We have a few cases where in validation rules we rely on a different rule ensuring that the definition for usage exists.

• Overlapping Fields, we use a Fragment Definition looked up by a SpreadName that potentially does not exist
• VariablesOfCorrectType, we look up a VariableDefinition from a VariableName which could potentially not exist.

In GraphQL JS we already guard against this in a few places

• https://github.com/graphql/graphql-js/blob/16.x.x/src/validation/rules/VariablesInAllowedPositionRule.ts#L46
• https://github.com/graphql/graphql-js/blob/8a16e93cb8a98b0f773164d7da6215aa47a85746/src/validation/rules/OverlappingFieldsCanBeMergedRule.ts#L266