw_fd()
@@ -437,11 +424,10 @@ cfg_if! {
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(windows)))]
+cfg_windows! {
-cfg_if! {
+    use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
+
     impl AsRawHandle for File {
         fn as_raw_handle(&self) -> RawHandle {
             self.file.as_raw_handle()
@@ -464,7 +450,6 @@ cfg_if! {
         }
     }
 }
-}
 
 /// An async mutex with non-borrowing lock guards.
 struct Lock<T>(Arc<LockState<T>>);

src/fs/file_type.rs

@@ -1,7 +1,8 @@
-use cfg_if::cfg_if;
+cfg_not_docs! {
+    pub use std::fs::FileType;
+}
 
-cfg_if! {
+cfg_docs! {
-    if #[cfg(feature = "docs")] {
     /// The type of a file or directory.
     ///
     /// A file type is returned by [`Metadata::file_type`].
@@ -80,7 +81,4 @@ cfg_if! {
             unimplemented!()
         }
     }
-    } else {
-        pub use std::fs::FileType;
-    }
 }

src/fs/metadata.rs

@@ -1,5 +1,3 @@
-use cfg_if::cfg_if;
-
 use crate::io;
 use crate::path::Path;
 use crate::task::blocking;
@@ -39,8 +37,11 @@ pub async fn metadata<P: AsRef<Path>>(path: P) -> io::Result<Metadata> {
     blocking::spawn(move || std::fs::metadata(path)).await
 }
 
-cfg_if! {
+cfg_not_docs! {
-    if #[cfg(feature = "docs")] {
+    pub use std::fs::Metadata;
+}
+
+cfg_docs! {
     use std::time::SystemTime;
 
     use crate::fs::{FileType, Permissions};
@@ -225,7 +226,4 @@ cfg_if! {
             unimplemented!()
         }
     }
-    } else {
-        pub use std::fs::Metadata;
-    }
 }

src/fs/open_options.rs

@@ -1,7 +1,5 @@
 use std::future::Future;
 
-use cfg_if::cfg_if;
-
 use crate::fs::File;
 use crate::io;
 use crate::path::Path;
@@ -296,17 +294,9 @@ impl Default for OpenOptions {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::fs::OpenOptionsExt;
-    } else if #[cfg(unix)] {
-        use std::os::unix::fs::OpenOptionsExt;
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl OpenOptionsExt for OpenOptions {
         fn mode(&mut self, mode: u32) -> &mut Self {
             self.0.mode(mode);
@@ -319,4 +309,3 @@ cfg_if! {
         }
     }
 }
-}

src/fs/permissions.rs

@@ -1,7 +1,8 @@
-use cfg_if::cfg_if;
+cfg_not_docs! {
+    pub use std::fs::Permissions;
+}
 
-cfg_if! {
+cfg_docs! {
-    if #[cfg(feature = "docs")] {
     /// A set of permissions on a file or directory.
     ///
     /// This type is a re-export of [`std::fs::Permissions`].
@@ -52,7 +53,4 @@ cfg_if! {
             unimplemented!()
         }
     }
-    } else {
-        pub use std::fs::Permissions;
-    }
 }

src/future/future.rs

@@ -1,24 +1,17 @@
-use crate::utils::extension_trait;
+cfg_unstable! {
+    mod delay;
 
-cfg_if::cfg_if! {
+    use std::time::Duration;
-    if #[cfg(feature = "docs")] {
+
+    use delay::DelayFuture;
+}
+
+extension_trait! {
     use std::pin::Pin;
     use std::ops::{Deref, DerefMut};
 
     use crate::task::{Context, Poll};
-    }
-}
 
-cfg_if::cfg_if! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
-        mod delay;
-
-        use std::time::Duration;
-        use delay::DelayFuture;
-    }
-}
-
-extension_trait! {
     #[doc = r#"
         A future represents an asynchronous computation.
 

src/future/into_future.rs

@@ -30,7 +30,7 @@ use crate::future::Future;
 ///     }
 /// }
 /// ```
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 pub trait IntoFuture {
     /// The type of value produced on completion.

src/future/mod.rs

@@ -44,12 +44,6 @@
 #[doc(inline)]
 pub use async_macros::{join, try_join};
 
-#[doc(inline)]
-#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-pub use async_macros::{select, try_select};
-
-use cfg_if::cfg_if;
-
 pub use future::Future;
 pub use pending::pending;
 pub use poll_fn::poll_fn;
@@ -62,10 +56,10 @@ mod poll_fn;
 mod ready;
 mod timeout;
 
-cfg_if! {
+cfg_unstable! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
+    #[doc(inline)]
-        mod into_future;
+    pub use async_macros::{select, try_select};
 
     pub use into_future::IntoFuture;
-    }
+    mod into_future;
 }

src/future/timeout.rs

@@ -4,6 +4,7 @@ use std::pin::Pin;
 use std::time::Duration;
 
 use futures_timer::Delay;
+use pin_project_lite::pin_project;
 
 use crate::future::Future;
 use crate::task::{Context, Poll};
@@ -39,24 +40,24 @@ where
     f.await
 }
 
+pin_project! {
     /// A future that times out after a duration of time.
     struct TimeoutFuture<F> {
+        #[pin]
         future: F,
+        #[pin]
         delay: Delay,
     }
-
-impl<F> TimeoutFuture<F> {
-    pin_utils::unsafe_pinned!(future: F);
-    pin_utils::unsafe_pinned!(delay: Delay);
 }
 
 impl<F: Future> Future for TimeoutFuture<F> {
     type Output = Result<F::Output, TimeoutError>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        match self.as_mut().future().poll(cx) {
+        let this = self.project();
+        match this.future.poll(cx) {
             Poll::Ready(v) => Poll::Ready(Ok(v)),
-            Poll::Pending => match self.delay().poll(cx) {
+            Poll::Pending => match this.delay.poll(cx) {
                 Poll::Ready(_) => Poll::Ready(Err(TimeoutError { _private: () })),
                 Poll::Pending => Poll::Pending,
             },

src/io/buf_read/lines.rs

@@ -2,11 +2,14 @@ use std::mem;
 use std::pin::Pin;
 use std::str;
 
+use pin_project_lite::pin_project;
+
 use super::read_until_internal;
 use crate::io::{self, BufRead};
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream of lines in a byte stream.
     ///
     /// This stream is created by the [`lines`] method on types that implement [`BufRead`].
@@ -18,34 +21,36 @@ use crate::task::{Context, Poll};
     /// [`std::io::Lines`]: https://doc.rust-lang.org/std/io/struct.Lines.html
     #[derive(Debug)]
     pub struct Lines<R> {
+        #[pin]
         pub(crate) reader: R,
         pub(crate) buf: String,
         pub(crate) bytes: Vec<u8>,
         pub(crate) read: usize,
     }
+}
 
 impl<R: BufRead> Stream for Lines<R> {
     type Item = io::Result<String>;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        let Self {
+        let this = self.project();
-            reader,
+        let n = futures_core::ready!(read_line_internal(
-            buf,
+            this.reader,
-            bytes,
+            cx,
-            read,
+            this.buf,
-        } = unsafe { self.get_unchecked_mut() };
+            this.bytes,
-        let reader = unsafe { Pin::new_unchecked(reader) };
+            this.read
-        let n = futures_core::ready!(read_line_internal(reader, cx, buf, bytes, read))?;
+        ))?;
-        if n == 0 && buf.is_empty() {
+        if n == 0 && this.buf.is_empty() {
             return Poll::Ready(None);
         }
-        if buf.ends_with('\n') {
+        if this.buf.ends_with('\n') {
-            buf.pop();
+            this.buf.pop();
-            if buf.ends_with('\r') {
+            if this.buf.ends_with('\r') {
-                buf.pop();
+                this.buf.pop();
             }
         }
-        Poll::Ready(Some(Ok(mem::replace(buf, String::new()))))
+        Poll::Ready(Some(Ok(mem::replace(this.buf, String::new()))))
     }
 }
 

src/io/buf_read/mod.rs

@@ -12,19 +12,12 @@ use read_until::ReadUntilFuture;
 use std::mem;
 use std::pin::Pin;
 
-use cfg_if::cfg_if;
-
 use crate::io;
 use crate::task::{Context, Poll};
-use crate::utils::extension_trait;
-
-cfg_if! {
-    if #[cfg(feature = "docs")] {
-        use std::ops::{Deref, DerefMut};
-    }
-}
 
 extension_trait! {
+    use std::ops::{Deref, DerefMut};
+
     #[doc = r#"
         Allows reading from a buffered byte stream.
 

src/io/buf_read/split.rs

@@ -1,11 +1,14 @@
 use std::mem;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::read_until_internal;
 use crate::io::{self, BufRead};
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream over the contents of an instance of [`BufRead`] split on a particular byte.
     ///
     /// This stream is created by the [`split`] method on types that implement [`BufRead`].
@@ -17,30 +20,32 @@ use crate::task::{Context, Poll};
     /// [`std::io::Split`]: https://doc.rust-lang.org/std/io/struct.Split.html
     #[derive(Debug)]
     pub struct Split<R> {
+        #[pin]
         pub(crate) reader: R,
         pub(crate) buf: Vec<u8>,
         pub(crate) read: usize,
         pub(crate) delim: u8,
     }
+}
 
 impl<R: BufRead> Stream for Split<R> {
     type Item = io::Result<Vec<u8>>;
 
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        let Self {
+        let this = self.project();
-            reader,
+        let n = futures_core::ready!(read_until_internal(
-            buf,
+            this.reader,
-            read,
+            cx,
-            delim,
+            *this.delim,
-        } = unsafe { self.get_unchecked_mut() };
+            this.buf,
-        let reader = unsafe { Pin::new_unchecked(reader) };
+            this.read
-        let n = futures_core::ready!(read_until_internal(reader, cx, *delim, buf, read))?;
+        ))?;
-        if n == 0 && buf.is_empty() {
+        if n == 0 && this.buf.is_empty() {
             return Poll::Ready(None);
         }
-        if buf[buf.len() - 1] == *delim {
+        if this.buf[this.buf.len() - 1] == *this.delim {
-            buf.pop();
+            this.buf.pop();
         }
-        Poll::Ready(Some(Ok(mem::replace(buf, vec![]))))
+        Poll::Ready(Some(Ok(mem::replace(this.buf, vec![]))))
     }
 }

src/io/buf_reader.rs

@@ -2,11 +2,14 @@ use std::io::{IoSliceMut, Read as _};
 use std::pin::Pin;
 use std::{cmp, fmt};
 
+use pin_project_lite::pin_project;
+
 use crate::io::{self, BufRead, Read, Seek, SeekFrom};
 use crate::task::{Context, Poll};
 
 const DEFAULT_CAPACITY: usize = 8 * 1024;
 
+pin_project! {
     /// Adds buffering to any reader.
     ///
     /// It can be excessively inefficient to work directly with a [`Read`] instance. A `BufReader`
@@ -43,11 +46,13 @@ const DEFAULT_CAPACITY: usize = 8 * 1024;
     /// # Ok(()) }) }
     /// ```
     pub struct BufReader<R> {
+        #[pin]
         inner: R,
         buf: Box<[u8]>,
         pos: usize,
         cap: usize,
     }
+}
 
 impl<R: io::Read> BufReader<R> {
     /// Creates a buffered reader with default buffer capacity.
@@ -95,10 +100,6 @@ impl<R: io::Read> BufReader<R> {
 }
 
 impl<R> BufReader<R> {
-    pin_utils::unsafe_pinned!(inner: R);
-    pin_utils::unsafe_unpinned!(pos: usize);
-    pin_utils::unsafe_unpinned!(cap: usize);
-
     /// Gets a reference to the underlying reader.
     ///
     /// It is inadvisable to directly read from the underlying reader.
@@ -141,6 +142,13 @@ impl<R> BufReader<R> {
         &mut self.inner
     }
 
+    /// Gets a pinned mutable reference to the underlying reader.
+    ///
+    /// It is inadvisable to directly read from the underlying reader.
+    fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
+        self.project().inner
+    }
+
     /// Returns a reference to the internal buffer.
     ///
     /// This function will not attempt to fill the buffer if it is empty.
@@ -185,9 +193,10 @@ impl<R> BufReader<R> {
 
     /// Invalidates all data in the internal buffer.
     #[inline]
-    fn discard_buffer(mut self: Pin<&mut Self>) {
+    fn discard_buffer(self: Pin<&mut Self>) {
-        *self.as_mut().pos() = 0;
+        let this = self.project();
-        *self.cap() = 0;
+        *this.pos = 0;
+        *this.cap = 0;
     }
 }
 
@@ -201,7 +210,7 @@ impl<R: Read> Read for BufReader<R> {
         // (larger than our internal buffer), bypass our internal buffer
         // entirely.
         if self.pos == self.cap && buf.len() >= self.buf.len() {
-            let res = futures_core::ready!(self.as_mut().inner().poll_read(cx, buf));
+            let res = futures_core::ready!(self.as_mut().get_pin_mut().poll_read(cx, buf));
             self.discard_buffer();
             return Poll::Ready(res);
         }
@@ -218,7 +227,8 @@ impl<R: Read> Read for BufReader<R> {
     ) -> Poll<io::Result<usize>> {
         let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
         if self.pos == self.cap && total_len >= self.buf.len() {
-            let res = futures_core::ready!(self.as_mut().inner().poll_read_vectored(cx, bufs));
+            let res =
+                futures_core::ready!(self.as_mut().get_pin_mut().poll_read_vectored(cx, bufs));
             self.discard_buffer();
             return Poll::Ready(res);
         }
@@ -234,28 +244,23 @@ impl<R: Read> BufRead for BufReader<R> {
         self: Pin<&'a mut Self>,
         cx: &mut Context<'_>,
     ) -> Poll<io::Result<&'a [u8]>> {
-        let Self {
+        let mut this = self.project();
-            inner,
-            buf,
-            cap,
-            pos,
-        } = unsafe { self.get_unchecked_mut() };
-        let mut inner = unsafe { Pin::new_unchecked(inner) };
 
         // If we've reached the end of our internal buffer then we need to fetch
         // some more data from the underlying reader.
         // Branch using `>=` instead of the more correct `==`
         // to tell the compiler that the pos..cap slice is always valid.
-        if *pos >= *cap {
+        if *this.pos >= *this.cap {
-            debug_assert!(*pos == *cap);
+            debug_assert!(*this.pos == *this.cap);
-            *cap = futures_core::ready!(inner.as_mut().poll_read(cx, buf))?;
+            *this.cap = futures_core::ready!(this.inner.as_mut().poll_read(cx, this.buf))?;
-            *pos = 0;
+            *this.pos = 0;
         }
-        Poll::Ready(Ok(&buf[*pos..*cap]))
+        Poll::Ready(Ok(&this.buf[*this.pos..*this.cap]))
     }
 
-    fn consume(mut self: Pin<&mut Self>, amt: usize) {
+    fn consume(self: Pin<&mut Self>, amt: usize) {
-        *self.as_mut().pos() = cmp::min(self.pos + amt, self.cap);
+        let this = self.project();
+        *this.pos = cmp::min(*this.pos + amt, *this.cap);
     }
 }
 
