Use cfg(target_has_atomic) on no-std targets to support platforms without atomic CAS (no dep)

.github/workflows/ci.yml

@@ -128,6 +128,23 @@ jobs:
           cargo build --target ${{ matrix.target }}
         if: matrix.target == 'wasm32-unknown-unknown'
 
+  # Build for no_std environment.
+  no-std:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust
+        run: rustup update stable
+      - name: Install cargo-hack
+        uses: taiki-e/install-action@cargo-hack
+      # thumbv6m-none-eabi supports atomic, but not atomic CAS.
+      # thumbv7m-none-eabi supports atomic CAS.
+      - run: rustup target add thumbv6m-none-eabi thumbv7m-none-eabi
+      # * --optional-deps is needed for serde feature
+      # * --no-dev-deps is needed to avoid https://github.com/rust-lang/cargo/issues/4866
+      # Note that Bytes and BytesMut are provided only on platforms that supports atomic CAS.
+      - run: cargo hack build --target thumbv6m-none-eabi --target thumbv7m-none-eabi --feature-powerset --skip std,default --optional-deps --no-dev-deps
+
   # Sanitizers
   tsan:
     name: tsan

README.md

@@ -27,6 +27,18 @@ Next, add this to your crate:
 use bytes::{Bytes, BytesMut, Buf, BufMut};
 ```
 
+## no_std support
+
+To use `bytes` with no_std environment, disable the (enabled by default) `std` feature.
+
+```toml
+[dependencies]
+bytes = { version = "1", default-features = false }
+```
+
+On no_std environment without atomic CAS, such as thumbv6m, `Bytes` and `BytesMut` are currently not provided.
+See [#467](https://github.com/tokio-rs/bytes/pull/467) for the proposal about support these types on such a environment.
+
 ## Serde support
 
 Serde support is optional and disabled by default. To enable use the feature `serde`.

src/buf/buf_impl.rs

@@ -2355,6 +2355,7 @@ pub trait Buf {
     /// # Panics
     ///
     /// This function panics if `len > self.remaining()`.
+    #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
     fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
         use super::BufMut;
 
@@ -2872,6 +2873,7 @@ macro_rules! deref_forward_buf {
         }
 
         #[inline]
+        #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
         fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
             (**self).copy_to_bytes(len)
         }

src/buf/chain.rs

@@ -1,5 +1,5 @@
 use crate::buf::{IntoIter, UninitSlice};
-use crate::{Buf, BufMut, Bytes};
+use crate::{Buf, BufMut};
 
 #[cfg(feature = "std")]
 use std::io::IoSlice;
@@ -169,7 +169,8 @@ where
         n
     }
 
-    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
+    #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
+    fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
         let a_rem = self.a.remaining();
         if a_rem >= len {
             self.a.copy_to_bytes(len)

src/buf/take.rs

@@ -1,4 +1,4 @@
-use crate::{Buf, Bytes};
+use crate::Buf;
 
 use core::cmp;
 
@@ -148,7 +148,8 @@ impl<T: Buf> Buf for Take<T> {
         self.limit -= cnt;
     }
 
-    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
+    #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
+    fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
         assert!(len <= self.remaining(), "`len` greater than remaining");
 
         let r = self.inner.copy_to_bytes(len);

src/lib.rs

@@ -80,17 +80,25 @@ extern crate std;
 pub mod buf;
 pub use crate::buf::{Buf, BufMut};
 
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 mod bytes;
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 mod bytes_mut;
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 mod fmt;
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 mod loom;
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 pub use crate::bytes::Bytes;
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 pub use crate::bytes_mut::BytesMut;
 
 // Optional Serde support
 #[cfg(feature = "serde")]
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 mod serde;
 
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 #[inline(never)]
 #[cold]
 fn abort() -> ! {
@@ -194,6 +202,7 @@ fn panic_does_not_fit(size: usize, nbytes: usize) -> ! {
 ///
 /// But due to min rust is 1.39 and it is only stabilized
 /// in 1.47, we cannot use it.
+#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
 #[inline]
 fn offset_from(dst: *const u8, original: *const u8) -> usize {
     dst as usize - original as usize