Only add implementation for Arc if the target supports it

.github/workflows/build.yml

@@ -39,6 +39,10 @@ jobs:
         run: rustup target add i686-unknown-linux-gnu && sudo apt update && sudo apt install -y gcc-multilib
       - name: Test (32-bit all-features)
         run: cargo test --target i686-unknown-linux-gnu --all-features
+      - name: Install thumbv6m-none-eabi
+        run: rustup target add thumbv6m-none-eabi
+      - name: Check (no atomic ptr)
+        run: cargo check --target thumbv6m-none-eabi --no-default-features --features derive,serde,arrayvec
       - name: Setup Miri
         run: cargo miri setup
       - name: Test (miri all-features)

src/derive/impls.rs

@@ -109,6 +109,7 @@ macro_rules! impl_smart_ptr {
 }
 impl_smart_ptr!(::alloc::boxed::Box);
 impl_smart_ptr!(::alloc::rc::Rc);
+#[cfg(target_has_atomic = "ptr")]
 impl_smart_ptr!(::alloc::sync::Arc);
 
 impl<T: Encode, const N: usize> Encode for [T; N] {