diff --git a/.travis.yml b/.travis.yml
index 5bcf70a559..bb049a06bb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -136,14 +136,23 @@ matrix:
             --no-default-features
             --features async-await
 
-    - name: cargo check (futures-util)
+    - name: cargo check (sub crates)
       rust: nightly
       script:
         - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
 
+        # futures-io
+        # Check default-features, all-features
+        - cargo check --manifest-path futures-io/Cargo.toml
+        - cargo check --manifest-path futures-io/Cargo.toml --all-features
+        # Check each features
+        - cargo check --manifest-path futures-io/Cargo.toml --features read_initializer,unstable
+
+        # futures-util
+        # Check default-features, all-features
         - cargo check --manifest-path futures-util/Cargo.toml
         - cargo check --manifest-path futures-util/Cargo.toml --all-features
-
+        # Check each features
         - cargo check --manifest-path futures-util/Cargo.toml --features sink
         - cargo check --manifest-path futures-util/Cargo.toml --features io
         - cargo check --manifest-path futures-util/Cargo.toml --features channel
@@ -158,6 +167,7 @@ matrix:
         - cargo check --manifest-path futures-util/Cargo.toml --features sink,bilock,unstable
         - cargo check --manifest-path futures-util/Cargo.toml --features io,bilock,unstable
         - cargo check --manifest-path futures-util/Cargo.toml --features sink,io
+        - cargo check --manifest-path futures-util/Cargo.toml --features read_initializer,unstable
 
         - cargo check --manifest-path futures-util/Cargo.toml --no-default-features
         - cargo check --manifest-path futures-util/Cargo.toml --no-default-features --features sink
diff --git a/futures-io/Cargo.toml b/futures-io/Cargo.toml
index 8b5c4a1ff5..3b6be90c27 100644
--- a/futures-io/Cargo.toml
+++ b/futures-io/Cargo.toml
@@ -18,6 +18,12 @@ name = "futures_io"
 default = ["std"]
 std = []
 
+# Unstable features
+# These features are outside of the normal semver guarantees and require the
+# `unstable` feature as an explicit opt-in to unstable API.
+unstable = []
+read_initializer = []
+
 [dependencies]
 
 [dev-dependencies]
diff --git a/futures-io/src/lib.rs b/futures-io/src/lib.rs
index ff2aff9f46..551ee0700a 100644
--- a/futures-io/src/lib.rs
+++ b/futures-io/src/lib.rs
@@ -8,6 +8,8 @@
 //! All items of this library are only available when the `std` feature of this
 //! library is activated, and it is activated by default.
 
+#![cfg_attr(feature = "read_initializer", feature(read_initializer))]
+
 #![cfg_attr(not(feature = "std"), no_std)]
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -19,13 +21,15 @@
 
 #![doc(html_root_url = "https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.18/futures_io")]
 
+#[cfg(all(feature = "read_initializer", not(feature = "unstable")))]
+compile_error!("The `read_initializer` feature requires the `unstable` feature as an explicit opt-in to unstable features");
+
 #[cfg(feature = "std")]
 mod if_std {
     use std::cmp;
     use std::io;
     use std::ops::DerefMut;
     use std::pin::Pin;
-    use std::ptr;
     use std::task::{Context, Poll};
 
     // Re-export some types from `std::io` so that users don't have to deal
@@ -40,45 +44,9 @@ mod if_std {
         SeekFrom as SeekFrom,
     };
 
