Use autocfg instead of unstable feature(cfg_target_has_atomic)

.github/workflows/ci.yml

@@ -127,59 +127,24 @@ jobs:
       - run: cargo update -Z minimal-versions
       - run: cargo build --workspace --all-features
 
-  thumbv6m:
-    name: cargo build --target thumbv6m-none-eabi
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - name: Install Rust
-        run: rustup update nightly && rustup default nightly
-      - run: rustup target add thumbv6m-none-eabi
-      - run: cargo install cargo-hack
-      # remove dev-dependencies to avoid https://github.com/rust-lang/cargo/issues/4866
-      - run: cargo hack --remove-dev-deps --workspace
-      - run: |
-          cargo build --manifest-path futures/Cargo.toml \
-            --target thumbv6m-none-eabi \
-            --no-default-features \
-            --features unstable,cfg-target-has-atomic
-      - run: |
-          cargo build --manifest-path futures/Cargo.toml \
-            --target thumbv6m-none-eabi \
-            --no-default-features \
-            --features alloc,unstable,cfg-target-has-atomic
-      - run: |
-          cargo build --manifest-path futures/Cargo.toml \
-            --target thumbv6m-none-eabi \
-            --no-default-features \
-            --features async-await,unstable,cfg-target-has-atomic
-
-  thumbv7m:
-    name: cargo build --target thumbv7m-none-eabi
+  no-std:
+    name: cargo build --target ${{ matrix.target }}
+    strategy:
+      matrix:
+        target:
+          - thumbv6m-none-eabi
+          - thumbv7m-none-eabi
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust
         run: rustup update nightly && rustup default nightly
-      - run: rustup target add thumbv7m-none-eabi
+      - run: rustup target add ${{ matrix.target }}
       - run: cargo install cargo-hack
-      # remove dev-dependencies to avoid https://github.com/rust-lang/cargo/issues/4866
-      - run: cargo hack --remove-dev-deps --workspace
-      - run: |
-          cargo build --manifest-path futures/Cargo.toml \
-            --target thumbv7m-none-eabi \
-            --no-default-features \
-            --features unstable,cfg-target-has-atomic
-      - run: |
-          cargo build --manifest-path futures/Cargo.toml \
-            --target thumbv7m-none-eabi \
-            --no-default-features \
-            --features alloc
       - run: |
-          cargo build --manifest-path futures/Cargo.toml \
-            --target thumbv7m-none-eabi \
-            --no-default-features \
-            --features async-await
+          cargo hack build --manifest-path futures/tests/no-std/Cargo.toml \
+            --each-feature --optional-deps \
+            --target ${{ matrix.target }}
 
   bench:
     name: cargo bench

Cargo.toml

@@ -13,6 +13,7 @@ members = [
 
   "futures/tests/macro-tests",
   "futures/tests/macro-reexport",
+  "futures/tests/no-std",
 
   "examples/functional",
   "examples/imperative",

futures-channel/Cargo.toml

@@ -17,11 +17,13 @@ std = ["alloc", "futures-core/std"]
 alloc = ["futures-core/alloc"]
 sink = ["futures-sink"]
 
-# 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/unstable"]
-cfg-target-has-atomic = ["futures-core/cfg-target-has-atomic"]
+# These features are no longer used.
+# TODO: remove in the next major version.
+unstable = []
+cfg-target-has-atomic = []
+
+[build-dependencies]
+autocfg = "1"
 
 [dependencies]
 futures-core = { path = "../futures-core", version = "0.3.8", default-features = false }

futures-channel/build.rs

@@ -0,0 +1,27 @@
+use autocfg::AutoCfg;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_atomic_cas`, not `has_atomic_cas`. This allows
+    // treating `cfg(target_has_atomic = "ptr")` as true when the build script
+    // doesn't run. This is needed for compatibility with non-cargo build
+    // systems that don't run build scripts.
+    if !cfg.probe_expression("core::sync::atomic::AtomicPtr::<()>::compare_exchange") {
+        println!("cargo:rustc-cfg=no_atomic_cas");
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}

futures-channel/src/lib.rs

@@ -11,8 +11,6 @@
 //! All items are only available when the `std` or `alloc` feature of this
 //! library is activated, and it is activated by default.
 
