x86: put Sync impl behind a Cargo feature

Cargo.toml

@@ -23,6 +23,8 @@ version = "0.5.4"
 default = ["cas"]
 cas = []
 ufmt-impl = ["ufmt-write"]
+# read the docs before enabling: makes `Pool` Sync on x86_64
+x86-sync-pool = []
 # only for tests
 __trybuild = []
 

ci/script.sh

@@ -5,8 +5,8 @@ main() {
     cargo check --target $TARGET --features 'serde'
 
     if [ $TARGET = x86_64-unknown-linux-gnu ]; then
-        cargo test --target $TARGET --features 'serde'
-        cargo test --target $TARGET --release --features 'serde'
+        cargo test --test cpass --target $TARGET --features 'serde'
+        cargo test --test cpass --target $TARGET --release --features 'serde'
 
         if [ $MSRV = 1 ]; then
             cd cfail
@@ -19,8 +19,8 @@ main() {
             export RUSTFLAGS="-Z sanitizer=thread"
             export TSAN_OPTIONS="suppressions=$(pwd)/suppressions.txt"
 
-            cargo test --test tsan --target $TARGET
-            cargo test --test tsan --target $TARGET --release
+            cargo test --test tsan --features x86-sync-pool --target $TARGET
+            cargo test --test tsan --features x86-sync-pool --target $TARGET --release
         fi
     fi
 }

src/pool/mod.rs

@@ -153,6 +153,8 @@
 //!
 //! # x86_64 support / limitations
 //!
+//! *NOTE* `Pool` is only `Sync` on `x86_64` if the Cargo feature "x86-sync-pool" is enabled
+//!
 //! x86_64 support is a gamble. Yes, a gamble. Do you feel lucky enough to use `Pool` on x86_64?
 //!
 //! As it's not possible to implement *ideal* LL/SC semantics (\*) on x86_64 the architecture is
@@ -253,7 +255,15 @@ pub struct Pool<T> {
     _not_send_or_sync: PhantomData<*const ()>,
 }
 
-#[cfg(any(armv7a, armv7r, armv7m, armv8m_main, target_arch = "x86_64"))]
+// NOTE(any(test)) makes testing easier (no need to enable Cargo features for testing)
+#[cfg(any(
+    armv7a,
+    armv7r,
+    armv7m,
+    armv8m_main,
+    all(target_arch = "x86_64", feature = "x86-sync-pool"),
+    test
+))]
 unsafe impl<T> Sync for Pool<T> {}
 
 unsafe impl<T> Send for Pool<T> {}

src/pool/singleton.rs

@@ -15,7 +15,13 @@ use as_slice::{AsMutSlice, AsSlice};
 use super::{Init, Node, Uninit};
 
 /// Instantiates a pool as a global singleton
-#[cfg(any(armv7a, armv7r, armv7m, armv8m_main, target_arch = "x86_64"))]
+#[cfg(any(
+    armv7a,
+    armv7r,
+    armv7m,
+    armv8m_main,
+    all(target_arch = "x86_64", feature = "x86-sync-pool"),
+))]
 #[macro_export]
 macro_rules! pool {
     ($(#[$($attr:tt)*])* $ident:ident: $ty:ty) => {