-    /// A type used to conditionally initialize buffers passed to `AsyncRead`
-    /// methods, modeled after `std`.
-    #[derive(Debug)]
-    pub struct Initializer(bool);
-
-    impl Initializer {
-        /// Returns a new `Initializer` which will zero out buffers.
-        #[inline]
-        pub fn zeroing() -> Initializer {
-            Initializer(true)
-        }
-
-        /// Returns a new `Initializer` which will not zero out buffers.
-        ///
-        /// # Safety
-        ///
-        /// This method may only be called by `AsyncRead`ers which guarantee
-        /// that they will not read from the buffers passed to `AsyncRead`
-        /// methods, and that the return value of the method accurately reflects
-        /// the number of bytes that have been written to the head of the buffer.
-        #[inline]
-        pub unsafe fn nop() -> Initializer {
-            Initializer(false)
-        }
-
-        /// Indicates if a buffer should be initialized.
-        #[inline]
-        pub fn should_initialize(&self) -> bool {
-            self.0
-        }
-
-        /// Initializes a buffer if necessary.
-        #[inline]
-        pub fn initialize(&self, buf: &mut [u8]) {
-            if self.should_initialize() {
-                unsafe { ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len()) }
-            }
-        }
-    }
+    #[cfg(feature = "read_initializer")]
+    #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+    pub use io::Initializer as Initializer;
 
     /// Read bytes asynchronously.
     ///
@@ -99,6 +67,7 @@ mod if_std {
         /// This method is `unsafe` because and `AsyncRead`er could otherwise
         /// return a non-zeroing `Initializer` from another `AsyncRead` type
         /// without an `unsafe` block.
+        #[cfg(feature = "read_initializer")]
         #[inline]
         unsafe fn initializer(&self) -> Initializer {
             Initializer::zeroing()
@@ -342,6 +311,7 @@ mod if_std {
 
     macro_rules! deref_async_read {
         () => {
+            #[cfg(feature = "read_initializer")]
             unsafe fn initializer(&self) -> Initializer {
                 (**self).initializer()
             }
@@ -373,6 +343,7 @@ mod if_std {
         P: DerefMut + Unpin,
         P::Target: AsyncRead,
     {
+        #[cfg(feature = "read_initializer")]
         unsafe fn initializer(&self) -> Initializer {
             (**self).initializer()
         }
@@ -390,12 +361,11 @@ mod if_std {
         }
     }
 
-    /// `unsafe` because the `io::Read` type must not access the buffer
-    /// before reading data into it.
-    macro_rules! unsafe_delegate_async_read_to_stdio {
+    macro_rules! delegate_async_read_to_stdio {
         () => {
+            #[cfg(feature = "read_initializer")]
             unsafe fn initializer(&self) -> Initializer {
-                Initializer::nop()
+                io::Read::initializer(self)
             }
 
             fn poll_read(mut self: Pin<&mut Self>, _: &mut Context<'_>, buf: &mut [u8])
@@ -413,11 +383,11 @@ mod if_std {
     }
 
     impl AsyncRead for &[u8] {
-        unsafe_delegate_async_read_to_stdio!();
+        delegate_async_read_to_stdio!();
     }
 
     impl<T: AsRef<[u8]> + Unpin> AsyncRead for io::Cursor<T> {
-        unsafe_delegate_async_read_to_stdio!();
+        delegate_async_read_to_stdio!();
     }
 
     macro_rules! deref_async_write {
diff --git a/futures-util/Cargo.toml b/futures-util/Cargo.toml
index 33f0428c50..1825ea83f3 100644
--- a/futures-util/Cargo.toml
+++ b/futures-util/Cargo.toml
@@ -33,6 +33,7 @@ select-macro = ["async-await", "futures-select-macro-preview", "proc-macro-hack"
 unstable = ["futures-core-preview/unstable"]
 cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic"]
 bilock = []
+read_initializer = ["io", "futures-io-preview/read_initializer", "futures-io-preview/unstable"]
 
