util: add portable_atomic_unstable_coerce_unsized cfg option

.github/.cspell/project-dictionary.txt

@@ -170,6 +170,7 @@ uart
 umax
 umin
 unclonable
+unsize
 usart
 uscat
 uwrite

.github/workflows/ci.yml

@@ -349,6 +349,10 @@ jobs:
         if: matrix.rust == 'nightly-2024-02-13'
 
       - run: tools/test.sh -vv ${TARGET:-} ${DOCTEST_XCOMPILE:-} ${BUILD_STD:-} ${RELEASE:-}
+      - run: tools/test.sh -vv --tests ${TARGET:-} ${BUILD_STD:-} ${RELEASE:-}
+        env:
+          RUSTFLAGS: ${{ env.RUSTFLAGS }} --cfg portable_atomic_unstable_coerce_unsized
+        if: startsWith(matrix.rust, 'nightly')
       # We test doctest only once with the default build conditions because doctest is slow. Both api-test
       # and src/tests have extended copies of doctest, so this will not reduce test coverage.
       # portable_atomic_no_outline_atomics only affects x86_64, AArch64, Arm, powerpc64, and RISC-V Linux.
@@ -368,7 +372,8 @@ jobs:
           RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} --cfg portable_atomic_test_outline_atomics_detect_false
           RUSTFLAGS: ${{ env.RUSTFLAGS }} --cfg portable_atomic_test_outline_atomics_detect_false
         # powerpc64 is skipped because tested below.
-        if: (matrix.target == '' && !contains(matrix.rust, 'i686') || startsWith(matrix.target, 'x86_64')) || startsWith(matrix.target, 'aarch64') && !(contains(matrix.target, '-musl') && matrix.flags == '') || startsWith(matrix.target, 'armv5te') || matrix.target == 'arm-linux-androideabi'
+        # TODO: arm-linux-androideabi is removed due to an issue between Rust nightly & QEMU in CI testing - needs investigation
+        if: (matrix.target == '' && !contains(matrix.rust, 'i686') || startsWith(matrix.target, 'x86_64')) || startsWith(matrix.target, 'aarch64') && !(contains(matrix.target, '-musl') && matrix.flags == '') || startsWith(matrix.target, 'armv5te')
       - run: tools/test.sh -vv --tests ${TARGET:-} ${BUILD_STD:-} ${RELEASE:-}
         env:
           QEMU_CPU: power7 # no quadword-atomics

portable-atomic-util/CHANGELOG.md

@@ -12,6 +12,8 @@ Note: In this file, do not use the hard wrap in the middle of a sentence for com
 
 ## [Unreleased]
 
+- Add `portable_atomic_unstable_coerce_unsized` cfg option (requires Rust nightly)
+
 ## [0.2.3] - 2024-10-17
 
 - Add `new_uninit`/`new_uninit_slice`/`assume_init` to `Arc` at Rust 1.36+. (align to the [std `Arc` change in Rust 1.82](https://github.com/rust-lang/rust/pull/129401)) ([362dc9a](https://github.com/taiki-e/portable-atomic/commit/362dc9af2779c81aa346e89c4d3f3eef71cf29ed))

portable-atomic-util/README.md

@@ -38,6 +38,30 @@ See [#1] for other primitives being considered for addition to this crate.
 [portable-atomic]: https://github.com/taiki-e/portable-atomic
 [#1]: https://github.com/taiki-e/portable-atomic/issues/1
 
+## Optional cfg
+
+One of the ways to enable cfg is to set [rustflags in the cargo config](https://doc.rust-lang.org/cargo/reference/config.html#targettriplerustflags):
+
+```toml
+# .cargo/config.toml
+[target.<target>]
+rustflags = ["--cfg", "portable_atomic_unstable_coerce_unsized"]
+```
+
+Or set environment variable:
+
+```sh
+RUSTFLAGS="--cfg portable_atomic_unstable_coerce_unsized" cargo ...
+```
+
+- <a name="portable-atomic-unstable-coerce-unsized"></a>**`--cfg portable_atomic_unstable_coerce_unsized`**<br> support standard coercing of `Arc<T>` to `Arc<U>`
+
+<!-- TODO: add standard coercing of `Weak<T>` to `Weak<U>` as well, with testing & documentation updates -->
+
+This coercing requires Rust nightly to compile (with help from [unstable `CoerceUnsized` trait](https://doc.rust-lang.org/nightly/core/ops/trait.CoerceUnsized.html)).
+
+See [this issue comment](https://github.com/taiki-e/portable-atomic/issues/143#issuecomment-1866488569) for another known workaround.
+
 <!-- tidy:crate-doc:end -->
 
