Remove uses of unstable feature(cfg_target_has_atomic)

Author: taiki-e

.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
@@ -212,6 +177,19 @@ jobs:
             --workspace --exclude futures-test \
             --features unstable --ignore-unknown-features
 
+  # When this job failed, run ci/no_atomic_cas.sh and commit result changes.
+  # TODO(taiki-e): Ideally, this should be automated using a bot that creates
+  #                PR when failed, but there is no bandwidth to implement it
+  #                right now...
+  codegen:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Rust
+        run: rustup update nightly && rustup default nightly
+      - run: ci/no_atomic_cas.sh
+      - run: git diff --exit-code
+
   san:
     name: cargo test -Z sanitizer=${{ matrix.sanitizer }}
     strategy:

Cargo.toml

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

ci/no_atomic_cas.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+set -euo pipefail
+IFS=$'\n\t'
+
+cd "$(cd "$(dirname "$0")" && pwd)"/..
+
+file="no_atomic_cas.rs"
+
+{
+    echo "// This file is @generated by $(basename "$0")."
+    echo "// It is not intended for manual editing."
+    echo ""
+    echo "const NO_ATOMIC_CAS_TARGETS: &[&str] = &["
+} >"$file"
+
+for target in $(rustc --print target-list); do
+    res=$(rustc --print target-spec-json -Z unstable-options --target "$target" \
+        | jq -r "select(.\"atomic-cas\" == false)")
+    [[ -z "$res" ]] || echo "    \"$target\"," >>"$file"
+done
+
+echo "];" >>"$file"

futures-channel/Cargo.toml

@@ -17,11 +17,10 @@ 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 = []
 
 [dependencies]
 futures-core = { path = "../futures-core", version = "=1.0.0-alpha.0", default-features = false }

futures-channel/build.rs

@@ -0,0 +1,42 @@
+#![warn(rust_2018_idioms, single_use_lifetimes)]
+
+use std::env;
+
+include!("no_atomic_cas.rs");
+
+// The rustc-cfg listed below are considered public API, but it is *unstable*
+// and outside of the normal semver guarantees:
+//
+// - `futures_no_atomic_cas`
+//      Assume the target does not have atomic CAS (compare-and-swap).
+//      This is usually detected automatically by the build script, but you may
+//      need to enable it manually when building for custom targets or using
+//      non-cargo build systems that don't run the build script.
+//
+// With the exceptions mentioned above, 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 target = match env::var("TARGET") {
+        Ok(target) => target,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to get TARGET environment variable: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_*`, not `has_*`. 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 the build script.
+    if NO_ATOMIC_CAS_TARGETS.contains(&&*target) {
+        println!("cargo:rustc-cfg=futures_no_atomic_cas");
+    }
+
+    println!("cargo:rerun-if-changed=no_atomic_cas.rs");
+}

futures-channel/no_atomic_cas.rs

@@ -0,0 +1 @@
+../no_atomic_cas.rs

futures-channel/src/lib.rs

@@ -11,20 +11,16 @@
 //! 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)]
 // It cannot be included in the published code because this lints have false positives in the minimum required version.
 #![cfg_attr(test, warn(single_use_lifetimes))]
 #![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(futures_no_atomic_cas))]
         $item
     )*};
 }

futures-core/Cargo.toml

@@ -16,9 +16,8 @@ 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 = []
 

futures-core/build.rs

@@ -0,0 +1,42 @@
+#![warn(rust_2018_idioms, single_use_lifetimes)]
+
+use std::env;
+
+include!("no_atomic_cas.rs");
+
+// The rustc-cfg listed below are considered public API, but it is *unstable*
+// and outside of the normal semver guarantees:
+//
+// - `futures_no_atomic_cas`
+//      Assume the target does not have atomic CAS (compare-and-swap).
+//      This is usually detected automatically by the build script, but you may
+//      need to enable it manually when building for custom targets or using
+//      non-cargo build systems that don't run the build script.
+//
+// With the exceptions mentioned above, 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 target = match env::var("TARGET") {
+        Ok(target) => target,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to get TARGET environment variable: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_*`, not `has_*`. 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 the build script.
+    if NO_ATOMIC_CAS_TARGETS.contains(&&*target) {
+        println!("cargo:rustc-cfg=futures_no_atomic_cas");
+    }
+
+    println!("cargo:rerun-if-changed=no_atomic_cas.rs");
+}

futures-core/no_atomic_cas.rs

@@ -0,0 +1 @@
+../no_atomic_cas.rs