-#![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
-
 #![cfg_attr(not(feature = "std"), no_std)]
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -21,12 +19,9 @@
 #![warn(clippy::all)]
 #![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]
 
-#[cfg(all(feature = "cfg-target-has-atomic", not(feature = "unstable")))]
-compile_error!("The `cfg-target-has-atomic` feature requires the `unstable` feature as an explicit opt-in to unstable features");
-
 macro_rules! cfg_target_has_atomic {
     ($($item:item)*) => {$(
-        #[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+        #[cfg(not(no_atomic_cas))]
         $item
     )*};
 }

futures-core/Cargo.toml

@@ -16,12 +16,14 @@ default = ["std"]
 std = ["alloc"]
 alloc = []
 
-# Unstable features
-# These features are outside of the normal semver guarantees and require the
-# `unstable` feature as an explicit opt-in to unstable API.
+# These features are no longer used.
+# TODO: remove in the next major version.
 unstable = []
 cfg-target-has-atomic = []
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 
 [dev-dependencies]

futures-core/build.rs

@@ -0,0 +1,27 @@
+use autocfg::AutoCfg;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_atomic_cas`, not `has_atomic_cas`. This allows
+    // treating `cfg(target_has_atomic = "ptr")` as true when the build script
+    // doesn't run. This is needed for compatibility with non-cargo build
+    // systems that don't run build scripts.
+    if !cfg.probe_expression("core::sync::atomic::AtomicPtr::<()>::compare_exchange") {
+        println!("cargo:rustc-cfg=no_atomic_cas");
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}

futures-core/src/lib.rs

@@ -1,7 +1,5 @@
 //! Core traits and types for asynchronous operations in Rust.
 
-#![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
-
 #![cfg_attr(not(feature = "std"), no_std)]
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -10,9 +8,6 @@
 #![warn(clippy::all)]
 #![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]
 
-#[cfg(all(feature = "cfg-target-has-atomic", not(feature = "unstable")))]
-compile_error!("The `cfg-target-has-atomic` feature requires the `unstable` feature as an explicit opt-in to unstable features");
-
 #[cfg(feature = "alloc")]
 extern crate alloc;
 

futures-core/src/task/__internal/mod.rs

@@ -1,4 +1,4 @@
-#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+#[cfg(not(no_atomic_cas))]
 mod atomic_waker;
-#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+#[cfg(not(no_atomic_cas))]
 pub use self::atomic_waker::AtomicWaker;

futures-task/Cargo.toml

@@ -16,12 +16,14 @@ default = ["std"]
 std = ["alloc", "once_cell"]
 alloc = []
 
-# Unstable features
-# These features are outside of the normal semver guarantees and require the
-# `unstable` feature as an explicit opt-in to unstable API.
+# These features are no longer used.
+# TODO: remove in the next major version.
 unstable = []
 cfg-target-has-atomic = []
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 once_cell = { version = "1.3.1", default-features = false, features = ["std"], optional = true }
 

futures-task/build.rs

@@ -0,0 +1,27 @@
+use autocfg::AutoCfg;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_atomic_cas`, not `has_atomic_cas`. This allows
+    // treating `cfg(target_has_atomic = "ptr")` as true when the build script
+    // doesn't run. This is needed for compatibility with non-cargo build
+    // systems that don't run build scripts.
+    if !cfg.probe_expression("core::sync::atomic::AtomicPtr::<()>::compare_exchange") {
+        println!("cargo:rustc-cfg=no_atomic_cas");
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}

futures-task/src/lib.rs

@@ -1,7 +1,5 @@
 //! Tools for working with tasks.
 
