Add Send + Sync + Unpin bound to PrimitiveAtom::Impl

This is not a breaking change as the trait is sealed. The `Impl`
associated type was `doc(hidden)` before as no one should care about it.
That changed when we needed a `const` way to create `Atomic`. This
change is useful as the whether `Atomic` implements `Send`, `Sync` and
`Unpin` currently depends on the `Impl` type. That's inconvenient.
`Atomic` should always implement `Send`, `Sync` and `Unpin`, so the
bound was added.

Author: LukasKalbertodt

src/impls.rs

@@ -24,7 +24,7 @@ mod sealed {
 /// the public API -- see the module docs.
 pub trait PrimitiveAtom: Sized + Copy + sealed::Sealed {
     /// The standard library type that is the atomic version of `Self`.
-    type Impl;
+    type Impl: Send + Sync + Unpin;
 
     #[doc(hidden)]
     fn into_impl(self) -> Self::Impl;

src/tests.rs

@@ -263,3 +263,9 @@ gen_tests_for_opt_non_zeroes!(nz_u64,   NonZeroU64);
 gen_tests_for_opt_non_zeroes!(nz_i64,   NonZeroI64);
 gen_tests_for_opt_non_zeroes!(nz_usize, NonZeroUsize);
 gen_tests_for_opt_non_zeroes!(nz_isize, NonZeroIsize);
+
+fn _atomic_is_always_send_sync<T: Atom>(a: Atomic<T>) {
+    fn requires_send_sync<X: Send + Sync>(_: X) {}
+
+    requires_send_sync(a);
+}