Add runtime trait to get the UUID from a goal request

C++ uses duck typing for this, knowing that for any `Action`, the type
`Action::Impl::SendGoalService::Request` will always have a `goal_id`
field of type `unique_identifier_msgs::msg::UUID` without having to
prove this to the compiler. Rust's generics are more strict, requiring
that this be proven using type bounds.

The `Request` type is also action-specific as it contains a `goal` field
containing the `Goal` message type of the action. We therefore cannot
enforce that all `Request`s are a specific type in `rclrs`.

This seems most easily represented using associated type bounds on the
`SendGoalService` associated type within `ActionImpl`. To avoid
introducing to `rosidl_runtime_rs` a circular dependency on message
packages like `unique_identifier_msgs`, the `ExtractUuid` trait only
operates on a byte array rather than a more nicely typed `UUID` message
type.

I'll likely revisit this as we introduce more similar bounds on the
generated types.

Author: nwn

rosidl_runtime_rs/src/lib.rs

@@ -9,4 +9,4 @@ mod string;
 pub use string::{BoundedString, BoundedWString, String, StringExceedsBoundsError, WString};
 
 mod traits;
-pub use traits::{Action, Message, RmwMessage, SequenceAlloc, Service};
+pub use traits::{Action, ActionImpl, ExtractUuid, Message, RmwMessage, SequenceAlloc, Service};

rosidl_runtime_rs/src/traits.rs

@@ -188,11 +188,19 @@ pub trait ActionImpl: 'static {
     type FeedbackMessage: Message;
 
     /// The send_goal service associated with this action.
-    type SendGoalService: Service;
+    type SendGoalService: Service<Request: Message<RmwMsg: ExtractUuid>>;
 
     /// The cancel_goal service associated with this action.
     type CancelGoalService: Service;
 
     /// The get_result service associated with this action.
     type GetResultService: Service;
 }
+
+/// Trait for types containing a special UUID field.
+///
+/// User code never needs to implement this trait, nor call its method.
+pub trait ExtractUuid: 'static {
+    /// Copies the UUID field from `self` into the provided buffer.
+    fn extract_uuid(&self, bytes: &mut [u8; 16]);
+}