api: make the memchr iterators implement Send/Sync

They did this previously, but due to some internally re-shuffling, these
impls got dropped accidentally. (Because the iterator switched to using
raw pointers.) They are still Send/Sync, we just need to explicitly opt
into it now. We also add a regression test to ensure we don't make this
mistake again.

Fixes #133, Closes #134

src/arch/generic/memchr.rs

@@ -1011,6 +1011,16 @@ pub(crate) struct Iter<'h> {
     haystack: core::marker::PhantomData<&'h [u8]>,
 }
 
+// SAFETY: Iter contains no shared references to anything that performs any
+// interior mutations. Also, the lifetime guarantees that Iter will not outlive
+// the haystack.
+unsafe impl<'h> Send for Iter<'h> {}
+
+// SAFETY: Iter perform no interior mutations, therefore no explicit
+// synchronization is necessary. Also, the lifetime guarantees that Iter will
+// not outlive the haystack.
+unsafe impl<'h> Sync for Iter<'h> {}
+
 impl<'h> Iter<'h> {
     /// Create a new generic memchr iterator.
     #[inline(always)]

src/memchr.rs

@@ -884,4 +884,20 @@ mod tests {
             },
         )
     }
+
+    // Prior to memchr 2.6, the memchr iterators both implemented Send and
+    // Sync. But in memchr 2.6, the iterator changed to use raw pointers
+    // internally and I didn't add explicit Send/Sync impls. This ended up
+    // regressing the API. This test ensures we don't do that again.
+    //
+    // See: https://github.com/BurntSushi/memchr/issues/133
+    #[test]
+    fn sync_regression() {
+        use core::panic::{RefUnwindSafe, UnwindSafe};
+
+        fn assert_send_sync<T: Send + Sync + UnwindSafe + RefUnwindSafe>() {}
+        assert_send_sync::<Memchr>();
+        assert_send_sync::<Memchr2>();
+        assert_send_sync::<Memchr3>()
+    }
 }