 [dependencies]
 futures-core-preview = { path = "../futures-core", version = "=0.3.0-alpha.18", default-features = false }
diff --git a/futures-util/src/compat/compat01as03.rs b/futures-util/src/compat/compat01as03.rs
index f9471031cc..dde42a4b54 100644
--- a/futures-util/src/compat/compat01as03.rs
+++ b/futures-util/src/compat/compat01as03.rs
@@ -368,9 +368,9 @@ unsafe impl UnsafeNotify01 for NotifyWaker {
 #[cfg(feature = "io-compat")]
 mod io {
     use super::*;
-    use futures_io::{
-        AsyncRead as AsyncRead03, AsyncWrite as AsyncWrite03, Initializer,
-    };
+    #[cfg(feature = "read_initializer")]
+    use futures_io::Initializer;
+    use futures_io::{AsyncRead as AsyncRead03, AsyncWrite as AsyncWrite03};
     use std::io::Error;
     use tokio_io::{AsyncRead as AsyncRead01, AsyncWrite as AsyncWrite01};
 
@@ -433,6 +433,7 @@ mod io {
     impl<W: AsyncWrite01> AsyncWrite01CompatExt for W {}
 
     impl<R: AsyncRead01> AsyncRead03 for Compat01As03<R> {
+        #[cfg(feature = "read_initializer")]
         unsafe fn initializer(&self) -> Initializer {
             // check if `prepare_uninitialized_buffer` needs zeroing
             if self.inner.get_ref().prepare_uninitialized_buffer(&mut [1]) {
diff --git a/futures-util/src/compat/compat03as01.rs b/futures-util/src/compat/compat03as01.rs
index 763d19b2be..b9632cede8 100644
--- a/futures-util/src/compat/compat03as01.rs
+++ b/futures-util/src/compat/compat03as01.rs
@@ -262,6 +262,7 @@ mod io {
     }
 
     impl<R: AsyncRead03 + Unpin> AsyncRead01 for Compat<R> {
+        #[cfg(feature = "read_initializer")]
         unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [u8]) -> bool {
             let initializer = self.inner.initializer();
             let does_init = initializer.should_initialize();
diff --git a/futures-util/src/future/either.rs b/futures-util/src/future/either.rs
index 24ff16da4f..12bdce623c 100644
--- a/futures-util/src/future/either.rs
+++ b/futures-util/src/future/either.rs
@@ -160,9 +160,10 @@ mod if_std {
     use super::Either;
     use core::pin::Pin;
     use core::task::{Context, Poll};
+    #[cfg(feature = "read_initializer")]
+    use futures_io::Initializer;
     use futures_io::{
-        AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, Initializer, IoSlice, IoSliceMut, Result,
-        SeekFrom,
+        AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, IoSlice, IoSliceMut, Result, SeekFrom,
     };
 
     impl<A, B> AsyncRead for Either<A, B>
@@ -170,6 +171,7 @@ mod if_std {
         A: AsyncRead,
         B: AsyncRead,
     {
+        #[cfg(feature = "read_initializer")]
         unsafe fn initializer(&self) -> Initializer {
             match self {
                 Either::Left(x) => x.initializer(),
diff --git a/futures-util/src/io/allow_std.rs b/futures-util/src/io/allow_std.rs
index 63cda335e2..f44d5b251c 100644
--- a/futures-util/src/io/allow_std.rs
+++ b/futures-util/src/io/allow_std.rs
@@ -1,4 +1,6 @@
 use futures_core::task::{Context, Poll};
+#[cfg(feature = "read_initializer")]
+use futures_io::Initializer;
 use futures_io::{AsyncRead, AsyncWrite, AsyncSeek, AsyncBufRead, IoSlice, IoSliceMut, SeekFrom};
 use std::{fmt, io};
 use std::pin::Pin;
@@ -106,8 +108,10 @@ impl<T> io::Read for AllowStdIo<T> where T: io::Read {
     fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         self.0.read_vectored(bufs)
     }
-    // TODO: implement the `initializer` fn when it stabilizes.
-    // See rust-lang/rust #42788
+    #[cfg(feature = "read_initializer")]
+    unsafe fn initializer(&self) -> Initializer {
+        self.0.initializer()
+    }
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
         self.0.read_to_end(buf)
     }
@@ -131,6 +135,11 @@ impl<T> AsyncRead for AllowStdIo<T> where T: io::Read {
     {
         Poll::Ready(Ok(try_with_interrupt!(self.0.read_vectored(bufs))))
     }
+
+    #[cfg(feature = "read_initializer")]
+    unsafe fn initializer(&self) -> Initializer {
+        self.0.initializer()
+    }
 }
 
