Allow tx payloads to be boxed (#1690)

* Allow tx payloads to be boxed

* Add example of boxed payloads

* explicit box import

* box import at top

* cargo fmt

core/src/tx/payload.rs

@@ -9,6 +9,7 @@ use crate::error::MetadataError;
 use crate::metadata::Metadata;
 use crate::Error;
 use alloc::borrow::{Cow, ToOwned};
+use alloc::boxed::Box;
 use alloc::string::String;
 
 use alloc::vec::Vec;
@@ -38,6 +39,32 @@ pub trait Payload {
     }
 }
 
+macro_rules! boxed_payload {
+    ($ty:path) => {
+        impl<T: Payload + ?Sized> Payload for $ty {
+            fn encode_call_data_to(
+                &self,
+                metadata: &Metadata,
+                out: &mut Vec<u8>,
+            ) -> Result<(), Error> {
+                self.as_ref().encode_call_data_to(metadata, out)
+            }
+            fn encode_call_data(&self, metadata: &Metadata) -> Result<Vec<u8>, Error> {
+                self.as_ref().encode_call_data(metadata)
+            }
+            fn validation_details(&self) -> Option<ValidationDetails<'_>> {
+                self.as_ref().validation_details()
+            }
+        }
+    };
+}
+
+boxed_payload!(Box<T>);
+#[cfg(feature = "std")]
+boxed_payload!(std::sync::Arc<T>);
+#[cfg(feature = "std")]
+boxed_payload!(std::rc::Rc<T>);
+
 /// Details required to validate the shape of a transaction payload against some metadata.
 pub struct ValidationDetails<'a> {
     /// The pallet name.