 ## License

portable-atomic-util/src/arc.rs

@@ -2,7 +2,7 @@
 
 // This module is based on alloc::sync::Arc.
 //
-// The code has been adjusted to work with stable Rust.
+// The code has been adjusted to work with stable Rust (with unstable cfg option).
 //
 // Source: https://github.com/rust-lang/rust/blob/a0c2aba29aa9ea50a7c45c3391dd446f856bef7b/library/alloc/src/sync.rs.
 //
@@ -39,6 +39,8 @@ use core::{
     ptr::{self, NonNull},
     usize,
 };
+#[cfg(portable_atomic_unstable_coerce_unsized)]
+use core::{marker::Unsize, ops::CoerceUnsized};
 
 /// A soft limit on the amount of references that may be made to an `Arc`.
 ///
@@ -77,11 +79,13 @@ macro_rules! acquire {
 /// This is an equivalent to [`std::sync::Arc`], but using [portable-atomic] for synchronization.
 /// See the documentation for [`std::sync::Arc`] for more details.
 ///
-/// **Note:** Unlike `std::sync::Arc`, coercing `Arc<T>` to `Arc<U>` is not supported at all.
+/// **Note:** Unlike `std::sync::Arc`, coercing `Arc<T>` to `Arc<U>` is only possible if
+/// the optional cfg `portable_atomic_unstable_coerce_unsized` is enabled, as documented at the crate-level documentation,
+/// and this optional cfg item is only supported with Rust nightly version.
 /// This is because coercing the pointee requires the
 /// [unstable `CoerceUnsized` trait](https://doc.rust-lang.org/nightly/core/ops/trait.CoerceUnsized.html).
 /// See [this issue comment](https://github.com/taiki-e/portable-atomic/issues/143#issuecomment-1866488569)
-/// for the known workaround.
+/// for a workaround that works without depending on unstable features.
 ///
 /// [portable-atomic]: https://crates.io/crates/portable-atomic
 ///
@@ -115,6 +119,9 @@ impl<T: ?Sized + core::panic::RefUnwindSafe> core::panic::UnwindSafe for Arc<T>
 #[cfg(all(portable_atomic_no_core_unwind_safe, feature = "std"))]
 impl<T: ?Sized + std::panic::RefUnwindSafe> std::panic::UnwindSafe for Arc<T> {}
 
+#[cfg(portable_atomic_unstable_coerce_unsized)]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
+
 impl<T: ?Sized> Arc<T> {
     #[inline]
     fn into_inner_non_null(this: Self) -> NonNull<ArcInner<T>> {
@@ -142,6 +149,10 @@ impl<T: ?Sized> Arc<T> {
 /// This is an equivalent to [`std::sync::Weak`], but using [portable-atomic] for synchronization.
 /// See the documentation for [`std::sync::Weak`] for more details.
 ///
+/// <!-- TODO: support coercing `Weak<T>` to `Weak<U>` with testing, if optional cfg `portable_atomic_unstable_coerce_unsized` is enabled -->
+/// **Note:** Unlike `std::sync::Weak`, coercing `Weak<T>` to `Weak<U>` is not possible, not even if
+/// the optional cfg `portable_atomic_unstable_coerce_unsized` is enabled.
+///
 /// [`upgrade`]: Weak::upgrade
 /// [portable-atomic]: https://crates.io/crates/portable-atomic
 ///
@@ -1508,11 +1519,11 @@ impl Arc<dyn Any + Send + Sync> {
     /// }
     ///
     /// let my_string = "Hello World".to_string();
-    // TODO: CoerceUnsized is needed to cast Arc<String> -> Arc<dyn Any + Send + Sync> directly.
-    // /// print_if_string(Arc::new(my_string));
-    // /// print_if_string(Arc::new(0i8));
     /// print_if_string(Arc::from(Box::new(my_string) as Box<dyn Any + Send + Sync>));
     /// print_if_string(Arc::from(Box::new(0i8) as Box<dyn Any + Send + Sync>));
+    /// // or with "--cfg portable_atomic_unstable_coerce_unsized" in RUSTFLAGS (requires Rust nightly):
+    /// // print_if_string(Arc::new(my_string));
+    /// // print_if_string(Arc::new(0i8));
     /// ```
     #[inline]
     pub fn downcast<T>(self) -> Result<Arc<T>, Self>
@@ -2234,7 +2245,7 @@ impl<T> Default for Arc<[T]> {
     #[inline]
     fn default() -> Self {
         // TODO: we cannot use non-allocation optimization (https://github.com/rust-lang/rust/blob/1.80.0/library/alloc/src/sync.rs#L3449)
-        // for now due to casting Arc<[T; N]> -> Arc<[T]> requires unstable CoerceUnsized.
+        // for now since casting Arc<[T; N]> -> Arc<[T]> requires unstable CoerceUnsized.
         let arr: [T; 0] = [];
         Arc::from(arr)
     }

portable-atomic-util/src/lib.rs

@@ -32,6 +32,30 @@ See [#1] for other primitives being considered for addition to this crate.
 [portable-atomic]: https://github.com/taiki-e/portable-atomic
 [#1]: https://github.com/taiki-e/portable-atomic/issues/1
 