 impl<T> io::Seek for AllowStdIo<T> where T: io::Seek {
diff --git a/futures-util/src/io/buf_reader.rs b/futures-util/src/io/buf_reader.rs
index 6f7a026c1c..e6df6abe8c 100644
--- a/futures-util/src/io/buf_reader.rs
+++ b/futures-util/src/io/buf_reader.rs
@@ -1,5 +1,7 @@
 use futures_core::task::{Context, Poll};
-use futures_io::{AsyncBufRead, AsyncRead, AsyncSeek, Initializer, IoSliceMut, SeekFrom};
+#[cfg(feature = "read_initializer")]
+use futures_io::Initializer;
+use futures_io::{AsyncBufRead, AsyncRead, AsyncSeek, IoSliceMut, SeekFrom};
 use pin_utils::{unsafe_pinned, unsafe_unpinned};
 use std::io::{self, Read};
 use std::pin::Pin;
@@ -48,7 +50,7 @@ impl<R: AsyncRead> BufReader<R> {
         unsafe {
             let mut buffer = Vec::with_capacity(capacity);
             buffer.set_len(capacity);
-            inner.initializer().initialize(&mut buffer);
+            super::initialize(&inner, &mut buffer);
             Self {
                 inner,
                 buf: buffer.into_boxed_slice(),
@@ -166,6 +168,7 @@ impl<R: AsyncRead> AsyncRead for BufReader<R> {
     }
 
     // we can't skip unconditionally because of the large buffer case in read.
+    #[cfg(feature = "read_initializer")]
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()
     }
diff --git a/futures-util/src/io/chain.rs b/futures-util/src/io/chain.rs
index 35adfdbbc4..ec6bbff760 100644
--- a/futures-util/src/io/chain.rs
+++ b/futures-util/src/io/chain.rs
@@ -1,5 +1,7 @@
 use futures_core::task::{Context, Poll};
-use futures_io::{AsyncBufRead, AsyncRead, Initializer, IoSliceMut};
+#[cfg(feature = "read_initializer")]
+use futures_io::Initializer;
+use futures_io::{AsyncBufRead, AsyncRead, IoSliceMut};
 use pin_utils::{unsafe_pinned, unsafe_unpinned};
 use std::fmt;
 use std::io;
@@ -117,6 +119,7 @@ where
         self.second().poll_read_vectored(cx, bufs)
     }
 
+    #[cfg(feature = "read_initializer")]
     unsafe fn initializer(&self) -> Initializer {
         let initializer = self.first.initializer();
         if initializer.should_initialize() {
diff --git a/futures-util/src/io/empty.rs b/futures-util/src/io/empty.rs
index 58e0d52fd6..60fa08a913 100644
--- a/futures-util/src/io/empty.rs
+++ b/futures-util/src/io/empty.rs
@@ -1,5 +1,7 @@
 use futures_core::task::{Context, Poll};
-use futures_io::{AsyncBufRead, AsyncRead, Initializer};
+#[cfg(feature = "read_initializer")]
+use futures_io::Initializer;
+use futures_io::{AsyncBufRead, AsyncRead};
 use std::fmt;
 use std::io;
 use std::pin::Pin;
@@ -42,6 +44,7 @@ impl AsyncRead for Empty {
         Poll::Ready(Ok(0))
     }
 
+    #[cfg(feature = "read_initializer")]
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
diff --git a/futures-util/src/io/mod.rs b/futures-util/src/io/mod.rs
index 3f2e6ba271..db8bd134d4 100644
--- a/futures-util/src/io/mod.rs
+++ b/futures-util/src/io/mod.rs
@@ -9,17 +9,35 @@
 //! This module is only available when the `io` and `std` features of this
 //! library is activated, and it is activated by default.
 