-#![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
-
 #![cfg_attr(not(feature = "std"), no_std)]
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -10,15 +8,12 @@
 #![warn(clippy::all)]
 #![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]
 
-#[cfg(all(feature = "cfg-target-has-atomic", not(feature = "unstable")))]
-compile_error!("The `cfg-target-has-atomic` feature requires the `unstable` feature as an explicit opt-in to unstable features");
-
 #[cfg(feature = "alloc")]
 extern crate alloc;
 
 macro_rules! cfg_target_has_atomic {
     ($($item:item)*) => {$(
-        #[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+        #[cfg(not(no_atomic_cas))]
         $item
     )*};
 }

futures-util/Cargo.toml

@@ -27,11 +27,17 @@ channel = ["std", "futures-channel"]
 # 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/unstable", "futures-task/unstable"]
-cfg-target-has-atomic = ["futures-core/cfg-target-has-atomic", "futures-task/cfg-target-has-atomic"]
 bilock = []
 read-initializer = ["io", "futures-io/read-initializer", "futures-io/unstable"]
 write-all-vectored = ["io"]
 
+# These features are no longer used.
+# TODO: remove in the next major version.
+cfg-target-has-atomic = []
+
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 futures-core = { path = "../futures-core", version = "0.3.8", default-features = false }
 futures-task = { path = "../futures-task", version = "0.3.8", default-features = false }

futures-util/build.rs

@@ -0,0 +1,27 @@
+use autocfg::AutoCfg;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_atomic_cas`, not `has_atomic_cas`. This allows
+    // treating `cfg(target_has_atomic = "ptr")` as true when the build script
+    // doesn't run. This is needed for compatibility with non-cargo build
+    // systems that don't run build scripts.
+    if !cfg.probe_expression("core::sync::atomic::AtomicPtr::<()>::compare_exchange") {
+        println!("cargo:rustc-cfg=no_atomic_cas");
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}

futures-util/src/lib.rs

@@ -1,7 +1,6 @@
 //! Combinators and utilities for working with `Future`s, `Stream`s, `Sink`s,
 //! 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(feature = "write-all-vectored", feature(io_slice_advance))]
 #![cfg_attr(not(feature = "std"), no_std)]
@@ -17,9 +16,6 @@
 #![doc(test(attr(deny(warnings), allow(dead_code, unused_assignments, unused_variables))))]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
-#[cfg(all(feature = "cfg-target-has-atomic", not(feature = "unstable")))]
-compile_error!("The `cfg-target-has-atomic` feature requires the `unstable` feature as an explicit opt-in to unstable features");
-
 #[cfg(all(feature = "bilock", not(feature = "unstable")))]
 compile_error!("The `bilock` feature requires the `unstable` feature as an explicit opt-in to unstable features");
 
@@ -58,7 +54,7 @@ pub mod __private {
 
 macro_rules! cfg_target_has_atomic {
     ($($item:item)*) => {$(
-        #[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+        #[cfg(not(no_atomic_cas))]
         $item
     )*};
 }

futures-util/src/stream/mod.rs

@@ -36,11 +36,11 @@ pub use self::stream::ReadyChunks;
 #[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
 pub use self::stream::Forward;
 
-#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+#[cfg(not(no_atomic_cas))]
 #[cfg(feature = "alloc")]
 pub use self::stream::{BufferUnordered, Buffered, ForEachConcurrent};
 
-#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+#[cfg(not(no_atomic_cas))]
 #[cfg(feature = "sink")]
 #[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
 #[cfg(feature = "alloc")]
@@ -58,7 +58,7 @@ pub use self::try_stream::{
 #[cfg(feature = "std")]
 pub use self::try_stream::IntoAsyncRead;
 
-#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
+#[cfg(not(no_atomic_cas))]
 #[cfg(feature = "alloc")]
 pub use self::try_stream::{TryBufferUnordered, TryBuffered, TryForEachConcurrent};