Function resolve_sql_function_drop_targets 

pub fn resolve_sql_function_drop_targets(
    catalog: &dyn RoutineNameCatalog,
    types: &dyn RoutineTypeCatalog,
    stmt: &DropFunctionStmt,
    registry: &BTreeMap<String, Vec<Arc<SQLUserFunction>>>,
    kind: &'static str,
    ensure_owner: impl FnMut(&SQLUserFunction, &DropFunctionItem) -> Result<(), SQLError>,
) -> Result<RoutineDropResolution, SQLError>

Find the routines a DROP names, item by item as RemoveObjects does: each item’s routine is looked up and then passed to ensure_owner, before the next item is looked up.