@@ -305,24 +310,26 @@ impl<R: Seek> Seek for BufReader<R> {
             if let Some(offset) = n.checked_sub(remainder) {
                 result = futures_core::ready!(
                     self.as_mut()
-                        .inner()
+                        .get_pin_mut()
                         .poll_seek(cx, SeekFrom::Current(offset))
                 )?;
             } else {
                 // seek backwards by our remainder, and then by the offset
                 futures_core::ready!(
                     self.as_mut()
-                        .inner()
+                        .get_pin_mut()
                         .poll_seek(cx, SeekFrom::Current(-remainder))
                 )?;
                 self.as_mut().discard_buffer();
                 result = futures_core::ready!(
-                    self.as_mut().inner().poll_seek(cx, SeekFrom::Current(n))
+                    self.as_mut()
+                        .get_pin_mut()
+                        .poll_seek(cx, SeekFrom::Current(n))
                 )?;
             }
         } else {
             // Seeking with Start/End doesn't care about our buffer length.
-            result = futures_core::ready!(self.as_mut().inner().poll_seek(cx, pos))?;
+            result = futures_core::ready!(self.as_mut().get_pin_mut().poll_seek(cx, pos))?;
         }
         self.discard_buffer();
         Poll::Ready(Ok(result))

src/io/buf_writer.rs

@@ -2,6 +2,7 @@ use std::fmt;
 use std::pin::Pin;
 
 use futures_core::ready;
+use pin_project_lite::pin_project;
 
 use crate::io::write::WriteExt;
 use crate::io::{self, Seek, SeekFrom, Write};
@@ -9,6 +10,7 @@ use crate::task::{Context, Poll};
 
 const DEFAULT_CAPACITY: usize = 8 * 1024;
 
+pin_project! {
     /// Wraps a writer and buffers its output.
     ///
     /// It can be excessively inefficient to work directly with something that
@@ -79,18 +81,17 @@ const DEFAULT_CAPACITY: usize = 8 * 1024;
     /// [`TcpStream`]: ../net/struct.TcpStream.html
     /// [`flush`]: trait.Write.html#tymethod.flush
     pub struct BufWriter<W> {
+        #[pin]
         inner: W,
         buf: Vec<u8>,
         written: usize,
     }
+}
 
 #[derive(Debug)]
 pub struct IntoInnerError<W>(W, std::io::Error);
 
 impl<W: Write> BufWriter<W> {
-    pin_utils::unsafe_pinned!(inner: W);
-    pin_utils::unsafe_unpinned!(buf: Vec<u8>);
-
     /// Creates a new `BufWriter` with a default buffer capacity. The default is currently 8 KB,
     /// but may change in the future.
     ///
@@ -178,6 +179,13 @@ impl<W: Write> BufWriter<W> {
         &mut self.inner
     }
 
+    /// Gets a pinned mutable reference to the underlying writer.
+    ///
+    /// It is inadvisable to directly write to the underlying writer.
+    fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut W> {
+        self.project().inner
+    }
+
     /// Consumes BufWriter, returning the underlying writer
     ///
     /// This method will not write leftover data, it will be lost.
@@ -234,16 +242,15 @@ impl<W: Write> BufWriter<W> {
     ///
     /// [`LineWriter`]: struct.LineWriter.html
     fn poll_flush_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
-        let Self {
+        let mut this = self.project();
-            inner,
+        let len = this.buf.len();
-            buf,
-            written,
-        } = unsafe { Pin::get_unchecked_mut(self) };
-        let mut inner = unsafe { Pin::new_unchecked(inner) };
-        let len = buf.len();
         let mut ret = Ok(());
-        while *written < len {
+        while *this.written < len {
-            match inner.as_mut().poll_write(cx, &buf[*written..]) {
+            match this
+                .inner
+                .as_mut()
+                .poll_write(cx, &this.buf[*this.written..])
+            {
                 Poll::Ready(Ok(0)) => {
                     ret = Err(io::Error::new(
                         io::ErrorKind::WriteZero,
@@ -251,7 +258,7 @@ impl<W: Write> BufWriter<W> {
                     ));
                     break;
                 }
-                Poll::Ready(Ok(n)) => *written += n,
+                Poll::Ready(Ok(n)) => *this.written += n,
                 Poll::Ready(Err(ref e)) if e.kind() == io::ErrorKind::Interrupted => {}
                 Poll::Ready(Err(e)) => {
                     ret = Err(e);
@@ -260,10 +267,10 @@ impl<W: Write> BufWriter<W> {
                 Poll::Pending => return Poll::Pending,
             }
         }
-        if *written > 0 {
+        if *this.written > 0 {
-            buf.drain(..*written);
+            this.buf.drain(..*this.written);
         }
-        *written = 0;
+        *this.written = 0;
         Poll::Ready(ret)
     }
 }
@@ -278,20 +285,20 @@ impl<W: Write> Write for BufWriter<W> {
             ready!(self.as_mut().poll_flush_buf(cx))?;
         }
         if buf.len() >= self.buf.capacity() {
-            self.inner().poll_write(cx, buf)
+            self.get_pin_mut().poll_write(cx, buf)
         } else {
-            Pin::new(&mut *self.buf()).poll_write(cx, buf)
+            Pin::new(&mut *self.project().buf).poll_write(cx, buf)
         }
     }
 
     fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
         ready!(self.as_mut().poll_flush_buf(cx))?;
-        self.inner().poll_flush(cx)
+        self.get_pin_mut().poll_flush(cx)
     }
 
     fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
         ready!(self.as_mut().poll_flush_buf(cx))?;
-        self.inner().poll_close(cx)
+        self.get_pin_mut().poll_close(cx)
     }
 }
 
@@ -314,6 +321,6 @@ impl<W: Write + Seek> Seek for BufWriter<W> {
         pos: SeekFrom,
     ) -> Poll<io::Result<u64>> {
         ready!(self.as_mut().poll_flush_buf(cx))?;
-        self.inner().poll_seek(cx, pos)
+        self.get_pin_mut().poll_seek(cx, pos)
     }
 }

src/io/copy.rs

@@ -1,5 +1,7 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::io::{self, BufRead, BufReader, Read, Write};
 use crate::task::{Context, Poll};
@@ -46,47 +48,38 @@ where
     R: Read + Unpin + ?Sized,
     W: Write + Unpin + ?Sized,
 {
-    pub struct CopyFuture<'a, R, W: ?Sized> {
+    pin_project! {
+        struct CopyFuture<R, W> {
+            #[pin]
             reader: R,
-        writer: &'a mut W,
+            #[pin]
+            writer: W,
             amt: u64,
         }
-
-    impl<R, W: Unpin + ?Sized> CopyFuture<'_, R, W> {
-        fn project(self: Pin<&mut Self>) -> (Pin<&mut R>, Pin<&mut W>, &mut u64) {
-            unsafe {
-                let this = self.get_unchecked_mut();
-                (
-                    Pin::new_unchecked(&mut this.reader),
-                    Pin::new(&mut *this.writer),
-                    &mut this.amt,
-                )
-            }
-        }
     }
 
-    impl<R, W> Future for CopyFuture<'_, R, W>
+    impl<R, W> Future for CopyFuture<R, W>
     where
         R: BufRead,
-        W: Write + Unpin + ?Sized,
+        W: Write + Unpin,
     {
         type Output = io::Result<u64>;
 
         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-            let (mut reader, mut writer, amt) = self.project();
+            let mut this = self.project();
             loop {
-                let buffer = futures_core::ready!(reader.as_mut().poll_fill_buf(cx))?;
+                let buffer = futures_core::ready!(this.reader.as_mut().poll_fill_buf(cx))?;
                 if buffer.is_empty() {
-                    futures_core::ready!(writer.as_mut().poll_flush(cx))?;
+                    futures_core::ready!(this.writer.as_mut().poll_flush(cx))?;
-                    return Poll::Ready(Ok(*amt));
+                    return Poll::Ready(Ok(*this.amt));
                 }
 
-                let i = futures_core::ready!(writer.as_mut().poll_write(cx, buffer))?;
+                let i = futures_core::ready!(this.writer.as_mut().poll_write(cx, buffer))?;
                 if i == 0 {
                     return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
                 }
-                *amt += i as u64;
+                *this.amt += i as u64;
-                reader.as_mut().consume(i);
+                this.reader.as_mut().consume(i);
             }
         }
     }

src/io/read/chain.rs

@@ -1,10 +1,12 @@
-use crate::io::IoSliceMut;
 use std::fmt;
 use std::pin::Pin;
 
-use crate::io::{self, BufRead, Read};
+use pin_project_lite::pin_project;
+
+use crate::io::{self, BufRead, IoSliceMut, Read};
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// Adaptor to chain together two readers.
     ///
     /// This struct is generally created by calling [`chain`] on a reader.
@@ -12,10 +14,13 @@ use crate::task::{Context, Poll};
     ///
     /// [`chain`]: trait.Read.html#method.chain
     pub struct Chain<T, U> {
+        #[pin]
         pub(crate) first: T,
+        #[pin]
         pub(crate) second: U,
         pub(crate) done_first: bool,
     }
+}
 
 impl<T, U> Chain<T, U> {
     /// Consumes the `Chain`, returning the wrapped readers.
@@ -98,76 +103,64 @@ impl<T: fmt::Debug, U: fmt::Debug> fmt::Debug for Chain<T, U> {
     }
 }
 
-impl<T: Read + Unpin, U: Read + Unpin> Read for Chain<T, U> {
+impl<T: Read, U: Read> Read for Chain<T, U> {
     fn poll_read(
-        mut self: Pin<&mut Self>,
+        self: Pin<&mut Self>,
         cx: &mut Context<'_>,
         buf: &mut [u8],
     ) -> Poll<io::Result<usize>> {
-        if !self.done_first {
+        let this = self.project();
-            let rd = Pin::new(&mut self.first);
+        if !*this.done_first {
-
+            match futures_core::ready!(this.first.poll_read(cx, buf)) {
-            match futures_core::ready!(rd.poll_read(cx, buf)) {
+                Ok(0) if !buf.is_empty() => *this.done_first = true,
-                Ok(0) if !buf.is_empty() => self.done_first = true,
                 Ok(n) => return Poll::Ready(Ok(n)),
                 Err(err) => return Poll::Ready(Err(err)),
             }
         }
 
-        let rd = Pin::new(&mut self.second);
+        this.second.poll_read(cx, buf)
-        rd.poll_read(cx, buf)
     }
 
     fn poll_read_vectored(
-        mut self: Pin<&mut Self>,
+        self: Pin<&mut Self>,
         cx: &mut Context<'_>,
         bufs: &mut [IoSliceMut<'_>],
     ) -> Poll<io::Result<usize>> {
-        if !self.done_first {
+        let this = self.project();
-            let rd = Pin::new(&mut self.first);
+        if !*this.done_first {
-
+            match futures_core::ready!(this.first.poll_read_vectored(cx, bufs)) {
-            match futures_core::ready!(rd.poll_read_vectored(cx, bufs)) {
+                Ok(0) if !bufs.is_empty() => *this.done_first = true,
-                Ok(0) if !bufs.is_empty() => self.done_first = true,
                 Ok(n) => return Poll::Ready(Ok(n)),
                 Err(err) => return Poll::Ready(Err(err)),
             }
         }
 
-        let rd = Pin::new(&mut self.second);
+        this.second.poll_read_vectored(cx, bufs)
-        rd.poll_read_vectored(cx, bufs)
     }
 }
 
-impl<T: BufRead + Unpin, U: BufRead + Unpin> BufRead for Chain<T, U> {
+impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
-        let Self {
+        let this = self.project();
-            first,
+        if !*this.done_first {
-            second,
+            match futures_core::ready!(this.first.poll_fill_buf(cx)) {
-            done_first,
-        } = unsafe { self.get_unchecked_mut() };
-
-        if !*done_first {
-            let first = unsafe { Pin::new_unchecked(first) };
-            match futures_core::ready!(first.poll_fill_buf(cx)) {
                 Ok(buf) if buf.is_empty() => {
-                    *done_first = true;
+                    *this.done_first = true;
                 }
                 Ok(buf) => return Poll::Ready(Ok(buf)),
                 Err(err) => return Poll::Ready(Err(err)),
             }
         }
 
-        let second = unsafe { Pin::new_unchecked(second) };
+        this.second.poll_fill_buf(cx)
-        second.poll_fill_buf(cx)
     }
 
-    fn consume(mut self: Pin<&mut Self>, amt: usize) {
+    fn consume(self: Pin<&mut Self>, amt: usize) {
-        if !self.done_first {
+        let this = self.project();
-            let rd = Pin::new(&mut self.first);
+        if !*this.done_first {
-            rd.consume(amt)
+            this.first.consume(amt)
         } else {
-            let rd = Pin::new(&mut self.second);
+            this.second.consume(amt)
-            rd.consume(amt)
         }
     }
 }

src/io/read/mod.rs

@@ -13,23 +13,17 @@ use read_to_end::{read_to_end_internal, ReadToEndFuture};
 use read_to_string::ReadToStringFuture;
 use read_vectored::ReadVectoredFuture;
 
-use cfg_if::cfg_if;
 use std::mem;
 
 use crate::io::IoSliceMut;
-use crate::utils::extension_trait;
 
-cfg_if! {
+extension_trait! {
-    if #[cfg(feature = "docs")] {
     use std::pin::Pin;
     use std::ops::{Deref, DerefMut};
 
     use crate::io;
     use crate::task::{Context, Poll};
-    }
-}
 
-extension_trait! {
     #[doc = r#"
         Allows reading from a byte stream.
 

src/io/read/take.rs

@@ -1,9 +1,12 @@
 use std::cmp;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::io::{self, BufRead, Read};
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// Reader adaptor which limits the bytes read from an underlying reader.
     ///
     /// This struct is generally created by calling [`take`] on a reader.
@@ -12,9 +15,11 @@ use crate::task::{Context, Poll};
     /// [`take`]: trait.Read.html#method.take
     #[derive(Debug)]
     pub struct Take<T> {
+        #[pin]
         pub(crate) inner: T,
         pub(crate) limit: u64,
     }
+}
 
 impl<T> Take<T> {
     /// Returns the number of bytes that can be read before this instance will
@@ -152,15 +157,15 @@ impl<T> Take<T> {
     }
 }
 
-impl<T: Read + Unpin> Read for Take<T> {
+impl<T: Read> Read for Take<T> {
     /// Attempt to read from the `AsyncRead` into `buf`.
     fn poll_read(
-        mut self: Pin<&mut Self>,
+        self: Pin<&mut Self>,
         cx: &mut Context<'_>,
         buf: &mut [u8],
     ) -> Poll<io::Result<usize>> {
-        let Self { inner, limit } = &mut *self;
+        let this = self.project();
-        take_read_internal(Pin::new(inner), cx, buf, limit)
+        take_read_internal(this.inner, cx, buf, this.limit)
     }
 }
 
@@ -186,31 +191,30 @@ pub fn take_read_internal<R: Read + ?Sized>(
     }
 }
 
-impl<T: BufRead + Unpin> BufRead for Take<T> {
+impl<T: BufRead> BufRead for Take<T> {
     fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
-        let Self { inner, limit } = unsafe { self.get_unchecked_mut() };
+        let this = self.project();
-        let inner = unsafe { Pin::new_unchecked(inner) };
 
-        if *limit == 0 {
+        if *this.limit == 0 {
             return Poll::Ready(Ok(&[]));
         }
 
-        match futures_core::ready!(inner.poll_fill_buf(cx)) {
+        match futures_core::ready!(this.inner.poll_fill_buf(cx)) {
             Ok(buf) => {
-                let cap = cmp::min(buf.len() as u64, *limit) as usize;
+                let cap = cmp::min(buf.len() as u64, *this.limit) as usize;
                 Poll::Ready(Ok(&buf[..cap]))
             }
             Err(e) => Poll::Ready(Err(e)),
         }
     }
 
-    fn consume(mut self: Pin<&mut Self>, amt: usize) {
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let this = self.project();
         // Don't let callers reset the limit by passing an overlarge value
-        let amt = cmp::min(amt as u64, self.limit) as usize;
+        let amt = cmp::min(amt as u64, *this.limit) as usize;
-        self.limit -= amt as u64;
+        *this.limit -= amt as u64;
 
-        let rd = Pin::new(&mut self.inner);
+        this.inner.consume(amt);
-        rd.consume(amt);
     }
 }
 

src/io/seek/mod.rs

@@ -1,19 +1,16 @@
-use std::pin::Pin;
+mod seek;
 
-use cfg_if::cfg_if;
+use seek::SeekFuture;
 
-use crate::future::Future;
+use crate::io::SeekFrom;
-use crate::io::{self, SeekFrom};
-use crate::task::{Context, Poll};
-use crate::utils::extension_trait;
-
-cfg_if! {
-    if #[cfg(feature = "docs")] {
-        use std::ops::{Deref, DerefMut};
-    }
-}
 
 extension_trait! {
+    use std::ops::{Deref, DerefMut};
+    use std::pin::Pin;
+
+    use crate::io;
+    use crate::task::{Context, Poll};
+
     #[doc = r#"
         Allows seeking through a byte stream.
 
