Add ControlMessage::copy_to_slice; used with stack allocation.

text/3430-unix-socket-ancillary-data-v2.md

@@ -147,17 +147,26 @@ impl ControlMessage<'a> {
         cmsg_type: c_int,
         data: &'a [u8],
     ) -> ControlMessage<'a>;
+}
 
+impl ControlMessage<'_> {
     // Opaque platform-specific integers; same as `struct cmsghdr` fields.
     fn cmsg_level(&self) -> c_int;
     fn cmsg_type(&self) -> c_int;
 
     // The type-specific data of this control message.
     fn data(&self) -> &[u8];
 
+    // How big the encoded cmsg is; same as `CMSG_SPACE()`.
+    fn cmsg_space(&self) -> usize;
+
     // Whether this control message is truncated, such as by being received
     // into a too-short buffer.
     fn truncated(&self) -> bool;
+
+    // Encodes the cmsg according to local OS conventions. `dst` must be
+    // `cmsg_space()` bytes long.
+    fn copy_to_slice<'a>(&self, dst: &'a mut [MaybeUninit<u8>]) -> &'a [u8];
 }
 ```