class GraphQL::StaticValidation::VariableUsagesAreAllowed

def validate(context)
  # holds { name => ast_node } pairs
  declared_variables = {}
  context.visitor[GraphQL::Language::Nodes::OperationDefinition] << -> (node, parent) {
    declared_variables = node.variables.each_with_object({}) { |var, memo| memo[var.name] = var }
  }
  context.visitor[GraphQL::Language::Nodes::Argument] << -> (node, parent) {
    return if !node.value.is_a?(GraphQL::Language::Nodes::VariableIdentifier)
    if parent.is_a?(GraphQL::Language::Nodes::Field)
      arguments = context.field_definition.arguments
    elsif parent.is_a?(GraphQL::Language::Nodes::Directive)
      arguments = context.directive_definition.arguments
    elsif parent.is_a?(GraphQL::Language::Nodes::InputObject)
      arguments = context.argument_definition.type.unwrap.input_fields
    else
      raise("Unexpected argument parent: #{parent}")
    end
    var_defn_ast = declared_variables[node.value.name]
    # Might be undefined :(
    # VariablesAreUsedAndDefined can't finalize its search until the end of the document.
    var_defn_ast && validate_usage(arguments, node, var_defn_ast, context)
  }
end