+#[cfg(feature = "io-compat")]
+use crate::compat::Compat;
+use std::ptr;
+
 pub use futures_io::{
     AsyncRead, AsyncWrite, AsyncSeek, AsyncBufRead, Error, ErrorKind,
     IoSlice, IoSliceMut, Result, SeekFrom,
 };
-
-#[cfg(feature = "io-compat")] use crate::compat::Compat;
+#[cfg(feature = "read_initializer")]
+pub use futures_io::Initializer;
 
 // used by `BufReader` and `BufWriter`
 // https://github.com/rust-lang/rust/blob/master/src/libstd/sys_common/io.rs#L1
 const DEFAULT_BUF_SIZE: usize = 8 * 1024;
 
+/// Initializes a buffer if necessary.
+///
+/// A buffer is always initialized if `read_initializer` feature is disabled.
+#[inline]
+unsafe fn initialize<R: AsyncRead>(_reader: &R, buf: &mut [u8]) {
+    #[cfg(feature = "read_initializer")]
+    {
+        if !_reader.initializer().should_initialize() {
+            return;
+        }
+    }
+    ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len())
+}
+
 mod allow_std;
 pub use self::allow_std::AllowStdIo;
 
diff --git a/futures-util/src/io/read_to_end.rs b/futures-util/src/io/read_to_end.rs
index 4e342e0574..c4dab99e0c 100644
--- a/futures-util/src/io/read_to_end.rs
+++ b/futures-util/src/io/read_to_end.rs
@@ -58,7 +58,7 @@ pub(super) fn read_to_end_internal<R: AsyncRead + ?Sized>(
                 g.buf.reserve(32);
                 let capacity = g.buf.capacity();
                 g.buf.set_len(capacity);
-                rd.initializer().initialize(&mut g.buf[g.len..]);
+                super::initialize(&rd, &mut g.buf[g.len..]);
             }
         }
 
diff --git a/futures-util/src/io/repeat.rs b/futures-util/src/io/repeat.rs
index 0821ec9b04..9f493d1a49 100644
--- a/futures-util/src/io/repeat.rs
+++ b/futures-util/src/io/repeat.rs
@@ -1,5 +1,7 @@
 use futures_core::task::{Context, Poll};
-use futures_io::{AsyncRead, Initializer, IoSliceMut};
+#[cfg(feature = "read_initializer")]
+use futures_io::Initializer;
+use futures_io::{AsyncRead, IoSliceMut};
 use std::fmt;
 use std::io;
 use std::pin::Pin;
@@ -57,6 +59,7 @@ impl AsyncRead for Repeat {
         Poll::Ready(Ok(nwritten))
     }
 
+    #[cfg(feature = "read_initializer")]
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
diff --git a/futures-util/src/io/take.rs b/futures-util/src/io/take.rs
index 3951371458..3eaa5f5ced 100644
--- a/futures-util/src/io/take.rs
+++ b/futures-util/src/io/take.rs
@@ -1,5 +1,7 @@
 use futures_core::task::{Context, Poll};
-use futures_io::{AsyncRead, AsyncBufRead, Initializer};
+#[cfg(feature = "read_initializer")]
+use futures_io::Initializer;
+use futures_io::{AsyncRead, AsyncBufRead};
 use pin_utils::{unsafe_pinned, unsafe_unpinned};
 use std::{cmp, io};
 use std::pin::Pin;
@@ -183,6 +185,7 @@ impl<R: AsyncRead> AsyncRead for Take<R> {
         Poll::Ready(Ok(n))
     }
 
+    #[cfg(feature = "read_initializer")]
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()
     }
diff --git a/futures-util/src/lib.rs b/futures-util/src/lib.rs
index 4446c1476d..3f91a9eb96 100644
--- a/futures-util/src/lib.rs
+++ b/futures-util/src/lib.rs
@@ -2,6 +2,7 @@
 //! and the `AsyncRead` and `AsyncWrite` traits.
 
