feature: Adds NoUndefinedVariablesRule

Author: NeedleInAJayStack

Sources/GraphQL/Validation/Rules/NoUndefinedVariablesRule.swift

@@ -0,0 +1,42 @@
+
+/**
+ * No undefined variables
+ *
+ * A GraphQL operation is only valid if all variables encountered, both directly
+ * and via fragment spreads, are defined by that operation.
+ *
+ * See https://spec.graphql.org/draft/#sec-All-Variable-Uses-Defined
+ */
+func NoUndefinedVariablesRule(context: ValidationContext) -> Visitor {
+    return Visitor(
+        enter: { node, _, _, _, _ in
+            if let operation = node as? OperationDefinition {
+                let variableNameDefined = Set<String>(
+                    operation.variableDefinitions.map { $0.variable.name.value }
+                )
+
+                let usages = context.getRecursiveVariableUsages(operation: operation)
+                for usage in usages {
+                    let node = usage.node
+                    let varName = node.name.value
+                    if !variableNameDefined.contains(varName) {
+                        let message: String
+                        if let operationName = operation.name {
+                            message =
+                                "Variable \"$\(varName)\" is not defined by operation \"\(operationName.value)\"."
+                        } else {
+                            message = "Variable \"$\(varName)\" is not defined."
+                        }
+                        context.report(
+                            error: GraphQLError(
+                                message: message,
+                                nodes: [node, operation]
+                            )
+                        )
+                    }
+                }
+            }
+            return .continue
+        }
+    )
+}

Sources/GraphQL/Validation/SpecifiedRules.swift

@@ -17,7 +17,7 @@ public let specifiedRules: [(ValidationContext) -> Visitor] = [
     PossibleFragmentSpreadsRule,
     NoFragmentCyclesRule,
     UniqueVariableNamesRule,
-//    NoUndefinedVariablesRule,
+    NoUndefinedVariablesRule,
     NoUnusedVariablesRule,
 //    KnownDirectivesRule,
 //    UniqueDirectivesPerLocationRule,