pub(crate) trait ChildBySource {
// Required method
fn child_by_source_to(
&self,
db: &dyn DefDatabase,
map: &mut DynMap,
file_id: HirFileId,
);
// Provided method
fn child_by_source(
&self,
db: &dyn DefDatabase,
file_id: HirFileId,
) -> DynMap { ... }
}