[jextract] Mark Sendable types as @ThreadSafe

Sources/JExtractSwiftLib/FFM/FFMSwift2JavaGenerator.swift

@@ -278,6 +278,9 @@ extension FFMSwift2JavaGenerator {
       parentProtocol = "SwiftValue"
     }
 
+    if decl.swiftNominal.isSendable {
+      printer.print("@ThreadSafe // Sendable")
+    }
     printer.printBraceBlock("public final class \(decl.swiftNominal.name) extends FFMSwiftInstance implements \(parentProtocol)") {
       printer in
       // Constants

Sources/JExtractSwiftLib/JNI/JNISwift2JavaGenerator+JavaBindingsPrinting.swift

@@ -171,6 +171,9 @@ extension JNISwift2JavaGenerator {
   private func printNominal(
     _ printer: inout CodePrinter, _ decl: ImportedNominalType, body: (inout CodePrinter) -> Void
   ) {
+    if decl.swiftNominal.isSendable {
+      printer.print("@ThreadSafe // Sendable")
+    }
     printer.printBraceBlock("public final class \(decl.swiftNominal.name) extends JNISwiftInstance") { printer in
       body(&printer)
     }

Sources/JExtractSwiftLib/SwiftTypes/SwiftNominalTypeDeclaration.swift

@@ -85,6 +85,22 @@ package class SwiftNominalTypeDeclaration: SwiftTypeDeclaration {
     super.init(moduleName: moduleName, name: node.name.text)
   }
 
+  /// Returns true if this type conforms to `Sendable` and therefore is "threadsafe".
+  lazy var isSendable: Bool = {
+    // Check if Sendable is in the inheritance list
+    guard let inheritanceClause = self.syntax?.inheritanceClause else {
+      return false
+    }
+
+    for inheritedType in inheritanceClause.inheritedTypes {
+      if inheritedType.type.trimmedDescription == "Sendable" {
+        return true
+      }
+    }
+
+    return false
+  }()
+
   /// Determine the known standard library type for this nominal type
   /// declaration.
   private func computeKnownStandardLibraryType() -> SwiftKnownTypeDeclKind? {

SwiftKitCore/src/main/java/org/swift/swiftkit/core/annotations/ThreadSafe.java

@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+package org.swift.swiftkit.core.annotations;
+
+import jdk.jfr.Description;
+import jdk.jfr.Label;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.*;
+
+/**
+ * Used to mark a type as thread-safe, i.e. no additional synchronization is necessary when accessing it
+ * from multiple threads.
+ *
+ * <p> In SwiftJava specifically, this attribute is applied when an extracted Swift type conforms to the Swift
+ * {@code Sendable} protocol, which is a compiler enforced mechanism to enforce thread-safety in Swift.
+ *
+ * @see <a href="https://developer.apple.com/documentation/Swift/Sendable">Swift Sendable API documentation</a>.
+ */
+@Documented
+@Label("Thread-safe")
+@Description("Value should be interpreted as safe to be shared across threads.")
+@Target({TYPE_USE})
+@Retention(RetentionPolicy.RUNTIME)
+public @interface ThreadSafe {
+}

Tests/JExtractSwiftTests/SendableTests.swift

@@ -0,0 +1,56 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import JExtractSwiftLib
+import Testing
+
+final class SendableTests {
+  let source =
+    """
+    public struct SendableStruct: Sendable {}
+    """
+
+
+  @Test("Import: Sendable struct (ffm)")
+  func sendableStruct_ffm() throws {
+
+    try assertOutput(
+      input: source, .ffm, .java,
+      expectedChunks: [
+        """
+        @ThreadSafe // Sendable
+        public final class SendableStruct extends FFMSwiftInstance implements SwiftValue {
+          static final String LIB_NAME = "SwiftModule";
+          static final Arena LIBRARY_ARENA = Arena.ofAuto();
+        """,
+      ]
+    )
+  }
+
+  @Test("Import: Sendable struct (jni)")
+  func sendableStruct_jni() throws {
+
+    try assertOutput(
+      input: source, .jni, .java,
+      expectedChunks: [
+        """
+        @ThreadSafe // Sendable
+        public final class SendableStruct extends JNISwiftInstance {
+          static final String LIB_NAME = "SwiftModule";
+        """,
+      ]
+    )
+  }
+
+}