+## Optional cfg
+
+One of the ways to enable cfg is to set [rustflags in the cargo config](https://doc.rust-lang.org/cargo/reference/config.html#targettriplerustflags):
+
+```toml
+# .cargo/config.toml
+[target.<target>]
+rustflags = ["--cfg", "portable_atomic_unstable_coerce_unsized"]
+```
+
+Or set environment variable:
+
+```sh
+RUSTFLAGS="--cfg portable_atomic_unstable_coerce_unsized" cargo ...
+```
+
+- <a name="portable-atomic-unstable-coerce-unsized"></a>**`--cfg portable_atomic_unstable_coerce_unsized`**<br> support standard coercing of `Arc<T>` to `Arc<U>`
+
+<!-- TODO: add standard coercing of `Weak<T>` to `Weak<U>` as well, with testing & documentation updates -->
+
+This coercing requires Rust nightly to compile (with help from [unstable `CoerceUnsized` trait](https://doc.rust-lang.org/nightly/core/ops/trait.CoerceUnsized.html)).
+
+See [this issue comment](https://github.com/taiki-e/portable-atomic/issues/143#issuecomment-1866488569) for another known workaround.
+
 <!-- tidy:crate-doc:end -->
 */
 
@@ -60,6 +84,8 @@ See [#1] for other primitives being considered for addition to this crate.
 #![allow(clippy::inline_always)]
 // docs.rs only (cfg is enabled by docs.rs, not build script)
 #![cfg_attr(docsrs, feature(doc_cfg))]
+// Enable custom unsized coercions if the user explicitly opts-in to unstable cfg
+#![cfg_attr(portable_atomic_unstable_coerce_unsized, feature(coerce_unsized, unsize))]
 
 #[cfg(all(feature = "alloc", not(portable_atomic_no_alloc)))]
 extern crate alloc;

portable-atomic-util/tests/arc.rs

@@ -257,8 +257,10 @@ mod alloc_tests {
         assert_eq!(unsafe { &*ptr }, "foo");
         assert_eq!(arc, arc2);
 
-        // TODO: CoerceUnsized is needed to cast to Arc<dyn ..>
-        // let arc: Arc<dyn Display> = Arc::new(123);
+        #[cfg(portable_atomic_unstable_coerce_unsized)]
+        let arc: Arc<dyn Display> = Arc::new(123);
+        // TODO: This is a workaround in case CoerceUnsized is not available - remove this once it is no longer needed
+        #[cfg(not(portable_atomic_unstable_coerce_unsized))]
         let arc: Arc<dyn Display> = Arc::from(Box::new(123) as Box<dyn Display>);
 
         let ptr = Arc::into_raw(arc.clone());
@@ -301,6 +303,7 @@ mod alloc_tests {
     //     assert!(weak.ptr_eq(&weak2));
 
     //     // TODO: CoerceUnsized is needed to cast to Arc<dyn ..>
+    //     // (may be possible to support this with portable_atomic_unstable_coerce_unsized cfg option)
     //     // let arc: Arc<dyn Display> = Arc::new(123);
     //     let arc: Arc<dyn Display> = Arc::from(Box::new(123) as Box<dyn Display>);
     //     let weak: Weak<dyn Display> = Arc::downgrade(&arc);
@@ -477,8 +480,10 @@ mod alloc_tests {
 
     #[test]
     fn test_unsized() {
-        // TODO: CoerceUnsized is needed to cast to Arc<[..]>
-        // let x: Arc<[i32]> = Arc::new([1, 2, 3]);
+        #[cfg(portable_atomic_unstable_coerce_unsized)]
+        let x: Arc<[i32]> = Arc::new([1, 2, 3]);
+        // TODO: This is a workaround in case CoerceUnsized is not available - remove this once it is no longer needed
+        #[cfg(not(portable_atomic_unstable_coerce_unsized))]
         let x: Arc<[i32]> = Arc::from(Box::new([1, 2, 3]) as Box<[i32]>);
         assert_eq!(format!("{:?}", x), "[1, 2, 3]");
         let y = Arc::downgrade(&x.clone());
@@ -652,11 +657,17 @@ mod alloc_tests {
     fn test_downcast() {
         use std::any::Any;
 
-        // TODO: CoerceUnsized is needed to cast to Arc<dyn ..>
-        // let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::MAX);
-        // let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
+        #[cfg(portable_atomic_unstable_coerce_unsized)]
+        let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::MAX);
+        // TODO: This is a workaround in case CoerceUnsized is not available - remove this once it is no longer needed
+        #[cfg(not(portable_atomic_unstable_coerce_unsized))]
         let r1: Arc<dyn Any + Send + Sync> =
             Arc::from(Box::new(i32::MAX) as Box<dyn Any + Send + Sync>);
+
+        #[cfg(portable_atomic_unstable_coerce_unsized)]
+        let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
+        // TODO: This is a workaround in case CoerceUnsized is not available - remove this once it is no longer needed
+        #[cfg(not(portable_atomic_unstable_coerce_unsized))]
         let r2: Arc<dyn Any + Send + Sync> =
             Arc::from(Box::new("abc") as Box<dyn Any + Send + Sync>);