@@ -114,19 +111,3 @@ extension_trait! {
         }
     }
 }
-
-#[doc(hidden)]
-#[allow(missing_debug_implementations)]
-pub struct SeekFuture<'a, T: Unpin + ?Sized> {
-    seeker: &'a mut T,
-    pos: SeekFrom,
-}
-
-impl<T: SeekExt + Unpin + ?Sized> Future for SeekFuture<'_, T> {
-    type Output = io::Result<u64>;
-
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let pos = self.pos;
-        Pin::new(&mut *self.seeker).poll_seek(cx, pos)
-    }
-}

src/io/seek/seek.rs

@@ -0,0 +1,21 @@
+use std::pin::Pin;
+
+use crate::future::Future;
+use crate::io::{self, Seek, SeekFrom};
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct SeekFuture<'a, T: Unpin + ?Sized> {
+    pub(crate) seeker: &'a mut T,
+    pub(crate) pos: SeekFrom,
+}
+
+impl<T: Seek + Unpin + ?Sized> Future for SeekFuture<'_, T> {
+    type Output = io::Result<u64>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let pos = self.pos;
+        Pin::new(&mut *self.seeker).poll_seek(cx, pos)
+    }
+}

src/io/stderr.rs

@@ -1,8 +1,6 @@
 use std::pin::Pin;
 use std::sync::Mutex;
 
-use cfg_if::cfg_if;
-
 use crate::future::Future;
 use crate::io::{self, Write};
 use crate::task::{blocking, Context, JoinHandle, Poll};
@@ -162,35 +160,22 @@ impl Write for Stderr {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, RawFd};
-        use crate::os::windows::io::{AsRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        use std::os::windows::io::{AsRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for Stderr {
         fn as_raw_fd(&self) -> RawFd {
             std::io::stderr().as_raw_fd()
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
+cfg_windows! {
-cfg_if! {
+    use crate::os::windows::io::{AsRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
+
     impl AsRawHandle for Stderr {
         fn as_raw_handle(&self) -> RawHandle {
             std::io::stderr().as_raw_handle()
         }
     }
 }
-}

src/io/stdin.rs

@@ -1,8 +1,6 @@
 use std::pin::Pin;
 use std::sync::Mutex;
 
-use cfg_if::cfg_if;
-
 use crate::future::{self, Future};
 use crate::io::{self, Read};
 use crate::task::{blocking, Context, JoinHandle, Poll};
@@ -186,35 +184,22 @@ impl Read for Stdin {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, RawFd};
-        use crate::os::windows::io::{AsRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        use std::os::windows::io::{AsRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for Stdin {
         fn as_raw_fd(&self) -> RawFd {
             std::io::stdin().as_raw_fd()
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
+cfg_windows! {
-cfg_if! {
+    use crate::os::windows::io::{AsRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
+
     impl AsRawHandle for Stdin {
         fn as_raw_handle(&self) -> RawHandle {
             std::io::stdin().as_raw_handle()
         }
     }
 }
-}

src/io/stdout.rs

@@ -1,8 +1,6 @@
 use std::pin::Pin;
 use std::sync::Mutex;
 
-use cfg_if::cfg_if;
-
 use crate::future::Future;
 use crate::io::{self, Write};
 use crate::task::{blocking, Context, JoinHandle, Poll};
@@ -162,35 +160,22 @@ impl Write for Stdout {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, RawFd};
-        use crate::os::windows::io::{AsRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        use std::os::windows::io::{AsRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for Stdout {
         fn as_raw_fd(&self) -> RawFd {
             std::io::stdout().as_raw_fd()
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
+cfg_windows! {
-cfg_if! {
+    use crate::os::windows::io::{AsRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
+
     impl AsRawHandle for Stdout {
         fn as_raw_handle(&self) -> RawHandle {
             std::io::stdout().as_raw_handle()
         }
     }
 }
-}

src/io/timeout.rs

@@ -3,7 +3,7 @@ use std::task::{Context, Poll};
 use std::time::Duration;
 
 use futures_timer::Delay;
-use pin_utils::unsafe_pinned;
+use pin_project_lite::pin_project;
 
 use crate::future::Future;
 use crate::io;
@@ -43,22 +43,18 @@ where
     .await
 }
 
+pin_project! {
     /// Future returned by the `FutureExt::timeout` method.
     #[derive(Debug)]
     pub struct Timeout<F, T>
     where
         F: Future<Output = io::Result<T>>,
     {
+        #[pin]
         future: F,
+        #[pin]
         timeout: Delay,
     }
-
-impl<F, T> Timeout<F, T>
-where
-    F: Future<Output = io::Result<T>>,
-{
-    unsafe_pinned!(future: F);
-    unsafe_pinned!(timeout: Delay);
 }
 
 impl<F, T> Future for Timeout<F, T>
@@ -67,14 +63,15 @@ where
 {
     type Output = io::Result<T>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        match self.as_mut().future().poll(cx) {
+        let this = self.project();
+        match this.future.poll(cx) {
             Poll::Pending => {}
             other => return other,
         }
 
-        if self.timeout().poll(cx).is_ready() {
+        if this.timeout.poll(cx).is_ready() {
-            let err = Err(io::Error::new(io::ErrorKind::TimedOut, "future timed out").into());
+            let err = Err(io::Error::new(io::ErrorKind::TimedOut, "future timed out"));
             Poll::Ready(err)
         } else {
             Poll::Pending

src/io/write/mod.rs

@@ -10,22 +10,14 @@ use write_all::WriteAllFuture;
 use write_fmt::WriteFmtFuture;
 use write_vectored::WriteVectoredFuture;
 
-use cfg_if::cfg_if;
+use crate::io::{self, IoSlice};
-
-use crate::io::IoSlice;
-use crate::utils::extension_trait;
-
-use crate::io;
-
-cfg_if! {
-    if #[cfg(feature = "docs")] {
-        use std::pin::Pin;
-        use std::ops::{Deref, DerefMut};
-        use crate::task::{Context, Poll};
-    }
-}
 
 extension_trait! {
+    use std::pin::Pin;
+    use std::ops::{Deref, DerefMut};
+
+    use crate::task::{Context, Poll};
+
     #[doc = r#"
         Allows writing to a byte stream.
 

src/io/write/write_fmt.rs

@@ -32,7 +32,7 @@ impl<T: Write + Unpin + ?Sized> Future for WriteFmtFuture<'_, T> {
             buffer,
             ..
         } = &mut *self;
-        let mut buffer = buffer.as_mut().unwrap();
+        let buffer = buffer.as_mut().unwrap();
 
         // Copy the data from the buffer into the writer until it's done.
         loop {
@@ -40,7 +40,7 @@ impl<T: Write + Unpin + ?Sized> Future for WriteFmtFuture<'_, T> {
                 futures_core::ready!(Pin::new(&mut **writer).poll_flush(cx))?;
                 return Poll::Ready(Ok(()));
             }
-            let i = futures_core::ready!(Pin::new(&mut **writer).poll_write(cx, &mut buffer))?;
+            let i = futures_core::ready!(Pin::new(&mut **writer).poll_write(cx, buffer))?;
             if i == 0 {
                 return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
             }

src/lib.rs

@@ -48,7 +48,8 @@
 #![doc(html_logo_url = "https://async.rs/images/logo--hero.svg")]
 #![recursion_limit = "1024"]
 
-use cfg_if::cfg_if;
+#[macro_use]
+mod utils;
 
 pub mod fs;
 pub mod future;
@@ -61,11 +62,8 @@ pub mod stream;
 pub mod sync;
 pub mod task;
 
-cfg_if! {
+cfg_unstable! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
-        #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
     pub mod pin;
-        #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
     pub mod process;
 
     mod unit;
@@ -74,13 +72,9 @@ cfg_if! {
     mod option;
     mod string;
     mod collections;
-    }
+
+    #[doc(inline)]
+    pub use std::{write, writeln};
 }
 
 mod macros;
-pub(crate) mod utils;
-
-#[cfg(any(feature = "unstable", feature = "docs"))]
-#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[doc(inline)]
-pub use std::{write, writeln};

src/macros.rs

@@ -43,7 +43,7 @@
 /// #
 /// # })
 /// ```
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[macro_export]
 macro_rules! print {
@@ -81,12 +81,15 @@ macro_rules! print {
 /// #
 /// # })
 /// ```
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[macro_export]
 macro_rules! println {
     () => ($crate::print!("\n"));
-    ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)))
+    ($($arg:tt)*) => (async {
+        $crate::io::_print(format_args!($($arg)*)).await;
+        $crate::io::_print(format_args!("\n")).await;
+    })
 }
 
 /// Prints to the standard error.
@@ -116,7 +119,7 @@ macro_rules! println {
 /// #
 /// # })
 /// ```
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[macro_export]
 macro_rules! eprint {
@@ -150,7 +153,7 @@ macro_rules! eprint {
 /// #
 /// # })
 /// ```
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[macro_export]
 macro_rules! eprintln {
@@ -158,6 +161,7 @@ macro_rules! eprintln {
     ($($arg:tt)*) => (
         async {
             $crate::io::_eprint(format_args!($($arg)*)).await;
+            $crate::io::_eprint(format_args!("\n")).await;
         }
     );
 }

src/net/addr.rs

@@ -3,25 +3,23 @@ use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
 use std::net::{SocketAddr, SocketAddrV4, SocketAddrV6};
 use std::pin::Pin;
 
-use cfg_if::cfg_if;
-
 use crate::future::Future;
 use crate::io;
 use crate::task::{blocking, Context, JoinHandle, Poll};
 
-cfg_if! {
+cfg_not_docs! {
-    if #[cfg(feature = "docs")] {
+    macro_rules! ret {
+        (impl Future<Output = $out:ty>, $fut:ty) => ($fut);
+    }
+}
+
+cfg_docs! {
     #[doc(hidden)]
     pub struct ImplFuture<T>(std::marker::PhantomData<T>);
 
     macro_rules! ret {
         (impl Future<Output = $out:ty>, $fut:ty) => (ImplFuture<$out>);
     }
-    } else {
-        macro_rules! ret {
-            (impl Future<Output = $out:ty>, $fut:ty) => ($fut);
-        }
-    }
 }
 
 /// Converts or resolves addresses to [`SocketAddr`] values.
@@ -212,7 +210,7 @@ impl ToSocketAddrs for str {
         impl Future<Output = Self::Iter>,
         ToSocketAddrsFuture<Self::Iter>
     ) {
-        if let Some(addr) = self.parse().ok() {
+        if let Ok(addr) = self.parse() {
             return ToSocketAddrsFuture::Ready(Ok(vec![addr].into_iter()));
         }
 

src/net/tcp/listener.rs

@@ -1,13 +1,10 @@
 use std::net::SocketAddr;
 use std::pin::Pin;
 
-use cfg_if::cfg_if;
-
-use super::TcpStream;
 use crate::future::{self, Future};
 use crate::io;
 use crate::net::driver::Watcher;
-use crate::net::ToSocketAddrs;
+use crate::net::{TcpStream, ToSocketAddrs};
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
@@ -213,20 +210,9 @@ impl From<std::net::TcpListener> for TcpListener {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-        // use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        // use std::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for TcpListener {
         fn as_raw_fd(&self) -> RawFd {
             self.watcher.get_ref().as_raw_fd()
@@ -245,11 +231,10 @@ cfg_if! {
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(windows)))]
+cfg_windows! {
-cfg_if! {
+    // use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
+    //
     // impl AsRawSocket for TcpListener {
     //     fn as_raw_socket(&self) -> RawSocket {
     //         self.raw_socket
@@ -268,4 +253,3 @@ cfg_if! {
     //     }
     // }
 }
-}

src/net/tcp/stream.rs

@@ -2,8 +2,6 @@ use std::io::{IoSlice, IoSliceMut, Read as _, Write as _};
 use std::net::SocketAddr;
 use std::pin::Pin;
 
-use cfg_if::cfg_if;
-
 use crate::future;
 use crate::io::{self, Read, Write};
 use crate::net::driver::Watcher;
@@ -367,20 +365,9 @@ impl From<std::net::TcpStream> for TcpStream {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-        // use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        // use std::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for TcpStream {
         fn as_raw_fd(&self) -> RawFd {
             self.watcher.get_ref().as_raw_fd()
@@ -399,11 +386,10 @@ cfg_if! {
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(windows)))]
+cfg_windows! {
-cfg_if! {
+    // use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
+    //
     // impl AsRawSocket for TcpStream {
     //     fn as_raw_socket(&self) -> RawSocket {
     //         self.raw_socket
@@ -422,4 +408,3 @@ cfg_if! {
     //     }
     // }
 }
-}

src/net/udp/mod.rs

@@ -1,7 +1,5 @@
 use std::io;
 use std::net::SocketAddr;
-
-use cfg_if::cfg_if;
 use std::net::{Ipv4Addr, Ipv6Addr};
 
 use crate::future;
@@ -463,20 +461,9 @@ impl From<std::net::UdpSocket> for UdpSocket {
     }
 }
 
-cfg_if! {
+cfg_unix! {
-    if #[cfg(feature = "docs")] {
     use crate::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-        // use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    } else if #[cfg(unix)] {
-        use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-    } else if #[cfg(windows)] {
-        // use std::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
-cfg_if! {
-    if #[cfg(any(unix, feature = "docs"))] {
     impl AsRawFd for UdpSocket {
         fn as_raw_fd(&self) -> RawFd {
             self.watcher.get_ref().as_raw_fd()
@@ -495,12 +482,9 @@ cfg_if! {
         }
     }
 }
-}
 
-#[cfg_attr(feature = "docs", doc(cfg(windows)))]
+cfg_windows! {
-cfg_if! {
+    // use crate::os::windows::io::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
-    if #[cfg(any(windows, feature = "docs"))] {
-        // use std::os::windows::io::{AsRawSocket, FromRawSocket, IntoRawSocket, RawSocket};
     //
     // impl AsRawSocket for UdpSocket {
     //     fn as_raw_socket(&self) -> RawSocket {
@@ -520,4 +504,3 @@ cfg_if! {
     //     }
     // }
 }
-}

src/os/mod.rs

@@ -1,9 +1,9 @@
 //! OS-specific extensions.
 
-#[cfg(any(unix, feature = "docs"))]
+cfg_unix! {
-#[cfg_attr(feature = "docs", doc(cfg(unix)))]
     pub mod unix;
+}
 
-#[cfg(any(windows, feature = "docs"))]
+cfg_windows! {
-#[cfg_attr(feature = "docs", doc(cfg(windows)))]
     pub mod windows;
+}

src/os/unix/fs.rs

@@ -1,7 +1,5 @@
 //! Unix-specific filesystem extensions.
 
