class GraphQL::StaticValidation::VariablesAreUsedAndDefined

def follow_spreads(node, parent_variables, spreads_for_context, fragment_definitions, visited_fragments)

Avoid infinite loops by skipping anything in `visited_fragments`.
Use those fragments to update {VariableUsage}s in `parent_variables`.
Follow spreads in `node`, looking them up from `spreads_for_context` and finding their match in `fragment_definitions`. 
def follow_spreads(node, parent_variables, spreads_for_context, fragment_definitions, visited_fragments)
  spreads = spreads_for_context[node] - visited_fragments
  spreads.each do |spread_name|
    def_node, variables = fragment_definitions.find { |def_node, vars| def_node.name == spread_name }
    next if !def_node
    visited_fragments << spread_name
    variables.each do |name, child_usage|
      parent_usage = parent_variables[name]
      if child_usage.used?
        parent_usage.ast_node   = child_usage.ast_node
        parent_usage.used_by    = child_usage.used_by
        parent_usage.path       = child_usage.path
      end
    end
    follow_spreads(def_node, parent_variables, spreads_for_context, fragment_definitions, visited_fragments)
  end
end