Add no_std support, by adding an std feature

I was really hopeful initially that this could be done without
having an `std` feature, but it turns out that `bytes` has a bunch
of special additional integration with `std::io` stuff, e.g.
`std::io::Reader` and `std::io::IoSlice`.

To make the library work as `no_std` we add an `std` feature which
is on by default. When it is off, we compile as `no_std` and make
parts of the API that require `std::io` conditional on the `std`
feature.

Cargo.toml

@@ -19,6 +19,10 @@ edition = "2018"
 
 publish = false
 
+[features]
+default = ["std"]
+std = []
+
 [dependencies]
 serde = { version = "1.0", optional = true }
 either = { version = "1.5", default-features = false, optional = true }

src/buf/buf.rs

@@ -1,6 +1,12 @@
-use super::{IntoBuf, Take, Reader, FromBuf, Chain};
+use super::{IntoBuf, Take, FromBuf, Chain};
 
-use std::{cmp, io::IoSlice, ptr, mem};
+#[cfg(feature = "std")]
+use super::Reader;
+
+use core::{cmp, ptr, mem};
+
+#[cfg(feature = "std")]
+use std::io::IoSlice;
 
 macro_rules! buf_get_impl {
     ($this:ident, $typ:tt::$conv:tt) => ({
@@ -148,6 +154,7 @@ pub trait Buf {
     /// with `dst` being a zero length slice.
     ///
     /// [`writev`]: http://man7.org/linux/man-pages/man2/readv.2.html
+    #[cfg(feature = "std")]
     fn bytes_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
         if dst.is_empty() {
             return 0;
@@ -909,6 +916,7 @@ pub trait Buf {
     /// assert_eq!(11, num);
     /// assert_eq!(&dst[..11], &b"hello world"[..]);
     /// ```
+    #[cfg(feature = "std")]
     fn reader(self) -> Reader<Self> where Self: Sized {
         super::reader::new(self)
     }
@@ -923,6 +931,7 @@ impl<T: Buf + ?Sized> Buf for &mut T {
         (**self).bytes()
     }
 
+    #[cfg(feature = "std")]
     fn bytes_vectored<'b>(&'b self, dst: &mut [IoSlice<'b>]) -> usize {
         (**self).bytes_vectored(dst)
     }
@@ -941,6 +950,7 @@ impl<T: Buf + ?Sized> Buf for Box<T> {
         (**self).bytes()
     }
 
+    #[cfg(feature = "std")]
     fn bytes_vectored<'b>(&'b self, dst: &mut [IoSlice<'b>]) -> usize {
         (**self).bytes_vectored(dst)
     }

src/buf/buf_mut.rs

@@ -1,6 +1,12 @@
-use super::{IntoBuf, Writer};
+use super::{IntoBuf};
 
-use std::{mem, cmp, io::IoSliceMut, ptr, usize};
+#[cfg(feature = "std")]
+use super::Writer;
+
+use core::{mem, cmp, ptr, usize};
+
+#[cfg(feature = "std")]
+use std::io::IoSliceMut;
 
 /// A trait for values that provide sequential write access to bytes.
 ///
@@ -917,6 +923,7 @@ pub trait BufMut {
     ///
     /// assert_eq!(*buf, b"hello world"[..]);
     /// ```
+    #[cfg(feature = "std")]
     fn writer(self) -> Writer<Self> where Self: Sized {
         super::writer::new(self)
     }

src/buf/chain.rs

@@ -1,5 +1,7 @@
 use crate::{Buf, BufMut};
 use crate::buf::IntoIter;
+
+#[cfg(feature = "std")]
 use std::io::{IoSlice, IoSliceMut};
 
 /// A `Chain` sequences two buffers.
@@ -178,6 +180,7 @@ impl<T, U> Buf for Chain<T, U>
         self.b.advance(cnt);
     }
 
+    #[cfg(feature = "std")]
     fn bytes_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
         let mut n = self.a.bytes_vectored(dst);
         n += self.b.bytes_vectored(&mut dst[n..]);
@@ -219,6 +222,7 @@ impl<T, U> BufMut for Chain<T, U>
         self.b.advance_mut(cnt);
     }
 
+    #[cfg(feature = "std")]
     unsafe fn bytes_vectored_mut<'a>(&'a mut self, dst: &mut [IoSliceMut<'a>]) -> usize {
         let mut n = self.a.bytes_vectored_mut(dst);
         n += self.b.bytes_vectored_mut(&mut dst[n..]);

src/buf/mod.rs

@@ -22,9 +22,13 @@ mod from_buf;
 mod chain;
 mod into_buf;
 mod iter;
-mod reader;
 mod take;
 mod vec_deque;
+
+// When std::io::Reader etc. traits are not available, skip these
+#[cfg(feature = "std")]
+mod reader;
+#[cfg(feature = "std")]
 mod writer;
 
 pub use self::buf::Buf;
@@ -33,6 +37,8 @@ pub use self::from_buf::FromBuf;
 pub use self::chain::Chain;
 pub use self::into_buf::IntoBuf;
 pub use self::iter::IntoIter;
+#[cfg(feature = "std")]
 pub use self::reader::Reader;
 pub use self::take::Take;
+#[cfg(feature = "std")]
 pub use self::writer::Writer;

src/buf/take.rs

@@ -1,6 +1,6 @@
 use crate::Buf;
 
-use std::cmp;
+use core::cmp;
 
