Fall back to Rc on targets without atomics

`alloc::sync::Arc` is unavailable on targets without atomics, such as
ESP32-C3, which is riscv32imc. In this case, use `Rc` instead, and make
the user responsible for synchronization.

Author: smaeul

src/lib.rs

@@ -279,6 +279,11 @@
 #[macro_use]
 extern crate alloc;
 
+#[cfg(target_has_atomic = "ptr")]
+type Arc<T> = alloc::sync::Arc<T>;
+#[cfg(not(target_has_atomic = "ptr"))]
+type Arc<T> = alloc::rc::Rc<T>;
+
 // Common prelude with alloc types
 pub mod prelude {
     pub use alloc::{boxed::Box, format, string::String, vec, vec::Vec};

src/mqtt/common/arc_payload.rs

@@ -20,7 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-use alloc::{string::String, sync::Arc, vec::Vec};
+use crate::Arc;
+use alloc::{string::String, vec::Vec};
 use serde::{Serialize, Serializer};
 
 // SSO buffer size configuration - priority-based selection for maximum size

src/mqtt/connection/packet_builder.rs

@@ -21,7 +21,8 @@
 // SOFTWARE.
 use crate::mqtt::common::Cursor;
 use crate::mqtt::result_code::MqttError;
-use alloc::{sync::Arc, vec::Vec};
+use crate::Arc;
+use alloc::vec::Vec;
 
 #[derive(Debug, Clone)]
 pub enum PacketData {

src/mqtt/packet/v3_1_1/publish.rs

@@ -20,7 +20,6 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-use alloc::sync::Arc;
 use alloc::vec::Vec;
 use core::fmt;
 use core::mem;
@@ -43,6 +42,7 @@ use crate::mqtt::packet::GenericPacketTrait;
 use crate::mqtt::packet::IsPacketId;
 use crate::mqtt::result_code::MqttError;
 use crate::mqtt::{ArcPayload, IntoPayload};
+use crate::Arc;
 
 /// MQTT 3.1.1 PUBLISH packet representation
 ///

src/mqtt/packet/v5_0/publish.rs

@@ -21,7 +21,6 @@
 // SOFTWARE.
 
 use alloc::string::String;
-use alloc::sync::Arc;
 use alloc::vec::Vec;
 use core::fmt;
 use core::mem;
@@ -49,6 +48,7 @@ use crate::mqtt::packet::PropertiesToBuffers;
 use crate::mqtt::packet::{Properties, PropertiesParse, PropertiesSize, Property};
 use crate::mqtt::result_code::MqttError;
 use crate::mqtt::{ArcPayload, IntoPayload};
+use crate::Arc;
 
 /// MQTT 5.0 PUBLISH packet representation
 ///