Add variables are input types validation rule

Author: lukel97

Sources/GraphQL/Validation/Rules/VariablesAreInputTypesRule.swift

@@ -0,0 +1,21 @@
+/**
+ * Variables are input types
+ *
+ * A GraphQL operation is only valid if all the variables it defines are of
+ * input types (scalar, enum, or input object).
+ *
+ * See https://spec.graphql.org/draft/#sec-Variables-Are-Input-Types
+ */
+struct VariablesAreInputTypesRule: ValidationRule {
+    let context: ValidationContext
+    func enter(variableDefinition: VariableDefinition, key: AnyKeyPath?, parent: VisitorParent?, ancestors: [VisitorParent]) -> VisitResult<VariableDefinition> {
+        if context.inputType == nil {
+            let error = GraphQLError(
+                message: "Variable \"$\(variableDefinition.variable.name.value)\" cannot be non-input type \"\(variableDefinition.type.printed)\".",
+                nodes: [variableDefinition.type]
+            )
+            context.report(error: error)
+        }
+        return .continue
+    }
+}

Sources/GraphQL/Validation/SpecifiedRules.swift

@@ -26,4 +26,5 @@ let specifiedRules: [ValidationRule.Type] = [
     //    VariablesInAllowedPosition,
     //    OverlappingFieldsCanBeMerged,
     //    UniqueInputFieldNames,
+    VariablesAreInputTypesRule.self
 ]