 #![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
+#![cfg_attr(feature = "read_initializer", feature(read_initializer))]
 
 #![cfg_attr(not(feature = "std"), no_std)]
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -19,6 +20,9 @@ compile_error!("The `cfg-target-has-atomic` feature requires the `unstable` feat
 #[cfg(all(feature = "bilock", not(feature = "unstable")))]
 compile_error!("The `bilock` feature requires the `unstable` feature as an explicit opt-in to unstable features");
 
+#[cfg(all(feature = "read_initializer", not(feature = "unstable")))]
+compile_error!("The `read_initializer` feature requires the `unstable` feature as an explicit opt-in to unstable features");
+
 #[cfg(feature = "alloc")]
 extern crate alloc;
 
diff --git a/futures/Cargo.toml b/futures/Cargo.toml
index e17f4833d1..71e99ea63e 100644
--- a/futures/Cargo.toml
+++ b/futures/Cargo.toml
@@ -46,9 +46,10 @@ io-compat = ["compat", "futures-util-preview/io-compat"]
 # Unstable features
 # These features are outside of the normal semver guarantees and require the
 # `unstable` feature as an explicit opt-in to unstable API.
-unstable = ["futures-core-preview/unstable", "futures-channel-preview/unstable", "futures-util-preview/unstable"]
+unstable = ["futures-core-preview/unstable", "futures-channel-preview/unstable", "futures-io-preview/unstable", "futures-util-preview/unstable"]
 cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic", "futures-channel-preview/cfg-target-has-atomic", "futures-util-preview/cfg-target-has-atomic"]
 bilock = ["futures-util-preview/bilock"]
+read_initializer = ["futures-io-preview/read_initializer", "futures-util-preview/read_initializer"]
 
 [package.metadata.docs.rs]
 all-features = true
diff --git a/futures/src/lib.rs b/futures/src/lib.rs
index 33f6e79a0a..f4973ff7ab 100644
--- a/futures/src/lib.rs
+++ b/futures/src/lib.rs
@@ -22,6 +22,7 @@
 //! completion, but *do not block* the thread running them.
 
 #![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
+#![cfg_attr(feature = "read_initializer", feature(read_initializer))]
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
@@ -40,6 +41,9 @@ compile_error!("The `cfg-target-has-atomic` feature requires the `unstable` feat
 #[cfg(all(feature = "bilock", not(feature = "unstable")))]
 compile_error!("The `bilock` feature requires the `unstable` feature as an explicit opt-in to unstable features");
 
+#[cfg(all(feature = "read_initializer", not(feature = "unstable")))]
+compile_error!("The `read_initializer` feature requires the `unstable` feature as an explicit opt-in to unstable features");
+
 #[doc(hidden)] pub use futures_core::core_reexport;
 
 #[doc(hidden)] pub use futures_core::future::Future;
@@ -298,9 +302,12 @@ pub mod io {
 
     pub use futures_io::{
         AsyncRead, AsyncWrite, AsyncSeek, AsyncBufRead, Error, ErrorKind,
-        Initializer, IoSlice, IoSliceMut, Result, SeekFrom,
+        IoSlice, IoSliceMut, Result, SeekFrom,
     };
 
+    #[cfg(feature = "read_initializer")]
+    pub use futures_io::Initializer;
+
     pub use futures_util::io::{
         AsyncReadExt, AsyncWriteExt, AsyncSeekExt, AsyncBufReadExt, AllowStdIo,
         BufReader, BufWriter, Chain, Close, CopyInto, CopyBufInto, empty, Empty,