-use cfg_if::cfg_if;
-
 use crate::io;
 use crate::path::Path;
 use crate::task::blocking;
@@ -31,8 +29,11 @@ pub async fn symlink<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Resu
     blocking::spawn(move || std::os::unix::fs::symlink(&src, &dst)).await
 }
 
-cfg_if! {
+cfg_not_docs! {
-    if #[cfg(feature = "docs")] {
+    pub use std::os::unix::fs::{DirBuilderExt, DirEntryExt, OpenOptionsExt};
+}
+
+cfg_docs! {
     /// Unix-specific extensions to `DirBuilder`.
     pub trait DirBuilderExt {
         /// Sets the mode to create new directories with. This option defaults to
@@ -67,7 +68,4 @@ cfg_if! {
         /// This options overwrites any previously set custom flags.
         fn custom_flags(&mut self, flags: i32) -> &mut Self;
     }
-    } else {
-        pub use std::os::unix::fs::{DirBuilderExt, OpenOptionsExt};
-    }
 }

src/os/unix/io.rs

@@ -1,9 +1,10 @@
 //! Unix-specific I/O extensions.
 
-use cfg_if::cfg_if;
+cfg_not_docs! {
+    pub use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
+}
 
-cfg_if! {
+cfg_docs! {
-    if #[cfg(feature = "docs")] {
     /// Raw file descriptors.
     pub type RawFd = std::os::raw::c_int;
 
@@ -50,7 +51,4 @@ cfg_if! {
         /// and must close the descriptor once it's no longer needed.
         fn into_raw_fd(self) -> RawFd;
     }
-    } else {
-        pub use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
-    }
 }

src/os/unix/net/mod.rs

@@ -1,7 +1,5 @@
 //! Unix-specific networking extensions.
 
-use cfg_if::cfg_if;
-
 pub use datagram::UnixDatagram;
 pub use listener::{Incoming, UnixListener};
 pub use stream::UnixStream;
@@ -10,8 +8,11 @@ mod datagram;
 mod listener;
 mod stream;
 
-cfg_if! {
+cfg_not_docs! {
-    if #[cfg(feature = "docs")] {
+    pub use std::os::unix::net::SocketAddr;
+}
+
+cfg_docs! {
     use std::fmt;
 
     use crate::path::Path;
@@ -93,7 +94,4 @@ cfg_if! {
             unreachable!("this impl only appears in the rendered docs")
         }
     }
-    } else {
-        pub use std::os::unix::net::SocketAddr;
-    }
 }

src/os/windows/io.rs

@@ -1,9 +1,12 @@
 //! Windows-specific I/O extensions.
 
-use cfg_if::cfg_if;
+cfg_not_docs! {
+    pub use std::os::windows::io::{
+        AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle, RawSocket,
+    };
+}
 
-cfg_if! {
+cfg_docs! {
-    if #[cfg(feature = "docs")] {
     /// Raw HANDLEs.
     pub type RawHandle = *mut std::os::raw::c_void;
 
@@ -42,9 +45,4 @@ cfg_if! {
         /// it once it's no longer needed.
         fn into_raw_handle(self) -> RawHandle;
     }
-    } else {
-        pub use std::os::windows::io::{
-            AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle, RawSocket,
-        };
-    }
 }

src/path/path.rs

@@ -799,7 +799,7 @@ impl AsRef<Path> for String {
 
 impl AsRef<Path> for std::path::PathBuf {
     fn as_ref(&self) -> &Path {
-        Path::new(self.into())
+        Path::new(self)
     }
 }
 

src/path/pathbuf.rs

@@ -5,7 +5,7 @@ use crate::path::Path;
 /// This struct is an async version of [`std::path::PathBuf`].
 ///
 /// [`std::path::Path`]: https://doc.rust-lang.org/std/path/struct.PathBuf.html
-#[derive(Debug, PartialEq)]
+#[derive(Debug, PartialEq, Default)]
 pub struct PathBuf {
     inner: std::path::PathBuf,
 }
@@ -206,7 +206,7 @@ impl From<std::path::PathBuf> for PathBuf {
 
 impl Into<std::path::PathBuf> for PathBuf {
     fn into(self) -> std::path::PathBuf {
-        self.inner.into()
+        self.inner
     }
 }
 

src/prelude.rs

@@ -11,8 +11,6 @@
 //! use async_std::prelude::*;
 //! ```
 
-use cfg_if::cfg_if;
-
 #[doc(no_inline)]
 pub use crate::future::Future;
 #[doc(no_inline)]
@@ -41,12 +39,9 @@ pub use crate::io::write::WriteExt as _;
 #[doc(hidden)]
 pub use crate::stream::stream::StreamExt as _;
 
-cfg_if! {
+cfg_unstable! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
     #[doc(no_inline)]
     pub use crate::stream::DoubleEndedStream;
-
     #[doc(no_inline)]
     pub use crate::stream::ExactSizeStream;
 }
-}

src/stream/double_ended_stream.rs

@@ -10,8 +10,8 @@ use std::task::{Context, Poll};
 /// `Item`s from the back, as well as the front.
 ///
 /// [`Stream`]: trait.Stream.html
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[cfg(any(feature = "unstable", feature = "docs"))]
 pub trait DoubleEndedStream: Stream {
     /// Removes and returns an element from the end of the stream.
     ///

src/stream/exact_size_stream.rs

@@ -59,7 +59,7 @@ pub use crate::stream::Stream;
 /// #         }
 /// #     }
 /// # }
-/// # fn main() { async_std::task::block_on(async {
+/// # async_std::task::block_on(async {
 /// #
 /// impl ExactSizeStream for Counter {
 ///     // We can easily calculate the remaining number of iterations.
@@ -74,10 +74,9 @@ pub use crate::stream::Stream;
 ///
 /// assert_eq!(5, counter.len());
 /// # });
-/// # }
 /// ```
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[cfg(any(feature = "unstable", feature = "docs"))]
 pub trait ExactSizeStream: Stream {
     /// Returns the exact number of times the stream will iterate.
     ///

src/stream/extend.rs

@@ -14,7 +14,7 @@ use crate::stream::IntoStream;
 /// ## Examples
 ///
 /// ```
-/// # fn main() { async_std::task::block_on(async {
+/// # async_std::task::block_on(async {
 /// #
 /// use async_std::prelude::*;
 /// use async_std::stream::{self, Extend};
@@ -25,8 +25,9 @@ use crate::stream::IntoStream;
 ///
 /// assert_eq!(v, vec![1, 2, 3, 3, 3]);
 /// #
-/// # }) }
+/// # })
 /// ```
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 pub trait Extend<A> {
     /// Extends a collection with the contents of a stream.

src/stream/from_fn.rs

@@ -1,10 +1,13 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream that yields elements by calling a closure.
     ///
     /// This stream is constructed by [`from_fn`] function.
@@ -13,9 +16,11 @@ use crate::task::{Context, Poll};
     #[derive(Debug)]
     pub struct FromFn<F, Fut, T> {
         f: F,
+        #[pin]
         future: Option<Fut>,
         __t: PhantomData<T>,
     }
+}
 
 /// Creates a new stream where to produce each new element a provided closure is called.
 ///
@@ -25,7 +30,7 @@ pub struct FromFn<F, Fut, T> {
 /// # Examples
 ///
 /// ```
-/// # fn main() { async_std::task::block_on(async {
+/// # async_std::task::block_on(async {
 /// #
 /// use async_std::prelude::*;
 /// use async_std::sync::Mutex;
@@ -53,8 +58,7 @@ pub struct FromFn<F, Fut, T> {
 /// assert_eq!(s.next().await, Some(3));
 /// assert_eq!(s.next().await, None);
 /// #
-/// # }) }
+/// # })
-///
 /// ```
 pub fn from_fn<T, F, Fut>(f: F) -> FromFn<F, Fut, T>
 where
@@ -68,11 +72,6 @@ where
     }
 }
 
-impl<F, Fut, T> FromFn<F, Fut, T> {
-    pin_utils::unsafe_unpinned!(f: F);
-    pin_utils::unsafe_pinned!(future: Option<Fut>);
-}
-
 impl<F, Fut, T> Stream for FromFn<F, Fut, T>
 where
     F: FnMut() -> Fut,
@@ -80,20 +79,18 @@ where
 {
     type Item = T;
 
-    fn poll_next(mut 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();
         loop {
-            match &self.future {
+            if this.future.is_some() {
-                Some(_) => {
                 let next =
-                        futures_core::ready!(self.as_mut().future().as_pin_mut().unwrap().poll(cx));
+                    futures_core::ready!(this.future.as_mut().as_pin_mut().unwrap().poll(cx));
-                    self.as_mut().future().set(None);
+                this.future.set(None);
 
                 return Poll::Ready(next);
-                }
+            } else {
-                None => {
+                let fut = (this.f)();
-                    let fut = (self.as_mut().f())();
+                this.future.set(Some(fut));
-                    self.as_mut().future().set(Some(fut));
-                }
             }
         }
     }

src/stream/from_stream.rs

@@ -106,8 +106,8 @@ use std::pin::Pin;
 ///```
 ///
 /// [`IntoStream`]: trait.IntoStream.html
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[cfg(any(feature = "unstable", feature = "docs"))]
 pub trait FromStream<T> {
     /// Creates a value from a stream.
     ///

src/stream/fused_stream.rs

@@ -14,7 +14,7 @@ use crate::stream::Stream;
 /// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
 /// [`Stream::fuse`]: trait.Stream.html#method.fuse
 /// [`Fuse`]: struct.Fuse.html
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 pub trait FusedStream: Stream {}
 

src/stream/interval.rs

@@ -4,8 +4,6 @@ use std::time::{Duration, Instant};
 
 use futures_core::future::Future;
 use futures_core::stream::Stream;
-use pin_utils::unsafe_pinned;
-
 use futures_timer::Delay;
 
 /// Creates a new stream that yields at a set interval.
@@ -43,8 +41,8 @@ use futures_timer::Delay;
 /// #
 /// # Ok(()) }) }
 /// ```
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[doc(inline)]
 pub fn interval(dur: Duration) -> Interval {
     Interval {
         delay: Delay::new(dur),
@@ -54,23 +52,19 @@ pub fn interval(dur: Duration) -> Interval {
 
 /// A stream representing notifications at fixed interval
 ///
-#[derive(Debug)]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[doc(inline)]
+#[derive(Debug)]
 pub struct Interval {
     delay: Delay,
     interval: Duration,
 }
 
-impl Interval {
-    unsafe_pinned!(delay: Delay);
-}
-
 impl Stream for Interval {
     type Item = ();
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        if Pin::new(&mut *self).delay().poll(cx).is_pending() {
+        if Pin::new(&mut self.delay).poll(cx).is_pending() {
             return Poll::Pending;
         }
         let when = Instant::now();

src/stream/into_stream.rs

@@ -13,8 +13,8 @@ use crate::stream::Stream;
 /// See also: [`FromStream`].
 ///
 /// [`FromStream`]: trait.FromStream.html
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[cfg(any(feature = "unstable", feature = "docs"))]
 pub trait IntoStream {
     /// The type of the elements being iterated over.
     type Item;

src/stream/mod.rs

@@ -1,4 +1,4 @@
-//! Asynchronous iteration.
+//! Composable asynchronous iteration.
 //!
 //! This module is an async version of [`std::iter`].
 //!
@@ -21,12 +21,11 @@
 //! # })
 //! ```
 
-use cfg_if::cfg_if;
-
 pub use empty::{empty, Empty};
 pub use from_fn::{from_fn, FromFn};
 pub use once::{once, Once};
 pub use repeat::{repeat, Repeat};
+pub use repeat_with::{repeat_with, RepeatWith};
 pub use stream::{
     Chain, Filter, Fuse, Inspect, Scan, Skip, SkipWhile, StepBy, Stream, Take, TakeWhile, Zip,
 };
@@ -37,9 +36,9 @@ mod empty;
 mod from_fn;
 mod once;
 mod repeat;
+mod repeat_with;
 
-cfg_if! {
+cfg_unstable! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
     mod double_ended_stream;
     mod exact_size_stream;
     mod extend;
@@ -58,8 +57,6 @@ cfg_if! {
     pub use interval::{interval, Interval};
     pub use into_stream::IntoStream;
     pub use product::Product;
-        pub use sum::Sum;
-
     pub use stream::Merge;
-    }
+    pub use sum::Sum;
 }

src/stream/once.rs

@@ -1,5 +1,7 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
@@ -24,6 +26,7 @@ pub fn once<T>(t: T) -> Once<T> {
     Once { value: Some(t) }
 }
 
+pin_project! {
     /// A stream that yields a single item.
     ///
     /// This stream is constructed by the [`once`] function.
@@ -33,11 +36,12 @@ pub fn once<T>(t: T) -> Once<T> {
     pub struct Once<T> {
         value: Option<T>,
     }
+}
 
 impl<T: Unpin> Stream for Once<T> {
     type Item = T;
 
-    fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
+    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
-        Poll::Ready(self.value.take())
+        Poll::Ready(self.project().value.take())
     }
 }

src/stream/product.rs

@@ -11,8 +11,8 @@ use crate::stream::Stream;
 /// [`product`]: trait.Product.html#tymethod.product
 /// [`FromStream`]: trait.FromStream.html
 /// [`Stream::product`]: trait.Stream.html#method.product
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[cfg(any(feature = "unstable", feature = "docs"))]
 pub trait Product<A = Self>: Sized {
     /// Method which takes a stream and generates `Self` from the elements by
     /// multiplying the items.

src/stream/repeat_with.rs

@@ -0,0 +1,100 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use pin_project_lite::pin_project;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    /// A stream that repeats elements of type `T` endlessly by applying a provided closure.
+    ///
+    /// This stream is constructed by the [`repeat_with`] function.
+    ///
+    /// [`repeat_with`]: fn.repeat_with.html
+    #[derive(Debug)]
+    pub struct RepeatWith<F, Fut, A> {
+        f: F,
+        #[pin]
+        future: Option<Fut>,
+        __a: PhantomData<A>,
+    }
+}
+
+/// Creates a new stream that repeats elements of type `A` endlessly by applying the provided closure.
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// # async_std::task::block_on(async {
+/// #
+/// use async_std::prelude::*;
+/// use async_std::stream;
+///
+/// let s = stream::repeat_with(|| async { 1 });
+///
+/// pin_utils::pin_mut!(s);
+///
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// # })
+/// ```
+///
+/// Going finite:
+///
+/// ```
+/// # async_std::task::block_on(async {
+/// #
+/// use async_std::prelude::*;
+/// use async_std::stream;
+///
+/// let s = stream::repeat_with(|| async { 1u8 }).take(2);
+///
+/// pin_utils::pin_mut!(s);
+///
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, None);
+/// # })
+/// ```
+pub fn repeat_with<F, Fut, A>(repeater: F) -> RepeatWith<F, Fut, A>
+where
+    F: FnMut() -> Fut,
+    Fut: Future<Output = A>,
+{
+    RepeatWith {
+        f: repeater,
+        future: None,
+        __a: PhantomData,
+    }
+}
+
+impl<F, Fut, A> Stream for RepeatWith<F, Fut, A>
+where
+    F: FnMut() -> Fut,
+    Fut: Future<Output = A>,
+{
+    type Item = A;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+        loop {
+            if this.future.is_some() {
+                let res = futures_core::ready!(this.future.as_mut().as_pin_mut().unwrap().poll(cx));
+
+                this.future.set(None);
+
+                return Poll::Ready(Some(res));
+            } else {
+                let fut = (this.f)();
+
+                this.future.set(Some(fut));
+            }
+        }
+    }
+}

src/stream/stream/chain.rs

@@ -1,20 +1,23 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::fuse::Fuse;
 use crate::prelude::*;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// Chains two streams one after another.
     #[derive(Debug)]
     pub struct Chain<S, U> {
+        #[pin]
         first: Fuse<S>,
+        #[pin]
         second: Fuse<U>,
     }
+}
 
 impl<S: Stream, U: Stream> Chain<S, U> {
-    pin_utils::unsafe_pinned!(first: Fuse<S>);
-    pin_utils::unsafe_pinned!(second: Fuse<U>);
-
     pub(super) fn new(first: S, second: U) -> Self {
         Chain {
             first: first.fuse(),
@@ -26,22 +29,23 @@ impl<S: Stream, U: Stream> Chain<S, U> {
 impl<S: Stream, U: Stream<Item = S::Item>> Stream for Chain<S, U> {
     type Item = S::Item;
 
