Add executorFactory option to package manifest.

Sources/PackageDescription/BuildSettings.swift

@@ -486,6 +486,23 @@ public struct SwiftSetting: Sendable {
         return SwiftSetting(
             name: "defaultIsolation", value: [isolationString], condition: condition)
     }
+
+    /// Defines an `-executor-factory` to pass to the
+    /// corresponding build tool.
+    ///
+    /// - Since: First available in PackageDescription 6.2
+    ///
+    /// - Parameters:
+    ///   - factory: The type name of the executor factory that should be used.
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func executorFactory(
+      _ factory: String,
+      _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        return SwiftSetting(
+          name: "swiftExecutorFactory", value: [name], condition: condition)
+    }
 }
 
 /// A linker build setting.

Sources/PackageLoading/ManifestJSONParser.swift

@@ -758,6 +758,11 @@ extension TargetBuildSettingDescription.Kind {
             }
 
             return .defaultIsolation(isolation)
+        case "swiftExecutorFactory":
+            guard let value = values.first else {
+                throw InternalError("invalid (empty) build settings value")
+            }
+            return .swiftExecutorFactory(value)
         default:
             throw InternalError("invalid build setting \(name)")
         }

Sources/PackageLoading/PackageBuilder.swift

@@ -1263,6 +1263,17 @@ public final class PackageBuilder {
                 }
 
                 values = ["-default-isolation", isolation.rawValue]
+
+            case .swiftExecutorFactory(let factory):
+                switch setting.tool {
+                case .c, .cxx, .linker:
+                    throw InternalError("only Swift supports executor factory")
+
+                case .swift:
+                  decl = .OTHER_SWIFT_FLAGS
+                }
+
+                values = ["-executor-factory ", factory]
             }
 
             // Create an assignment for this setting.

Sources/PackageModel/Manifest/TargetBuildSettingDescription.swift

@@ -49,14 +49,16 @@ public enum TargetBuildSettingDescription {
 
         case defaultIsolation(DefaultIsolation)
 
+        case executorFactory(String)
+
         public var isUnsafeFlags: Bool {
             switch self {
             case .unsafeFlags(let flags):
                 // If `.unsafeFlags` is used, but doesn't specify any flags, we treat it the same way as not specifying it.
                 return !flags.isEmpty
             case .headerSearchPath, .define, .linkedLibrary, .linkedFramework, .interoperabilityMode,
                  .enableUpcomingFeature, .enableExperimentalFeature, .strictMemorySafety, .swiftLanguageMode,
-                 .defaultIsolation:
+                 .defaultIsolation, executorFactory:
                 return false
             }
         }

Sources/PackageModel/ManifestSourceGeneration.swift

@@ -584,6 +584,9 @@ fileprivate extension SourceCodeFragment {
                 params.append(SourceCodeFragment(from: condition))
             }
             self.init(enum: setting.kind.name, subnodes: params)
+        case .swiftExecutorFactory(let factory):
+            params.append(SourceCodeFragment(string: factory))
+            self.init(enum: setting.kind.name, subnodes: params)
         }
     }