add try_attach_all method alongside attach_all

src/types/message.rs

@@ -1,5 +1,6 @@
 use crate::{Address, Capability, LazyLoadBlob, ProcessId};
 use serde::{Deserialize, Serialize};
+use thiserror::Error;
 
 /// The basic `Message` type.
 /// A `Message` is either a [`crate::Request`] or a [`crate::Response`].
@@ -26,23 +27,14 @@ pub enum Message {
     },
 }
 
-#[derive(Debug, Serialize, Deserialize)]
+#[derive(Debug, Error, Serialize, Deserialize)]
 pub enum BuildError {
+    #[error("no body set for message")]
     NoBody,
+    #[error("no target set for message")]
     NoTarget,
 }
 
-impl std::fmt::Display for BuildError {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        match self {
-            BuildError::NoBody => write!(f, "no body set for message"),
-            BuildError::NoTarget => write!(f, "no target set for  message"),
-        }
-    }
-}
-
-impl std::error::Error for BuildError {}
-
 impl Message {
     /// Get the `source` [`Address`] of a `Message`.
     pub fn source(&self) -> &Address {

src/types/request.rs

@@ -34,7 +34,7 @@ impl Request {
             capabilities: vec![],
         }
     }
-    /// Start building a new Request with the `target` [`Address`]. In order
+    /// Start building a new `Request` with the `target` [`Address`]. In order
     /// to successfully send, you must still fill out at least the `body` field
     /// by calling [`Request::body()`] or [`Request::try_body()`] next.
     pub fn to<T>(target: T) -> Self
@@ -52,7 +52,7 @@ impl Request {
             capabilities: vec![],
         }
     }
-    /// Set the target [`Address`] that this request will go to.
+    /// Set the `target` [`Address`] that this `Request` will go to.
     pub fn target<T>(mut self, target: T) -> Self
     where
         T: Into<Address>,
@@ -242,6 +242,13 @@ impl Request {
         }]);
         self
     }
+    /// Attach all capabilities we have that were issued by `target` (if set) to the next message.
+    pub fn try_attach_all(mut self) -> Result<Self, BuildError> {
+        let Some(ref target) = self.target else {
+            return Err(BuildError::NoTarget);
+        };
+        Ok(self.attach_all(target))
+    }
     /// Attach all capabilities we have that were issued by `target` to the next message.
     pub fn attach_all(mut self, target: &Address) -> Self {
         let target = target.clone();

src/types/response.rs

@@ -150,6 +150,13 @@ impl Response {
         self.capabilities = capabilities;
         self
     }
+    /// Attach all capabilities we have that were issued by `target` (if set) to the next message.
+    pub fn try_attach_all(mut self) -> Result<Self, BuildError> {
+        let Some(ref target) = self.target else {
+            return Err(BuildError::NoTarget);
+        };
+        Ok(self.attach_all(target))
+    }
     /// Attach all capabilities we have that were issued by `target` to the next message.
     pub fn attach_all(mut self, target: &Address) -> Self {
         let target = target.clone();