-    fn poll_next(mut 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>> {
-        if !self.first.done {
+        let mut this = self.project();
-            let next = futures_core::ready!(self.as_mut().first().poll_next(cx));
+        if !this.first.done {
+            let next = futures_core::ready!(this.first.as_mut().poll_next(cx));
             if let Some(next) = next {
                 return Poll::Ready(Some(next));
             }
         }
 
-        if !self.second.done {
+        if !this.second.done {
-            let next = futures_core::ready!(self.as_mut().second().poll_next(cx));
+            let next = futures_core::ready!(this.second.as_mut().poll_next(cx));
             if let Some(next) = next {
                 return Poll::Ready(Some(next));
             }
         }
 
-        if self.first.done && self.second.done {
+        if this.first.done && this.second.done {
             return Poll::Ready(None);
         }
 

src/stream/stream/cmp.rs

@@ -1,29 +1,30 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::fuse::Fuse;
 use crate::future::Future;
 use crate::prelude::*;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     // Lexicographically compares the elements of this `Stream` with those
     // of another using `Ord`.
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct CmpFuture<L: Stream, R: Stream> {
+        #[pin]
         l: Fuse<L>,
+        #[pin]
         r: Fuse<R>,
         l_cache: Option<L::Item>,
         r_cache: Option<R::Item>,
     }
+}
 
 impl<L: Stream, R: Stream> CmpFuture<L, R> {
-    pin_utils::unsafe_pinned!(l: Fuse<L>);
-    pin_utils::unsafe_pinned!(r: Fuse<R>);
-    pin_utils::unsafe_unpinned!(l_cache: Option<L::Item>);
-    pin_utils::unsafe_unpinned!(r_cache: Option<R::Item>);
-
     pub(super) fn new(l: L, r: R) -> Self {
         CmpFuture {
             l: l.fuse(),
@@ -42,11 +43,12 @@ where
 {
     type Output = Ordering;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
         loop {
             // Stream that completes earliest can be considered Less, etc
-            let l_complete = self.l.done && self.as_mut().l_cache.is_none();
+            let l_complete = this.l.done && this.l_cache.is_none();
-            let r_complete = self.r.done && self.as_mut().r_cache.is_none();
+            let r_complete = this.r.done && this.r_cache.is_none();
 
             if l_complete && r_complete {
                 return Poll::Ready(Ordering::Equal);
@@ -57,30 +59,30 @@ where
             }
 
             // Get next value if possible and necesary
-            if !self.l.done && self.as_mut().l_cache.is_none() {
+            if !this.l.done && this.l_cache.is_none() {
-                let l_next = futures_core::ready!(self.as_mut().l().poll_next(cx));
+                let l_next = futures_core::ready!(this.l.as_mut().poll_next(cx));
                 if let Some(item) = l_next {
-                    *self.as_mut().l_cache() = Some(item);
+                    *this.l_cache = Some(item);
                 }
             }
 
-            if !self.r.done && self.as_mut().r_cache.is_none() {
+            if !this.r.done && this.r_cache.is_none() {
-                let r_next = futures_core::ready!(self.as_mut().r().poll_next(cx));
+                let r_next = futures_core::ready!(this.r.as_mut().poll_next(cx));
                 if let Some(item) = r_next {
-                    *self.as_mut().r_cache() = Some(item);
+                    *this.r_cache = Some(item);
                 }
             }
 
             // Compare if both values are available.
-            if self.as_mut().l_cache.is_some() && self.as_mut().r_cache.is_some() {
+            if this.l_cache.is_some() && this.r_cache.is_some() {
-                let l_value = self.as_mut().l_cache().take().unwrap();
+                let l_value = this.l_cache.take().unwrap();
-                let r_value = self.as_mut().r_cache().take().unwrap();
+                let r_value = this.r_cache.take().unwrap();
                 let result = l_value.cmp(&r_value);
 
                 if let Ordering::Equal = result {
                     // Reset cache to prepare for next comparison
-                    *self.as_mut().l_cache() = None;
+                    *this.l_cache = None;
-                    *self.as_mut().r_cache() = None;
+                    *this.r_cache = None;
                 } else {
                     // Return non equal value
                     return Poll::Ready(result);

src/stream/stream/enumerate.rs

@@ -1,19 +1,21 @@
-use crate::task::{Context, Poll};
 use std::pin::Pin;
 
-use crate::stream::Stream;
+use pin_project_lite::pin_project;
 
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct Enumerate<S> {
+        #[pin]
         stream: S,
         i: usize,
     }
+}
 
 impl<S> Enumerate<S> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(i: usize);
-
     pub(super) fn new(stream: S) -> Self {
         Enumerate { stream, i: 0 }
     }
@@ -25,13 +27,14 @@ where
 {
     type Item = (usize, S::Item);
 
-    fn poll_next(mut 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 next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
 
         match next {
             Some(v) => {
-                let ret = (self.i, v);
+                let ret = (*this.i, v);
-                *self.as_mut().i() += 1;
+                *this.i += 1;
                 Poll::Ready(Some(ret))
             }
             None => Poll::Ready(None),

src/stream/stream/filter.rs

@@ -1,21 +1,23 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream to filter elements of another stream with a predicate.
     #[derive(Debug)]
     pub struct Filter<S, P, T> {
+        #[pin]
         stream: S,
         predicate: P,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, P, T> Filter<S, P, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(predicate: P);
-
     pub(super) fn new(stream: S, predicate: P) -> Self {
         Filter {
             stream,
@@ -32,11 +34,12 @@ where
 {
     type Item = S::Item;
 
-    fn poll_next(mut 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 next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
 
         match next {
-            Some(v) if (self.as_mut().predicate())(&v) => Poll::Ready(Some(v)),
+            Some(v) if (this.predicate)(&v) => Poll::Ready(Some(v)),
             Some(_) => {
                 cx.waker().wake_by_ref();
                 Poll::Pending

src/stream/stream/filter_map.rs

@@ -2,21 +2,23 @@ use std::marker::PhantomData;
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct FilterMap<S, F, T, B> {
+        #[pin]
         stream: S,
         f: F,
         __from: PhantomData<T>,
         __to: PhantomData<B>,
     }
+}
 
 impl<S, F, T, B> FilterMap<S, F, T, B> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-
     pub(crate) fn new(stream: S, f: F) -> Self {
         FilterMap {
             stream,
@@ -34,10 +36,11 @@ where
 {
     type Item = B;
 
-    fn poll_next(mut 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 next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
         match next {
-            Some(v) => match (self.as_mut().f())(v) {
+            Some(v) => match (this.f)(v) {
                 Some(b) => Poll::Ready(Some(b)),
                 None => {
                     cx.waker().wake_by_ref();

src/stream/stream/fold.rs

@@ -1,24 +1,25 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct FoldFuture<S, F, T, B> {
+        #[pin]
         stream: S,
         f: F,
         acc: Option<B>,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, F, T, B> FoldFuture<S, F, T, B> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-    pin_utils::unsafe_unpinned!(acc: Option<B>);
-
     pub(super) fn new(stream: S, init: B, f: F) -> Self {
         FoldFuture {
             stream,
@@ -36,17 +37,18 @@ where
 {
     type Output = B;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
         loop {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match next {
                 Some(v) => {
-                    let old = self.as_mut().acc().take().unwrap();
+                    let old = this.acc.take().unwrap();
-                    let new = (self.as_mut().f())(old, v);
+                    let new = (this.f)(old, v);
-                    *self.as_mut().acc() = Some(new);
+                    *this.acc = Some(new);
                 }
-                None => return Poll::Ready(self.as_mut().acc().take().unwrap()),
+                None => return Poll::Ready(this.acc.take().unwrap()),
             }
         }
     }

src/stream/stream/for_each.rs

@@ -1,22 +1,24 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct ForEachFuture<S, F, T> {
+        #[pin]
         stream: S,
         f: F,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, F, T> ForEachFuture<S, F, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-
     pub(super) fn new(stream: S, f: F) -> Self {
         ForEachFuture {
             stream,
@@ -33,12 +35,13 @@ where
 {
     type Output = ();
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
         loop {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match next {
-                Some(v) => (self.as_mut().f())(v),
+                Some(v) => (this.f)(v),
                 None => return Poll::Ready(()),
             }
         }

src/stream/stream/fuse.rs

@@ -1,33 +1,32 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A `Stream` that is permanently closed once a single call to `poll` results in
     /// `Poll::Ready(None)`, returning `Poll::Ready(None)` for all future calls to `poll`.
     #[derive(Clone, Debug)]
     pub struct Fuse<S> {
+        #[pin]
         pub(crate) stream: S,
         pub(crate) done: bool,
     }
-
-impl<S: Unpin> Unpin for Fuse<S> {}
-
-impl<S: Stream> Fuse<S> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(done: bool);
 }
 
 impl<S: Stream> Stream for Fuse<S> {
     type Item = S::Item;
 
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
-        if self.done {
+        let this = self.project();
+        if *this.done {
             Poll::Ready(None)
         } else {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.poll_next(cx));
             if next.is_none() {
-                *self.as_mut().done() = true;
+                *this.done = true;
             }
             Poll::Ready(next)
         }

src/stream/stream/ge.rs

@@ -1,26 +1,29 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::partial_cmp::PartialCmpFuture;
 use crate::future::Future;
 use crate::prelude::*;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     // Determines if the elements of this `Stream` are lexicographically
     // greater than or equal to those of another.
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct GeFuture<L: Stream, R: Stream> {
+        #[pin]
         partial_cmp: PartialCmpFuture<L, R>,
     }
+}
 
 impl<L: Stream, R: Stream> GeFuture<L, R>
 where
     L::Item: PartialOrd<R::Item>,
 {
-    pin_utils::unsafe_pinned!(partial_cmp: PartialCmpFuture<L, R>);
-
     pub(super) fn new(l: L, r: R) -> Self {
         GeFuture {
             partial_cmp: l.partial_cmp(r),
@@ -30,14 +33,14 @@ where
 
 impl<L: Stream, R: Stream> Future for GeFuture<L, R>
 where
-    L: Stream + Sized,
+    L: Stream,
-    R: Stream + Sized,
+    R: Stream,
     L::Item: PartialOrd<R::Item>,
 {
     type Output = bool;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let result = futures_core::ready!(self.as_mut().partial_cmp().poll(cx));
+        let result = futures_core::ready!(self.project().partial_cmp.poll(cx));
 
         match result {
             Some(Ordering::Greater) | Some(Ordering::Equal) => Poll::Ready(true),

src/stream/stream/gt.rs

@@ -1,26 +1,29 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::partial_cmp::PartialCmpFuture;
 use crate::future::Future;
 use crate::prelude::*;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     // Determines if the elements of this `Stream` are lexicographically
     // greater than those of another.
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct GtFuture<L: Stream, R: Stream> {
+        #[pin]
         partial_cmp: PartialCmpFuture<L, R>,
     }
+}
 
 impl<L: Stream, R: Stream> GtFuture<L, R>
 where
     L::Item: PartialOrd<R::Item>,
 {
-    pin_utils::unsafe_pinned!(partial_cmp: PartialCmpFuture<L, R>);
-
     pub(super) fn new(l: L, r: R) -> Self {
         GtFuture {
             partial_cmp: l.partial_cmp(r),
@@ -36,8 +39,8 @@ where
 {
     type Output = bool;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let result = futures_core::ready!(self.as_mut().partial_cmp().poll(cx));
+        let result = futures_core::ready!(self.project().partial_cmp.poll(cx));
 
         match result {
             Some(Ordering::Greater) => Poll::Ready(true),

src/stream/stream/inspect.rs

@@ -1,21 +1,23 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream that does something with each element of another stream.
     #[derive(Debug)]
     pub struct Inspect<S, F, T> {
+        #[pin]
         stream: S,
         f: F,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, F, T> Inspect<S, F, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-
     pub(super) fn new(stream: S, f: F) -> Self {
         Inspect {
             stream,
@@ -32,11 +34,12 @@ where
 {
     type Item = S::Item;
 
-    fn poll_next(mut 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 next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let mut this = self.project();
+        let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
         Poll::Ready(next.and_then(|x| {
-            (self.as_mut().f())(&x);
+            (this.f)(&x);
             Some(x)
         }))
     }

src/stream/stream/last.rs

@@ -0,0 +1,45 @@
+use std::pin::Pin;
+
+use pin_project_lite::pin_project;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    #[doc(hidden)]
+    #[allow(missing_debug_implementations)]
+    pub struct LastFuture<S, T> {
+        #[pin]
+        stream: S,
+        last: Option<T>,
+    }
+}
+
+impl<S, T> LastFuture<S, T> {
+    pub(crate) fn new(stream: S) -> Self {
+        LastFuture { stream, last: None }
+    }
+}
+
+impl<S> Future for LastFuture<S, S::Item>
+where
+    S: Stream + Unpin + Sized,
+    S::Item: Copy,
+{
+    type Output = Option<S::Item>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
+
+        match next {
+            Some(new) => {
+                cx.waker().wake_by_ref();
+                *this.last = Some(new);
+                Poll::Pending
+            }
+            None => Poll::Ready(*this.last),
+        }
+    }
+}

src/stream/stream/le.rs

@@ -1,26 +1,29 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::partial_cmp::PartialCmpFuture;
 use crate::future::Future;
 use crate::prelude::*;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// Determines if the elements of this `Stream` are lexicographically
     /// less or equal to those of another.
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct LeFuture<L: Stream, R: Stream> {
+        #[pin]
         partial_cmp: PartialCmpFuture<L, R>,
     }
+}
 
 impl<L: Stream, R: Stream> LeFuture<L, R>
 where
     L::Item: PartialOrd<R::Item>,
 {
-    pin_utils::unsafe_pinned!(partial_cmp: PartialCmpFuture<L, R>);
-
     pub(super) fn new(l: L, r: R) -> Self {
         LeFuture {
             partial_cmp: l.partial_cmp(r),
@@ -36,8 +39,8 @@ where
 {
     type Output = bool;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let result = futures_core::ready!(self.as_mut().partial_cmp().poll(cx));
+        let result = futures_core::ready!(self.project().partial_cmp.poll(cx));
 
         match result {
             Some(Ordering::Less) | Some(Ordering::Equal) => Poll::Ready(true),

src/stream/stream/lt.rs

@@ -1,26 +1,29 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::partial_cmp::PartialCmpFuture;
 use crate::future::Future;
 use crate::prelude::*;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     // Determines if the elements of this `Stream` are lexicographically
     // less than those of another.
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct LtFuture<L: Stream, R: Stream> {
+        #[pin]
         partial_cmp: PartialCmpFuture<L, R>,
     }
+}
 
 impl<L: Stream, R: Stream> LtFuture<L, R>
 where
     L::Item: PartialOrd<R::Item>,
 {
-    pin_utils::unsafe_pinned!(partial_cmp: PartialCmpFuture<L, R>);
-
     pub(super) fn new(l: L, r: R) -> Self {
         LtFuture {
             partial_cmp: l.partial_cmp(r),
@@ -36,8 +39,8 @@ where
 {
     type Output = bool;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let result = futures_core::ready!(self.as_mut().partial_cmp().poll(cx));
+        let result = futures_core::ready!(self.project().partial_cmp.poll(cx));
 
         match result {
             Some(Ordering::Less) => Poll::Ready(true),

src/stream/stream/map.rs

@@ -1,22 +1,24 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct Map<S, F, T, B> {
+        #[pin]
         stream: S,
         f: F,
         __from: PhantomData<T>,
         __to: PhantomData<B>,
     }
+}
 
 impl<S, F, T, B> Map<S, F, T, B> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-
     pub(crate) fn new(stream: S, f: F) -> Self {
         Map {
             stream,
@@ -34,8 +36,9 @@ where
 {
     type Item = B;
 
