Warn when a target contains an unrecognized YAML key

Sources/ProjectSpec/SpecLoader.swift

@@ -51,7 +51,17 @@ public class SpecLoader {
 private extension Dictionary where Key == String, Value: Any {
 
     func validateWarnings() throws {
-        let errors: [SpecValidationError.ValidationError] = []
+        var errors: [SpecValidationError.ValidationError] = []
+
+        if let targets = self["targets"] as? [String: Any] {
+            for (targetName, targetValue) in targets {
+                guard let targetDict = targetValue as? JSONDictionary else { continue }
+                let unknownKeys = Set(targetDict.keys).subtracting(Target.validYAMLKeys)
+                for key in unknownKeys.sorted() {
+                    errors.append(.unknownTargetKey(target: targetName, key: key))
+                }
+            }
+        }
 
         if !errors.isEmpty {
             throw SpecValidationError(errors: errors)

Sources/ProjectSpec/SpecValidationError.swift

@@ -44,6 +44,7 @@ public struct SpecValidationError: Error, CustomStringConvertible {
         case invalidPluginPackageReference(plugin: String, package: String)
         case emptySourcePath(target: String)
         case invalidProjectFormat(String)
+        case unknownTargetKey(target: String, key: String)
 
         public var description: String {
             switch self {
@@ -116,6 +117,8 @@ public struct SpecValidationError: Error, CustomStringConvertible {
             case let .invalidProjectFormat(projectFormat):
                 let validValues = ProjectFormat.allCases.map(\.rawValue).joined(separator: ", ")
                 return "Invalid projectFormat \(projectFormat.quoted). Valid values are: \(validValues)"
+            case let .unknownTargetKey(target, key):
+                return "Target \(target.quoted) has an unrecognized key \(key.quoted). This key will be ignored. Check the XcodeGen documentation for valid target keys."
             }
         }
     }

Sources/ProjectSpec/Target.swift

@@ -275,6 +275,20 @@ extension LegacyTarget: JSONEncodable {
 
 extension Target: NamedJSONDictionaryConvertible {
 
+    /// All YAML keys recognized at the top level of a target definition.
+    public static let validYAMLKeys: Set<String> = [
+        "name", "productName", "type", "platform", "supportedDestinations",
+        "isMultiPlatformTarget", "deploymentTarget", "settings", "configFiles",
+        "sources", "dependencies", "buildToolPlugins", "info", "entitlements",
+        "transitivelyLinkDependencies", "directlyEmbedCarthageDependencies",
+        "requiresObjCLinking", "preBuildScripts", "prebuildScripts",
+        "postCompileScripts", "postBuildScripts", "postbuildScripts",
+        "buildRules", "scheme", "legacy", "attributes",
+        "onlyCopyFilesOnInstall", "putResourcesBeforeSourcesBuildPhase",
+        // Template-related keys consumed before Target.init is called.
+        "templates", "templateAttributes",
+    ]
+
     public init(name: String, jsonDictionary: JSONDictionary) throws {
         let resolvedName: String = jsonDictionary.json(atKeyPath: "name") ?? name
         self.name = resolvedName

Tests/ProjectSpecTests/SpecLoadingTests.swift

@@ -423,6 +423,36 @@ class SpecLoadingTests: XCTestCase {
                     throw failure("Expected to not throw a validation error. Got: \(error)")
                 }
             }
+
+            $0.it("warns about unrecognized target keys") {
+                let dictionary: [String: Any] = [
+                    "name": "TestUnknownTargetKey",
+                    "targets": [
+                        "App": [
+                            "type": "application",
+                            "platform": "iOS",
+                            "sources": ["Sources/App"],
+                            "resources": ["Resources"],
+                        ],
+                    ],
+                ]
+                try? Yams.dump(object: dictionary).write(toFile: path.string, atomically: true, encoding: .utf8)
+                let specLoader = SpecLoader(version: "1.1.0")
+                do {
+                    _ = try specLoader.loadProject(path: path)
+                } catch {
+                    throw failure("loadProject should not throw for unknown keys. Got: \(error)")
+                }
+                do {
+                    try specLoader.validateProjectDictionaryWarnings()
+                    throw failure("Expected a validation error for the unknown 'resources' key")
+                } catch let error as SpecValidationError {
+                    let expected = SpecValidationError.ValidationError.unknownTargetKey(target: "App", key: "resources")
+                    try expect(error.errors.contains(expected)) == true
+                } catch {
+                    throw failure("Unexpected error type: \(error)")
+                }
+            }
         }
     }