 /// A `Buf` adapter which limits the bytes read from an underlying buffer.
 ///

src/buf/vec_deque.rs

@@ -1,4 +1,4 @@
-use std::collections::VecDeque;
+use alloc::collections::VecDeque;
 
 use super::Buf;
 

src/bytes.rs

@@ -2,12 +2,12 @@ use crate::{Buf, BufMut, IntoBuf};
 use crate::buf::IntoIter;
 use crate::debug;
 
-use std::{cmp, fmt, mem, hash, slice, ptr, usize};
-use std::borrow::{Borrow, BorrowMut};
-use std::ops::{Deref, DerefMut, RangeBounds};
-use std::sync::atomic::{self, AtomicUsize, AtomicPtr};
-use std::sync::atomic::Ordering::{Relaxed, Acquire, Release, AcqRel};
-use std::iter::{FromIterator, Iterator};
+use core::{cmp, fmt, mem, hash, slice, ptr, usize};
+use alloc::borrow::{Borrow, BorrowMut};
+use core::ops::{Deref, DerefMut, RangeBounds};
+use core::sync::atomic::{self, AtomicUsize, AtomicPtr};
+use core::sync::atomic::Ordering::{Relaxed, Acquire, Release, AcqRel};
+use core::iter::{FromIterator, Iterator};
 
 /// A reference counted contiguous slice of memory.
 ///
@@ -316,10 +316,10 @@ struct Inner {
 }
 
 // Thread-safe reference-counted container for the shared storage. This mostly
-// the same as `std::sync::Arc` but without the weak counter. The ref counting
+// the same as `core::sync::Arc` but without the weak counter. The ref counting
 // fns are based on the ones found in `std`.
 //
-// The main reason to use `Shared` instead of `std::sync::Arc` is that it ends
+// The main reason to use `Shared` instead of `core::sync::Arc` is that it ends
 // up making the overall code simpler and easier to reason about. This is due to
 // some of the logic around setting `Inner::arc` and other ways the `arc` field
 // is used. Using `Arc` ended up requiring a number of funky transmutes and
@@ -522,7 +522,7 @@ impl Bytes {
     /// Requires that `begin <= end` and `end <= self.len()`, otherwise slicing
     /// will panic.
     pub fn slice(&self, range: impl RangeBounds<usize>) -> Bytes {
-        use std::ops::Bound;
+        use core::ops::Bound;
 
         let len = self.len();
 
@@ -852,7 +852,7 @@ impl Bytes {
     /// assert_eq!(iter.next().map(|b| *b), Some(b'c'));
     /// assert_eq!(iter.next(), None);
     /// ```
-    pub fn iter<'a>(&'a self) -> std::slice::Iter<'a, u8> {
+    pub fn iter<'a>(&'a self) -> core::slice::Iter<'a, u8> {
         self.bytes().iter()
     }
 }
@@ -1026,7 +1026,7 @@ impl IntoIterator for Bytes {
 
 impl<'a> IntoIterator for &'a Bytes {
     type Item = &'a u8;
-    type IntoIter = std::slice::Iter<'a, u8>;
+    type IntoIter = core::slice::Iter<'a, u8>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.as_ref().into_iter()
@@ -1534,7 +1534,7 @@ impl BytesMut {
     /// assert_eq!(iter.next().map(|b| *b), Some(b'c'));
     /// assert_eq!(iter.next(), None);
     /// ```
-    pub fn iter<'a>(&'a self) -> std::slice::Iter<'a, u8> {
+    pub fn iter<'a>(&'a self) -> core::slice::Iter<'a, u8> {
         self.bytes().iter()
     }
 }
@@ -1783,7 +1783,7 @@ impl IntoIterator for BytesMut {
 
 impl<'a> IntoIterator for &'a BytesMut {
     type Item = &'a u8;
-    type IntoIter = std::slice::Iter<'a, u8>;
+    type IntoIter = core::slice::Iter<'a, u8>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.as_ref().into_iter()

src/debug.rs

@@ -1,4 +1,4 @@
-use std::fmt;
+use core::fmt;
 
 /// Alternative implementation of `fmt::Debug` for byte slice.
 ///

src/either.rs

@@ -2,6 +2,8 @@ use crate::{Buf, BufMut};
 
 use either::Either;
 use either::Either::*;
+
+#[cfg(feature = "std")]
 use std::io::{IoSlice, IoSliceMut};
 
 impl<L, R> Buf for Either<L, R>
@@ -23,6 +25,7 @@ where
         }
     }
 
+    #[cfg(feature = "std")]
     fn bytes_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
         match *self {
             Left(ref b) => b.bytes_vectored(dst),
@@ -64,6 +67,7 @@ where
         }
     }
 
+    #[cfg(feature = "std")]
     unsafe fn bytes_vectored_mut<'a>(&'a mut self, dst: &mut [IoSliceMut<'a>]) -> usize {
         match *self {
             Left(ref mut b) => b.bytes_vectored_mut(dst),

src/lib.rs

@@ -71,6 +71,10 @@
 #![deny(warnings, missing_docs, missing_debug_implementations, rust_2018_idioms)]
 #![doc(html_root_url = "https://docs.rs/bytes/0.5.0")]
 
+#![cfg_attr(not(feature = "std"), no_std)]
+
+extern crate alloc;
+
 pub mod buf;
 pub use crate::buf::{
     Buf,

src/serde.rs

@@ -1,4 +1,4 @@
-use std::{cmp, fmt};
+use core::{cmp, fmt};
 use serde::{Serialize, Serializer, Deserialize, Deserializer, de};
 use super::{Bytes, BytesMut};