-    fn poll_next(mut 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 next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let this = self.project();
-        Poll::Ready(next.map(self.as_mut().f()))
+        let next = futures_core::ready!(this.stream.poll_next(cx));
+        Poll::Ready(next.map(this.f))
     }
 }

src/stream/stream/merge.rs

@@ -2,19 +2,24 @@ use std::pin::Pin;
 use std::task::{Context, Poll};
 
 use futures_core::Stream;
+use pin_project_lite::pin_project;
 
+pin_project! {
     /// A stream that merges two other streams into a single stream.
     ///
     /// This stream is returned by [`Stream::merge`].
     ///
     /// [`Stream::merge`]: trait.Stream.html#method.merge
+    #[cfg(feature = "unstable")]
+    #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
     #[derive(Debug)]
     pub struct Merge<L, R> {
+        #[pin]
         left: L,
+        #[pin]
         right: R,
     }
-
+}
-impl<L, R> Unpin for Merge<L, R> {}
 
 impl<L, R> Merge<L, R> {
     pub(crate) fn new(left: L, right: R) -> Self {
@@ -24,19 +29,20 @@ impl<L, R> Merge<L, R> {
 
 impl<L, R, T> Stream for Merge<L, R>
 where
-    L: Stream<Item = T> + Unpin,
+    L: Stream<Item = T>,
-    R: Stream<Item = T> + Unpin,
+    R: Stream<Item = T>,
 {
     type Item = T;
 
-    fn poll_next(mut 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>> {
-        if let Poll::Ready(Some(item)) = Pin::new(&mut self.left).poll_next(cx) {
+        let this = self.project();
+        if let Poll::Ready(Some(item)) = this.left.poll_next(cx) {
             // The first stream made progress. The Merge needs to be polled
             // again to check the progress of the second stream.
             cx.waker().wake_by_ref();
             Poll::Ready(Some(item))
         } else {
-            Pin::new(&mut self.right).poll_next(cx)
+            this.right.poll_next(cx)
         }
     }
 }

src/stream/stream/min_by.rs

@@ -1,23 +1,24 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct MinByFuture<S, F, T> {
+        #[pin]
         stream: S,
         compare: F,
         min: Option<T>,
     }
+}
 
 impl<S, F, T> MinByFuture<S, F, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(compare: F);
-    pin_utils::unsafe_unpinned!(min: Option<T>);
-
     pub(super) fn new(stream: S, compare: F) -> Self {
         MinByFuture {
             stream,
@@ -35,22 +36,23 @@ where
 {
     type Output = Option<S::Item>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
 
         match next {
             Some(new) => {
                 cx.waker().wake_by_ref();
-                match self.as_mut().min().take() {
+                match this.min.take() {
-                    None => *self.as_mut().min() = Some(new),
+                    None => *this.min = Some(new),
-                    Some(old) => match (&mut self.as_mut().compare())(&new, &old) {
+                    Some(old) => match (this.compare)(&new, &old) {
-                        Ordering::Less => *self.as_mut().min() = Some(new),
+                        Ordering::Less => *this.min = Some(new),
-                        _ => *self.as_mut().min() = Some(old),
+                        _ => *this.min = Some(old),
                     },
                 }
                 Poll::Pending
             }
-            None => Poll::Ready(self.min),
+            None => Poll::Ready(*this.min),
         }
     }
 }

src/stream/stream/mod.rs

@@ -36,6 +36,7 @@ mod fuse;
 mod ge;
 mod gt;
 mod inspect;
+mod last;
 mod le;
 mod lt;
 mod map;
@@ -64,6 +65,7 @@ use fold::FoldFuture;
 use for_each::ForEachFuture;
 use ge::GeFuture;
 use gt::GtFuture;
+use last::LastFuture;
 use le::LeFuture;
 use lt::LtFuture;
 use min_by::MinByFuture;
@@ -89,32 +91,22 @@ pub use zip::Zip;
 use std::cmp::Ordering;
 use std::marker::PhantomData;
 
-use cfg_if::cfg_if;
+cfg_unstable! {
-
-use crate::utils::extension_trait;
-
-cfg_if! {
-    if #[cfg(feature = "docs")] {
-        use std::ops::{Deref, DerefMut};
-
-        use crate::task::{Context, Poll};
-    }
-}
-
-cfg_if! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
-        mod merge;
-
     use std::pin::Pin;
 
     use crate::future::Future;
     use crate::stream::FromStream;
 
     pub use merge::Merge;
-    }
+
+    mod merge;
 }
 
 extension_trait! {
+    use std::ops::{Deref, DerefMut};
+
+    use crate::task::{Context, Poll};
+
     #[doc = r#"
         An asynchronous stream of values.
 
@@ -456,6 +448,54 @@ extension_trait! {
             Inspect::new(self, f)
         }
 
+        #[doc = r#"
+            Returns the last element of the stream.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use std::collections::VecDeque;
+
+            use async_std::prelude::*;
+
+            let s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
+
+            let last  = s.last().await;
+            assert_eq!(last, Some(3));
+            #
+            # }) }
+            ```
+
+            An empty stream will return `None:
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use std::collections::VecDeque;
+
+            use async_std::prelude::*;
+
+            let s: VecDeque<usize> = vec![].into_iter().collect();
+
+            let last  = s.last().await;
+            assert_eq!(last, None);
+            #
+            # }) }
+            ```
+
+        "#]
+        fn last(
+            self,
+        ) -> impl Future<Output = Option<Self::Item>> [LastFuture<Self, Self::Item>]
+        where
+            Self: Sized,
+        {
+            LastFuture::new(self)
+        }
+
         #[doc = r#"
             Transforms this `Stream` into a "fused" `Stream` such that after the first time
             `poll` returns `Poll::Ready(None)`, all future calls to `poll` will also return
@@ -1226,7 +1266,7 @@ extension_trait! {
 
             [`stream`]: trait.Stream.html#tymethod.next
         "#]
-        #[cfg(any(feature = "unstable", feature = "docs"))]
+        #[cfg(feature = "unstable")]
         #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
         #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead (TODO)"]
         fn collect<'a, B>(
@@ -1265,7 +1305,7 @@ extension_trait! {
             # });
             ```
         "#]
-        #[cfg(any(feature = "unstable", feature = "docs"))]
+        #[cfg(feature = "unstable")]
         #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
         fn merge<U>(self, other: U) -> Merge<Self, U>
         where

src/stream/stream/partial_cmp.rs

@@ -1,29 +1,30 @@
 use std::cmp::Ordering;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use super::fuse::Fuse;
 use crate::future::Future;
 use crate::prelude::*;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     // Lexicographically compares the elements of this `Stream` with those
     // of another.
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct PartialCmpFuture<L: Stream, R: Stream> {
+        #[pin]
         l: Fuse<L>,
+        #[pin]
         r: Fuse<R>,
         l_cache: Option<L::Item>,
         r_cache: Option<R::Item>,
     }
+}
 
 impl<L: Stream, R: Stream> PartialCmpFuture<L, R> {
-    pin_utils::unsafe_pinned!(l: Fuse<L>);
-    pin_utils::unsafe_pinned!(r: Fuse<R>);
-    pin_utils::unsafe_unpinned!(l_cache: Option<L::Item>);
-    pin_utils::unsafe_unpinned!(r_cache: Option<R::Item>);
-
     pub(super) fn new(l: L, r: R) -> Self {
         PartialCmpFuture {
             l: l.fuse(),
@@ -42,12 +43,13 @@ where
 {
     type Output = Option<Ordering>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
         loop {
             // Short circuit logic
             // Stream that completes earliest can be considered Less, etc
-            let l_complete = self.l.done && self.as_mut().l_cache.is_none();
+            let l_complete = this.l.done && this.l_cache.is_none();
-            let r_complete = self.r.done && self.as_mut().r_cache.is_none();
+            let r_complete = this.r.done && this.r_cache.is_none();
 
             if l_complete && r_complete {
                 return Poll::Ready(Some(Ordering::Equal));
@@ -58,30 +60,30 @@ where
             }
 
             // Get next value if possible and necesary
-            if !self.l.done && self.as_mut().l_cache.is_none() {
+            if !this.l.done && this.l_cache.is_none() {
-                let l_next = futures_core::ready!(self.as_mut().l().poll_next(cx));
+                let l_next = futures_core::ready!(this.l.as_mut().poll_next(cx));
                 if let Some(item) = l_next {
-                    *self.as_mut().l_cache() = Some(item);
+                    *this.l_cache = Some(item);
                 }
             }
 
-            if !self.r.done && self.as_mut().r_cache.is_none() {
+            if !this.r.done && this.r_cache.is_none() {
-                let r_next = futures_core::ready!(self.as_mut().r().poll_next(cx));
+                let r_next = futures_core::ready!(this.r.as_mut().poll_next(cx));
                 if let Some(item) = r_next {
-                    *self.as_mut().r_cache() = Some(item);
+                    *this.r_cache = Some(item);
                 }
             }
 
             // Compare if both values are available.
-            if self.as_mut().l_cache.is_some() && self.as_mut().r_cache.is_some() {
+            if this.l_cache.is_some() && this.r_cache.is_some() {
-                let l_value = self.as_mut().l_cache().take().unwrap();
+                let l_value = this.l_cache.as_mut().take().unwrap();
-                let r_value = self.as_mut().r_cache().take().unwrap();
+                let r_value = this.r_cache.as_mut().take().unwrap();
                 let result = l_value.partial_cmp(&r_value);
 
                 if let Some(Ordering::Equal) = result {
                     // Reset cache to prepare for next comparison
-                    *self.as_mut().l_cache() = None;
+                    *this.l_cache = None;
-                    *self.as_mut().r_cache() = None;
+                    *this.r_cache = None;
                 } else {
                     // Return non equal value
                     return Poll::Ready(result);

src/stream/stream/scan.rs

@@ -1,14 +1,19 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream to maintain state while polling another stream.
     #[derive(Debug)]
     pub struct Scan<S, St, F> {
+        #[pin]
         stream: S,
         state_f: (St, F),
     }
+}
 
 impl<S, St, F> Scan<S, St, F> {
     pub(crate) fn new(stream: S, initial_state: St, f: F) -> Self {
@@ -17,13 +22,8 @@ impl<S, St, F> Scan<S, St, F> {
             state_f: (initial_state, f),
         }
     }
-
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(state_f: (St, F));
 }
 
-impl<S: Unpin, St, F> Unpin for Scan<S, St, F> {}
-
 impl<S, St, F, B> Stream for Scan<S, St, F>
 where
     S: Stream,
@@ -31,11 +31,12 @@ where
 {
     type Item = B;
 
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>> {
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>> {
-        let poll_result = self.as_mut().stream().poll_next(cx);
+        let mut this = self.project();
+        let poll_result = this.stream.as_mut().poll_next(cx);
         poll_result.map(|item| {
             item.and_then(|item| {
-                let (state, f) = self.as_mut().state_f();
+                let (state, f) = this.state_f;
                 f(state, item)
             })
         })

src/stream/stream/skip.rs

@@ -1,19 +1,21 @@
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 
+pin_project! {
     /// A stream to skip first n elements of another stream.
     #[derive(Debug)]
     pub struct Skip<S> {
+        #[pin]
         stream: S,
         n: usize,
     }
+}
 
 impl<S> Skip<S> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(n: usize);
-
     pub(crate) fn new(stream: S, n: usize) -> Self {
         Skip { stream, n }
     }
@@ -25,14 +27,15 @@ where
 {
     type Item = S::Item;
 
-    fn poll_next(mut 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();
         loop {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match next {
-                Some(v) => match self.n {
+                Some(v) => match *this.n {
                     0 => return Poll::Ready(Some(v)),
-                    _ => *self.as_mut().n() -= 1,
+                    _ => *this.n -= 1,
                 },
                 None => return Poll::Ready(None),
             }

src/stream/stream/skip_while.rs

@@ -1,21 +1,23 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream to skip elements of another stream based on a predicate.
     #[derive(Debug)]
     pub struct SkipWhile<S, P, T> {
+        #[pin]
         stream: S,
         predicate: Option<P>,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, P, T> SkipWhile<S, P, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(predicate: Option<P>);
-
     pub(crate) fn new(stream: S, predicate: P) -> Self {
         SkipWhile {
             stream,
@@ -32,15 +34,16 @@ where
 {
     type Item = S::Item;
 
-    fn poll_next(mut 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();
         loop {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match next {
-                Some(v) => match self.as_mut().predicate() {
+                Some(v) => match this.predicate {
                     Some(p) => {
                         if !p(&v) {
-                            *self.as_mut().predicate() = None;
+                            *this.predicate = None;
                             return Poll::Ready(Some(v));
                         }
                     }

src/stream/stream/step_by.rs

@@ -1,21 +1,22 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream that steps a given amount of elements of another stream.
     #[derive(Debug)]
     pub struct StepBy<S> {
+        #[pin]
         stream: S,
         step: usize,
         i: usize,
     }
+}
 
 impl<S> StepBy<S> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(step: usize);
-    pin_utils::unsafe_unpinned!(i: usize);
-
     pub(crate) fn new(stream: S, step: usize) -> Self {
         StepBy {
             stream,
@@ -31,17 +32,18 @@ where
 {
     type Item = S::Item;
 
-    fn poll_next(mut 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();
         loop {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match next {
-                Some(v) => match self.i {
+                Some(v) => match this.i {
                     0 => {
-                        *self.as_mut().i() = self.step;
+                        *this.i = *this.step;
                         return Poll::Ready(Some(v));
                     }
-                    _ => *self.as_mut().i() -= 1,
+                    _ => *this.i -= 1,
                 },
                 None => return Poll::Ready(None),
             }

src/stream/stream/take.rs

@@ -1,33 +1,32 @@
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream that yields the first `n` items of another stream.
     #[derive(Clone, Debug)]
     pub struct Take<S> {
+        #[pin]
         pub(crate) stream: S,
         pub(crate) remaining: usize,
     }
-
-impl<S: Unpin> Unpin for Take<S> {}
-
-impl<S: Stream> Take<S> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(remaining: usize);
 }
 
 impl<S: Stream> Stream for Take<S> {
     type Item = S::Item;
 
