Function fully_qualified_name 

pub fn fully_qualified_name(
    node_name: &str,
    namespace: &str,
) -> Result<ResolvedName, ()>

A node’s fully-qualified name, joined from its two halves.

/robot + talker -> /robot/talker; the root namespace collapses, so / + talker is /talker and never //talker. The namespace may be given with or without a leading /, and empty means root — the same input rules expand_name already accepts, because this IS expand_name’s private-name branch under the name people look for.

Why a named function for one call

The join is three lines and the tree already contained THREE of them — Node::fully_qualified_name, the inline build in Executor::register_parameter_services, and this branch — which is the “one shared helper, never a second spelling” rule pointing at itself. It is also not discoverable: nothing says that expand_name("~", …) is how you ask for a node’s own name, so the fourth caller writes a fourth copy and gets the root case wrong. //talker is the failure this exists to stop.

The counterpart in ROS 2 is rcl_node_get_fully_qualified_name / rclcpp::Node::get_fully_qualified_name, but both answer only for the CALLER’S OWN node. Neither project has this one — the join for a node you DISCOVERED — because their graph APIs hand back two parallel arrays and leave correlating them to you. Ours delivers both halves in one visitor call, so the join is the only step left.