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
-        if self.remaining == 0 {
+        let this = self.project();
+        if *this.remaining == 0 {
             Poll::Ready(None)
         } else {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.poll_next(cx));
             match next {
-                Some(_) => *self.as_mut().remaining() -= 1,
+                Some(_) => *this.remaining -= 1,
-                None => *self.as_mut().remaining() = 0,
+                None => *this.remaining = 0,
             }
             Poll::Ready(next)
         }

src/stream/stream/take_while.rs

@@ -1,21 +1,23 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// A stream that yields elements based on a predicate.
     #[derive(Debug)]
     pub struct TakeWhile<S, P, T> {
+        #[pin]
         stream: S,
         predicate: P,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, P, T> TakeWhile<S, P, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(predicate: P);
-
     pub(super) fn new(stream: S, predicate: P) -> Self {
         TakeWhile {
             stream,
@@ -32,11 +34,12 @@ where
 {
     type Item = S::Item;
 
-    fn poll_next(mut 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 next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        let this = self.project();
+        let next = futures_core::ready!(this.stream.poll_next(cx));
 
         match next {
-            Some(v) if (self.as_mut().predicate())(&v) => Poll::Ready(Some(v)),
+            Some(v) if (this.predicate)(&v) => Poll::Ready(Some(v)),
             Some(_) => {
                 cx.waker().wake_by_ref();
                 Poll::Pending

src/stream/stream/try_fold.rs

@@ -1,24 +1,25 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct TryFoldFuture<S, F, T> {
+        #[pin]
         stream: S,
         f: F,
         acc: Option<T>,
         __t: PhantomData<T>,
     }
+}
 
 impl<S, F, T> TryFoldFuture<S, F, T> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-    pin_utils::unsafe_unpinned!(acc: Option<T>);
-
     pub(super) fn new(stream: S, init: T, f: F) -> Self {
         TryFoldFuture {
             stream,
@@ -36,23 +37,22 @@ where
 {
     type Output = Result<T, E>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
         loop {
-            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match next {
                 Some(v) => {
-                    let old = self.as_mut().acc().take().unwrap();
+                    let old = this.acc.take().unwrap();
-                    let new = (self.as_mut().f())(old, v);
+                    let new = (this.f)(old, v);
 
                     match new {
-                        Ok(o) => {
+                        Ok(o) => *this.acc = Some(o),
-                            *self.as_mut().acc() = Some(o);
-                        }
                         Err(e) => return Poll::Ready(Err(e)),
                     }
                 }
-                None => return Poll::Ready(Ok(self.as_mut().acc().take().unwrap())),
+                None => return Poll::Ready(Ok(this.acc.take().unwrap())),
             }
         }
     }

src/stream/stream/try_for_each.rs

@@ -1,23 +1,25 @@
 use std::marker::PhantomData;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::future::Future;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     #[doc(hidden)]
     #[allow(missing_debug_implementations)]
     pub struct TryForEeachFuture<S, F, T, R> {
+        #[pin]
         stream: S,
         f: F,
         __from: PhantomData<T>,
         __to: PhantomData<R>,
     }
+}
 
 impl<S, F, T, R> TryForEeachFuture<S, F, T, R> {
-    pin_utils::unsafe_pinned!(stream: S);
-    pin_utils::unsafe_unpinned!(f: F);
-
     pub(crate) fn new(stream: S, f: F) -> Self {
         TryForEeachFuture {
             stream,
@@ -36,14 +38,15 @@ where
 {
     type Output = Result<(), E>;
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
         loop {
-            let item = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+            let item = futures_core::ready!(this.stream.as_mut().poll_next(cx));
 
             match item {
                 None => return Poll::Ready(Ok(())),
                 Some(v) => {
-                    let res = (self.as_mut().f())(v);
+                    let res = (this.f)(v);
                     if let Err(e) = res {
                         return Poll::Ready(Err(e));
                     }

src/stream/stream/zip.rs

@@ -1,15 +1,21 @@
 use std::fmt;
 use std::pin::Pin;
 
+use pin_project_lite::pin_project;
+
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
+pin_project! {
     /// An iterator that iterates two other iterators simultaneously.
     pub struct Zip<A: Stream, B> {
         item_slot: Option<A::Item>,
+        #[pin]
         first: A,
+        #[pin]
         second: B,
     }
+}
 
 impl<A: Stream + fmt::Debug, B: fmt::Debug> fmt::Debug for Zip<A, B> {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -20,8 +26,6 @@ impl<A: Stream + fmt::Debug, B: fmt::Debug> fmt::Debug for Zip<A, B> {
     }
 }
 
-impl<A: Stream + Unpin, B: Unpin> Unpin for Zip<A, B> {}
-
 impl<A: Stream, B> Zip<A, B> {
     pub(crate) fn new(first: A, second: B) -> Self {
         Zip {
@@ -30,25 +34,22 @@ impl<A: Stream, B> Zip<A, B> {
             second,
         }
     }
-
-    pin_utils::unsafe_unpinned!(item_slot: Option<A::Item>);
-    pin_utils::unsafe_pinned!(first: A);
-    pin_utils::unsafe_pinned!(second: B);
 }
 
 impl<A: Stream, B: Stream> Stream for Zip<A, B> {
     type Item = (A::Item, B::Item);
 
-    fn poll_next(mut 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>> {
-        if self.as_mut().item_slot().is_none() {
+        let this = self.project();
-            match self.as_mut().first().poll_next(cx) {
+        if this.item_slot.is_none() {
+            match this.first.poll_next(cx) {
                 Poll::Pending => return Poll::Pending,
                 Poll::Ready(None) => return Poll::Ready(None),
-                Poll::Ready(Some(item)) => *self.as_mut().item_slot() = Some(item),
+                Poll::Ready(Some(item)) => *this.item_slot = Some(item),
             }
         }
-        let second_item = futures_core::ready!(self.as_mut().second().poll_next(cx));
+        let second_item = futures_core::ready!(this.second.poll_next(cx));
-        let first_item = self.as_mut().item_slot().take().unwrap();
+        let first_item = this.item_slot.take().unwrap();
         Poll::Ready(second_item.map(|second_item| (first_item, second_item)))
     }
 }

src/stream/sum.rs

@@ -11,8 +11,8 @@ use crate::stream::Stream;
 /// [`sum`]: trait.Sum.html#tymethod.sum
 /// [`FromStream`]: trait.FromStream.html
 /// [`Stream::sum`]: trait.Stream.html#method.sum
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-#[cfg(any(feature = "unstable", feature = "docs"))]
 pub trait Sum<A = Self>: Sized {
     /// Method which takes a stream and generates `Self` from the elements by
     /// "summing up" the items.

src/sync/barrier.rs

@@ -8,7 +8,7 @@ use crate::sync::Mutex;
 /// # Examples
 ///
 /// ```
-/// # fn main() { async_std::task::block_on(async {
+/// # async_std::task::block_on(async {
 /// #
 /// use async_std::sync::{Arc, Barrier};
 /// use async_std::task;
@@ -30,8 +30,8 @@ use crate::sync::Mutex;
 ///     handle.await;
 /// }
 /// # });
-/// # }
 /// ```
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[derive(Debug)]
 pub struct Barrier {
@@ -61,6 +61,7 @@ struct BarrierState {
 /// let barrier = Barrier::new(1);
 /// let barrier_wait_result = barrier.wait();
 /// ```
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[derive(Debug, Clone)]
 pub struct BarrierWaitResult(bool);
@@ -118,7 +119,7 @@ impl Barrier {
     /// # Examples
     ///
     /// ```
-    /// # fn main() { async_std::task::block_on(async {
+    /// # async_std::task::block_on(async {
     /// #
     /// use async_std::sync::{Arc, Barrier};
     /// use async_std::task;
@@ -140,7 +141,6 @@ impl Barrier {
     ///     handle.await;
     /// }
     /// # });
-    /// # }
     /// ```
     pub async fn wait(&self) -> BarrierWaitResult {
         let mut lock = self.state.lock().await;
@@ -188,7 +188,7 @@ impl BarrierWaitResult {
     /// # Examples
     ///
     /// ```
-    /// # fn main() { async_std::task::block_on(async {
+    /// # async_std::task::block_on(async {
     /// #
     /// use async_std::sync::Barrier;
     ///
@@ -196,7 +196,6 @@ impl BarrierWaitResult {
     /// let barrier_wait_result = barrier.wait().await;
     /// println!("{:?}", barrier_wait_result.is_leader());
     /// # });
-    /// # }
     /// ```
     pub fn is_leader(&self) -> bool {
         self.0

src/sync/mod.rs

@@ -32,14 +32,16 @@
 #[doc(inline)]
 pub use std::sync::{Arc, Weak};
 
-#[cfg(any(feature = "unstable", feature = "docs"))]
-pub use barrier::{Barrier, BarrierWaitResult};
-
 pub use mutex::{Mutex, MutexGuard};
 pub use rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
-#[cfg(any(feature = "unstable", feature = "docs"))]
-#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
-mod barrier;
 mod mutex;
 mod rwlock;
+
+cfg_unstable! {
+    pub use barrier::{Barrier, BarrierWaitResult};
+    pub use channel::{channel, Sender, Receiver};
+
+    mod barrier;
+    mod channel;
+}

src/task/block_on.rs

@@ -1,10 +1,13 @@
-use std::cell::UnsafeCell;
+use std::cell::{Cell, UnsafeCell};
 use std::mem::{self, ManuallyDrop};
 use std::panic::{self, AssertUnwindSafe, UnwindSafe};
 use std::pin::Pin;
 use std::sync::Arc;
 use std::task::{RawWaker, RawWakerVTable};
-use std::thread::{self, Thread};
+use std::thread;
+
+use crossbeam_utils::sync::Parker;
+use pin_project_lite::pin_project;
 
 use super::task;
 use super::task_local;
@@ -98,19 +101,18 @@ where
     }
 }
 
+pin_project! {
     struct CatchUnwindFuture<F> {
+        #[pin]
         future: F,
     }
-
-impl<F> CatchUnwindFuture<F> {
-    pin_utils::unsafe_pinned!(future: F);
 }
 
 impl<F: Future + UnwindSafe> Future for CatchUnwindFuture<F> {
     type Output = thread::Result<F::Output>;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        panic::catch_unwind(AssertUnwindSafe(|| self.future().poll(cx)))?.map(Ok)
+        panic::catch_unwind(AssertUnwindSafe(|| self.project().future.poll(cx)))?.map(Ok)
     }
 }
 
@@ -119,13 +121,21 @@ where
     F: Future<Output = T>,
 {
     thread_local! {
-        static ARC_THREAD: Arc<Thread> = Arc::new(thread::current());
+        // May hold a pre-allocated parker that can be reused for efficiency.
+        //
+        // Note that each invocation of `block` needs its own parker. In particular, if `block`
+        // recursively calls itself, we must make sure that each recursive call uses a distinct
+        // parker instance.
+        static CACHE: Cell<Option<Arc<Parker>>> = Cell::new(None);
     }
 
     pin_utils::pin_mut!(f);
 
-    ARC_THREAD.with(|arc_thread: &Arc<Thread>| {
+    CACHE.with(|cache| {
-        let ptr = (&**arc_thread as *const Thread) as *const ();
+        // Reuse a cached parker or create a new one for this invocation of `block`.
+        let arc_parker: Arc<Parker> = cache.take().unwrap_or_else(|| Arc::new(Parker::new()));
+
+        let ptr = (&*arc_parker as *const Parker) as *const ();
         let vt = vtable();
 
         let waker = unsafe { ManuallyDrop::new(Waker::from_raw(RawWaker::new(ptr, vt))) };
@@ -133,32 +143,34 @@ where
 
         loop {
             if let Poll::Ready(t) = f.as_mut().poll(cx) {
+                // Save the parker for the next invocation of `block`.
+                cache.set(Some(arc_parker));
                 return t;
             }
-            thread::park();
+            arc_parker.park();
         }
     })
 }
 
 fn vtable() -> &'static RawWakerVTable {
     unsafe fn clone_raw(ptr: *const ()) -> RawWaker {
-        let arc = ManuallyDrop::new(Arc::from_raw(ptr as *const Thread));
+        let arc = ManuallyDrop::new(Arc::from_raw(ptr as *const Parker));
         mem::forget(arc.clone());
         RawWaker::new(ptr, vtable())
     }
 
     unsafe fn wake_raw(ptr: *const ()) {
-        let arc = Arc::from_raw(ptr as *const Thread);
+        let arc = Arc::from_raw(ptr as *const Parker);
-        arc.unpark();
+        arc.unparker().unpark();
     }
 
     unsafe fn wake_by_ref_raw(ptr: *const ()) {
-        let arc = ManuallyDrop::new(Arc::from_raw(ptr as *const Thread));
+        let arc = ManuallyDrop::new(Arc::from_raw(ptr as *const Parker));
-        arc.unpark();
+        arc.unparker().unpark();
     }
 
     unsafe fn drop_raw(ptr: *const ()) {
-        drop(Arc::from_raw(ptr as *const Thread))
+        drop(Arc::from_raw(ptr as *const Parker))
     }
 
     &RawWakerVTable::new(clone_raw, wake_raw, wake_by_ref_raw, drop_raw)

src/task/mod.rs

@@ -1,26 +1,122 @@
-//! Asynchronous tasks.
+//! Types and traits for working with asynchronous tasks.
 //!
 //! This module is similar to [`std::thread`], except it uses asynchronous tasks in place of
 //! threads.
 //!
-//! [`std::thread`]: https://doc.rust-lang.org/std/thread/index.html
+//! [`std::thread`]: https://doc.rust-lang.org/std/thread
 //!
-//! # Examples
+//! ## The task model
 //!
-//! Spawn a task and await its result:
+//! An executing asynchronous Rust program consists of a collection of native OS threads, on top of
+//! which multiple stackless coroutines are multiplexed. We refer to these as "tasks".  Tasks can
+//! be named, and provide some built-in support for synchronization.
 //!
-//! ```
+//! Communication between tasks can be done through channels, Rust's message-passing types, along
-//! # async_std::task::block_on(async {
+//! with [other forms of tasks synchronization](../sync/index.html) and shared-memory data
-//! #
+//! structures. In particular, types that are guaranteed to be threadsafe are easily shared between
+//! tasks using the atomically-reference-counted container, [`Arc`].
+//!
+//! Fatal logic errors in Rust cause *thread panic*, during which a thread will unwind the stack,
+//! running destructors and freeing owned resources. If a panic occurs inside a task, there is no
+//! meaningful way of recovering, so the panic will propagate through any thread boundaries all the
+//! way to the root task. This is also known as a "panic = abort" model.
+//!
+//! ## Spawning a task
+//!
+//! A new task can be spawned using the [`task::spawn`][`spawn`] function:
+//!
+//! ```no_run
 //! use async_std::task;
 //!
-//! let handle = task::spawn(async {
+//! task::spawn(async {
-//!     1 + 2
+//!     // some work here
 //! });
-//! assert_eq!(handle.await, 3);
+//! ```
+//!
+//! In this example, the spawned task is "detached" from the current task. This means that it can
+//! outlive its parent (the task that spawned it), unless this parent is the root task.
+//!
+//! The root task can also wait on the completion of the child task; a call to [`spawn`] produces a
+//! [`JoinHandle`], which provides implements `Future` and can be `await`ed:
+//!
+//! ```
+//! use async_std::task;
+//!
+//! # async_std::task::block_on(async {
+//! #
+//! let child = task::spawn(async {
+//!     // some work here
+//! });
+//! // some work here
+//! let res = child.await;
 //! #
 //! # })
 //! ```
+//!
+//! The `await` operator returns the final value produced by the child task.
+//!
+//! ## Configuring tasks
+//!
+//! A new task can be configured before it is spawned via the [`Builder`] type,
+//! which currently allows you to set the name and stack size for the child task:
+//!
+//! ```
+//! # #![allow(unused_must_use)]
+//! use async_std::task;
+//!
+//! # async_std::task::block_on(async {
+//! #
+//! task::Builder::new().name("child1".to_string()).spawn(async {
+//!     println!("Hello, world!");
+//! });
+//! #
+//! # })
+//! ```
+//!
+//! ## The `Task` type
+//!
+//! Tasks are represented via the [`Task`] type, which you can get in one of
+//! two ways:
+//!
+//! * By spawning a new task, e.g., using the [`task::spawn`][`spawn`]
+//!   function, and calling [`task`][`JoinHandle::task`] on the [`JoinHandle`].
+//! * By requesting the current task, using the [`task::current`] function.
+//!
+//! ## Task-local storage
+//!
+//! This module also provides an implementation of task-local storage for Rust
+//! programs. Task-local storage is a method of storing data into a global
+//! variable that each task in the program will have its own copy of.
+//! Tasks do not share this data, so accesses do not need to be synchronized.
+//!
+//! A task-local key owns the value it contains and will destroy the value when the
+//! task exits. It is created with the [`task_local!`] macro and can contain any
+//! value that is `'static` (no borrowed pointers). It provides an accessor function,
+//! [`with`], that yields a shared reference to the value to the specified
+//! closure. Task-local keys allow only shared access to values, as there would be no
+//! way to guarantee uniqueness if mutable borrows were allowed.
+//!
+//! ## Naming tasks
+//!
+//! Tasks are able to have associated names for identification purposes. By default, spawned
+//! tasks are unnamed. To specify a name for a task, build the task with [`Builder`] and pass
+//! the desired task name to [`Builder::name`]. To retrieve the task name from within the
+//! task, use [`Task::name`].
+//!
+//! [`Arc`]: ../gsync/struct.Arc.html
+//! [`spawn`]: fn.spawn.html
+//! [`JoinHandle`]: struct.JoinHandle.html
+//! [`JoinHandle::task`]: struct.JoinHandle.html#method.task
+//! [`join`]: struct.JoinHandle.html#method.join
+//! [`panic!`]: https://doc.rust-lang.org/std/macro.panic.html
+//! [`Builder`]: struct.Builder.html
+//! [`Builder::stack_size`]: struct.Builder.html#method.stack_size
+//! [`Builder::name`]: struct.Builder.html#method.name
+//! [`task::current`]: fn.current.html
+//! [`Task`]: struct.Thread.html
+//! [`Task::name`]: struct.Task.html#method.name
+//! [`task_local!`]: ../macro.task_local.html
+//! [`with`]: struct.LocalKey.html#method.with
 
 #[doc(inline)]
 pub use std::task::{Context, Poll, Waker};
@@ -47,12 +143,10 @@ mod worker;
 
 pub(crate) mod blocking;
 
-cfg_if::cfg_if! {
+cfg_unstable! {
-    if #[cfg(any(feature = "unstable", feature = "docs"))] {
     mod yield_now;
     pub use yield_now::yield_now;
 }
-}
 
 /// Spawns a blocking task.
 ///
@@ -82,7 +176,7 @@ cfg_if::cfg_if! {
 /// ```
 // Once this function stabilizes we should merge `blocking::spawn` into this so
 // all code in our crate uses `task::blocking` too.
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[inline]
 pub fn spawn_blocking<F, R>(f: F) -> task::JoinHandle<R>

src/task/yield_now.rs

@@ -18,15 +18,15 @@ use std::pin::Pin;
 /// Basic usage:
 ///
 /// ```
-/// # fn main() { async_std::task::block_on(async {
+/// # async_std::task::block_on(async {
 /// #
 /// use async_std::task;
 ///
 /// task::yield_now().await;
 /// #
-/// # }) }
+/// # })
 /// ```
-#[cfg(any(feature = "unstable", feature = "docs"))]
+#[cfg(feature = "unstable")]
 #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
 #[inline]
 pub async fn yield_now() {

src/utils.rs

@@ -20,6 +20,64 @@ pub fn abort_on_panic<T>(f: impl FnOnce() -> T) -> T {
     t
 }
 
+/// Declares unstable items.
+#[doc(hidden)]
+macro_rules! cfg_unstable {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "unstable")]
+            #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+            $item
+        )*
+    }
+}
+
+/// Declares Unix-specific items.
+#[doc(hidden)]
+macro_rules! cfg_unix {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(unix, feature = "docs"))]
+            #[cfg_attr(feature = "docs", doc(cfg(unix)))]
+            $item
+        )*
+    }
+}
+
+/// Declares Windows-specific items.
+#[doc(hidden)]
+macro_rules! cfg_windows {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(windows, feature = "docs"))]
+            #[cfg_attr(feature = "docs", doc(cfg(windows)))]
+            $item
+        )*
+    }
+}
+
+/// Declares items when the "docs" feature is enabled.
+#[doc(hidden)]
+macro_rules! cfg_docs {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "docs")]
+            $item
+        )*
+    }
+}
+
+/// Declares items when the "docs" feature is disabled.
+#[doc(hidden)]
+macro_rules! cfg_not_docs {
+    ($($item:item)*) => {
+        $(
+            #[cfg(not(feature = "docs"))]
+            $item
+        )*
+    }
+}
+
 /// Defines an extension trait for a base trait.
 ///
 /// In generated docs, the base trait will contain methods from the extension trait. In actual
@@ -29,7 +87,6 @@ pub fn abort_on_panic<T>(f: impl FnOnce() -> T) -> T {
 /// Inside invocations of this macro, we write a definitions that looks similar to the final
 /// rendered docs, and the macro then generates all the boilerplate for us.
 #[doc(hidden)]
-#[macro_export]
 macro_rules! extension_trait {
     (
         // Interesting patterns:
@@ -113,6 +170,12 @@ macro_rules! extension_trait {
     // Handle the end of the token list.
     (@doc ($($head:tt)*)) => { $($head)* };
     (@ext ($($head:tt)*)) => { $($head)* };
-}
 
-pub use crate::extension_trait;
+    // Parse imports at the beginning of the macro.
+    ($import:item $($tail:tt)*) => {
+        #[cfg(feature = "docs")]
+        $import
+
+        extension_trait!($($tail)*);
+    };
+}

tests/buf_writer.rs

@@ -48,13 +48,13 @@ fn test_buffered_writer() {
 }
 
 #[test]
-fn test_buffered_writer_inner_into_inner_does_not_flush() {
+fn test_buffered_writer_inner_into_inner_flushes() {
     task::block_on(async {
         let mut w = BufWriter::with_capacity(3, Vec::new());
         w.write(&[0, 1]).await.unwrap();
         assert_eq!(*w.get_ref(), []);
         let w = w.into_inner().await.unwrap();
-        assert_eq!(w, []);
+        assert_eq!(w, [0, 1]);
     })
 }
 

tests/channel.rs

@@ -0,0 +1,350 @@
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Arc;
+use std::time::Duration;
+
+use async_std::sync::channel;
+use async_std::task;
+use rand::{thread_rng, Rng};
+
+fn ms(ms: u64) -> Duration {
+    Duration::from_millis(ms)
+}
+
+#[test]
+fn smoke() {
+    task::block_on(async {
+        let (s, r) = channel(1);
+
+        s.send(7).await;
+        assert_eq!(r.recv().await, Some(7));
+
+        s.send(8).await;
+        assert_eq!(r.recv().await, Some(8));
+
+        drop(s);
+        assert_eq!(r.recv().await, None);
+    })
+}
+
+#[test]
+fn capacity() {
+    for i in 1..10 {
+        let (s, r) = channel::<()>(i);
+        assert_eq!(s.capacity(), i);
+        assert_eq!(r.capacity(), i);
+    }
+}
+
+#[test]
+fn len_empty_full() {
+    task::block_on(async {
+        let (s, r) = channel(2);
+
+        assert_eq!(s.len(), 0);
+        assert_eq!(s.is_empty(), true);
+        assert_eq!(s.is_full(), false);
+        assert_eq!(r.len(), 0);
+        assert_eq!(r.is_empty(), true);
+        assert_eq!(r.is_full(), false);
+
+        s.send(()).await;
+
+        assert_eq!(s.len(), 1);
+        assert_eq!(s.is_empty(), false);
+        assert_eq!(s.is_full(), false);
+        assert_eq!(r.len(), 1);
+        assert_eq!(r.is_empty(), false);
+        assert_eq!(r.is_full(), false);
+
+        s.send(()).await;
+
+        assert_eq!(s.len(), 2);
+        assert_eq!(s.is_empty(), false);
+        assert_eq!(s.is_full(), true);
+        assert_eq!(r.len(), 2);
+        assert_eq!(r.is_empty(), false);
+        assert_eq!(r.is_full(), true);
+
+        r.recv().await;
+
+        assert_eq!(s.len(), 1);
+        assert_eq!(s.is_empty(), false);
+        assert_eq!(s.is_full(), false);
+        assert_eq!(r.len(), 1);
+        assert_eq!(r.is_empty(), false);
+        assert_eq!(r.is_full(), false);
+    })
+}
+
+#[test]
+fn recv() {
+    task::block_on(async {
+        let (s, r) = channel(100);
+
+        task::spawn(async move {
+            assert_eq!(r.recv().await, Some(7));
+            task::sleep(ms(1000)).await;
+            assert_eq!(r.recv().await, Some(8));
+            task::sleep(ms(1000)).await;
+            assert_eq!(r.recv().await, Some(9));
+            assert_eq!(r.recv().await, None);
+        });
+
+        task::sleep(ms(1500)).await;
+        s.send(7).await;
+        s.send(8).await;
+        s.send(9).await;
+    })
+}
+
+#[test]
+fn send() {
+    task::block_on(async {
+        let (s, r) = channel(1);
+
+        task::spawn(async move {
+            s.send(7).await;
+            task::sleep(ms(1000)).await;
+            s.send(8).await;
+            task::sleep(ms(1000)).await;
+            s.send(9).await;
+            task::sleep(ms(1000)).await;
+            s.send(10).await;
+        });
+
+        task::sleep(ms(1500)).await;
+        assert_eq!(r.recv().await, Some(7));
+        assert_eq!(r.recv().await, Some(8));
+        assert_eq!(r.recv().await, Some(9));
+    })
+}
+
+#[test]
+fn recv_after_disconnect() {
+    task::block_on(async {
+        let (s, r) = channel(100);
+
+        s.send(1).await;
+        s.send(2).await;
+        s.send(3).await;
+
+        drop(s);
+
+        assert_eq!(r.recv().await, Some(1));
+        assert_eq!(r.recv().await, Some(2));
+        assert_eq!(r.recv().await, Some(3));
+        assert_eq!(r.recv().await, None);
+    })
+}
+
+#[test]
+fn len() {
+    const COUNT: usize = 25_000;
+    const CAP: usize = 1000;
+
+    task::block_on(async {
+        let (s, r) = channel(CAP);
+
+        assert_eq!(s.len(), 0);
+        assert_eq!(r.len(), 0);
+
+        for _ in 0..CAP / 10 {
+            for i in 0..50 {
+                s.send(i).await;
+                assert_eq!(s.len(), i + 1);
+            }
+
+            for i in 0..50 {
+                r.recv().await;
+                assert_eq!(r.len(), 50 - i - 1);
+            }
+        }
+
+        assert_eq!(s.len(), 0);
+        assert_eq!(r.len(), 0);
+
+        for i in 0..CAP {
+            s.send(i).await;
+            assert_eq!(s.len(), i + 1);
+        }
+
+        for _ in 0..CAP {
+            r.recv().await.unwrap();
+        }
+
+        assert_eq!(s.len(), 0);
+        assert_eq!(r.len(), 0);
+
+        let child = task::spawn({
+            let r = r.clone();
+            async move {
+                for i in 0..COUNT {
+                    assert_eq!(r.recv().await, Some(i));
+                    let len = r.len();
+                    assert!(len <= CAP);
+                }
+            }
+        });
+
+        for i in 0..COUNT {
+            s.send(i).await;
+            let len = s.len();
+            assert!(len <= CAP);
+        }
+
+        child.await;
+
+        assert_eq!(s.len(), 0);
+        assert_eq!(r.len(), 0);
+    })
+}
+
+#[test]
+fn disconnect_wakes_receiver() {
+    task::block_on(async {
+        let (s, r) = channel::<()>(1);
+
+        let child = task::spawn(async move {
+            assert_eq!(r.recv().await, None);
+        });
+
+        task::sleep(ms(1000)).await;
+        drop(s);
+
+        child.await;
+    })
+}
+
+#[test]
+fn spsc() {
+    const COUNT: usize = 100_000;
+
+    task::block_on(async {
+        let (s, r) = channel(3);
+
+        let child = task::spawn(async move {
+            for i in 0..COUNT {
+                assert_eq!(r.recv().await, Some(i));
+            }
+            assert_eq!(r.recv().await, None);
+        });
+
+        for i in 0..COUNT {
+            s.send(i).await;
+        }
+        drop(s);
+
+        child.await;
+    })
+}
+
+#[test]
+fn mpmc() {
+    const COUNT: usize = 25_000;
+    const TASKS: usize = 4;
+
+    task::block_on(async {
+        let (s, r) = channel::<usize>(3);
+        let v = (0..COUNT).map(|_| AtomicUsize::new(0)).collect::<Vec<_>>();
+        let v = Arc::new(v);
+
+        let mut tasks = Vec::new();
+
+        for _ in 0..TASKS {
+            let r = r.clone();
+            let v = v.clone();
+            tasks.push(task::spawn(async move {
+                for _ in 0..COUNT {
+                    let n = r.recv().await.unwrap();
+                    v[n].fetch_add(1, Ordering::SeqCst);
+                }
+            }));
+        }
+
+        for _ in 0..TASKS {
+            let s = s.clone();
+            tasks.push(task::spawn(async move {
+                for i in 0..COUNT {
+                    s.send(i).await;
+                }
+            }));
+        }
+
+        for t in tasks {
+            t.await;
+        }
+
+        for c in v.iter() {
+            assert_eq!(c.load(Ordering::SeqCst), TASKS);
+        }
+    });
+}
+
+#[test]
+fn oneshot() {
+    const COUNT: usize = 10_000;
+
+    task::block_on(async {
+        for _ in 0..COUNT {
+            let (s, r) = channel(1);
+
+            let c1 = task::spawn(async move { r.recv().await.unwrap() });
+            let c2 = task::spawn(async move { s.send(0).await });
+
+            c1.await;
+            c2.await;
+        }
+    })
+}
+
+#[test]
+fn drops() {
+    const RUNS: usize = 100;
+
+    static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+    #[derive(Debug, PartialEq)]
+    struct DropCounter;
+
+    impl Drop for DropCounter {
+        fn drop(&mut self) {
+            DROPS.fetch_add(1, Ordering::SeqCst);
+        }
+    }
+
+    let mut rng = thread_rng();
+
+    for _ in 0..RUNS {
+        task::block_on(async {
+            let steps = rng.gen_range(0, 10_000);
+            let additional = rng.gen_range(0, 50);
+
+            DROPS.store(0, Ordering::SeqCst);
+            let (s, r) = channel::<DropCounter>(50);
+
+            let child = task::spawn({
+                let r = r.clone();
+                async move {
+                    for _ in 0..steps {
+                        r.recv().await.unwrap();
+                    }
+                }
+            });
+
+            for _ in 0..steps {
+                s.send(DropCounter).await;
+            }
+
+            child.await;
+
+            for _ in 0..additional {
+                s.send(DropCounter).await;
+            }
+
+            assert_eq!(DROPS.load(Ordering::SeqCst), steps);
+            drop(s);
+            drop(r);
+            assert_eq!(DROPS.load(Ordering::SeqCst), steps + additional);
+        })
+    }
+}