jextract: deduplicate protocol default JNI thunks

Sources/JExtractSwiftLib/JNI/JNISwift2JavaGenerator+SwiftThunkPrinting.swift

@@ -389,6 +389,7 @@ extension JNISwift2JavaGenerator {
   /// function (including requirements inherited from refined protocols).
   private func printExistentialBoxDispatchThunks(_ printer: inout SwiftPrinter, _ type: ExtractedNominalType) {
     let boxParentName = SwiftQualifiedTypeName(type.swiftNominal.javaExistentialBoxName)
+    var emittedCDeclSymbols: Set<String> = []
 
     for method in self.allProtocolRequirementMethods(of: type) {
       guard var translated = try? self.javaTranslator.translate(method) else {
@@ -397,6 +398,14 @@ extension JNISwift2JavaGenerator {
       }
       translated.parentName = boxParentName
 
+      let cName = cDeclSymbolName(for: translated)
+      // A protocol requirement and its default implementation can be represented
+      // by distinct extracted declarations while mapping to the same JNI symbol.
+      // The existential box only needs one dispatch thunk for that symbol.
+      guard emittedCDeclSymbols.insert(cName).inserted else {
+        continue
+      }
+
       printCDecl(&printer, translated) { printer in
         self.printFunctionDowncall(&printer, method)
       }
@@ -838,12 +847,15 @@ extension JNISwift2JavaGenerator {
     )
   }
 
-  private func printCDecl(
-    _ printer: inout SwiftPrinter,
-    _ translatedDecl: TranslatedFunctionDecl,
-    _ body: (inout SwiftPrinter) -> Void,
-  ) {
-    let nativeSignature = translatedDecl.nativeFunctionSignature
+  private func cDeclSymbolName(for translatedDecl: TranslatedFunctionDecl) -> String {
+    cDeclSymbolName(
+      javaMethodName: translatedDecl.nativeFunctionName,
+      parentName: translatedDecl.parentName,
+      parameters: nativeParameters(for: translatedDecl.nativeFunctionSignature)
+    )
+  }
+
+  private func nativeParameters(for nativeSignature: NativeFunctionSignature) -> [JavaParameter] {
     var parameters = nativeSignature.parameters.flatMap(\.parameters)
 
     if let selfParameter = nativeSignature.selfParameter {
@@ -854,6 +866,17 @@ extension JNISwift2JavaGenerator {
     }
     parameters += nativeSignature.result.outParameters
 
+    return parameters
+  }
+
+  private func printCDecl(
+    _ printer: inout SwiftPrinter,
+    _ translatedDecl: TranslatedFunctionDecl,
+    _ body: (inout SwiftPrinter) -> Void,
+  ) {
+    let nativeSignature = translatedDecl.nativeFunctionSignature
+    let parameters = nativeParameters(for: nativeSignature)
+
     printCDecl(
       &printer,
       javaMethodName: translatedDecl.nativeFunctionName,
@@ -873,15 +896,10 @@ extension JNISwift2JavaGenerator {
     resultType: JavaType,
     _ body: (inout SwiftPrinter) -> Void,
   ) {
-    let jniSignature = parameters.reduce(into: "") { signature, parameter in
-      signature += parameter.type.jniTypeSignature
-    }
-
-    let cName = String.jniSymbolName(
-      package: self.javaPackage,
-      parent: parentName,
-      method: javaMethodName,
-      signature: jniSignature,
+    let cName = cDeclSymbolName(
+      javaMethodName: javaMethodName,
+      parentName: parentName,
+      parameters: parameters
     )
 
     self.generatedCDeclSymbolNames.append(cName)
@@ -914,6 +932,23 @@ extension JNISwift2JavaGenerator {
     }
   }
 
+  private func cDeclSymbolName(
+    javaMethodName: String,
+    parentName: SwiftQualifiedTypeName,
+    parameters: [JavaParameter],
+  ) -> String {
+    let jniSignature = parameters.reduce(into: "") { signature, parameter in
+      signature += parameter.type.jniTypeSignature
+    }
+
+    return String.jniSymbolName(
+      package: self.javaPackage,
+      parent: parentName,
+      method: javaMethodName,
+      signature: jniSignature,
+    )
+  }
+
   private func printJNICache(_ printer: inout SwiftPrinter, _ type: ExtractedNominalType) {
     let cacheName = JNICaching.cacheName(for: type)
     let jniClassName = "\(javaPackagePath)/\(type.effectiveJavaTypeName.jniEscapedName)"

Tests/JExtractSwiftTests/Asserts/TextAssertions.swift

@@ -45,6 +45,7 @@ func assertOutput(
   moduleJavaPackages: [String: String] = [:],
   expectedChunks: [String],
   notExpectedChunks: [String] = [],
+  expectedOccurrences: [String: Int] = [:],
   fileID: String = #fileID,
   filePath: String = #filePath,
   line: Int = #line,
@@ -112,6 +113,7 @@ func assertOutput(
     dump: dump,
     expectedChunks: expectedChunks,
     notExpectedChunks: notExpectedChunks,
+    expectedOccurrences: expectedOccurrences,
     detectChunkByInitialLines: _detectChunkByInitialLines,
     fileID: fileID,
     filePath: filePath,
@@ -126,6 +128,7 @@ func assertOutput(
   dump: Bool = false,
   expectedChunks: [String],
   notExpectedChunks: [String] = [],
+  expectedOccurrences: [String: Int] = [:],
   detectChunkByInitialLines _detectChunkByInitialLines: Int = 4,
   fileID: String = #fileID,
   filePath: String = #filePath,
@@ -149,6 +152,15 @@ func assertOutput(
     )
   }
 
+  for (expectedChunk, expectedCount) in expectedOccurrences {
+    let actualCount = output.components(separatedBy: expectedChunk).count - 1
+    #expect(
+      actualCount == expectedCount,
+      "Expected '\(expectedChunk)' to occur \(expectedCount) time(s), but found \(actualCount).",
+      sourceLocation: sourceLocation
+    )
+  }
+
   let gotLines = output.split(separator: "\n").filter { l in
     l.trimmingCharacters(in: CharacterSet.whitespacesAndNewlines).count > 0
   }

Tests/JExtractSwiftTests/JNI/JNIProtocolTests.swift

@@ -82,6 +82,18 @@ struct JNIProtocolTests {
       public func makeChild() -> any ChildProtocol
     """
 
+  let protocolDefaultImplementationSource = """
+      public protocol Test {
+        public func action()
+      }
+
+      public extension Test {
+        public func action() {}
+      }
+
+      public func makeTest() -> any Test
+    """
+
   @Test
   func generatesJavaInterface() throws {
     try assertOutput(
@@ -542,6 +554,31 @@ struct JNIProtocolTests {
     )
   }
 
+  @Test
+  func existentialBoxDispatchThunkWithDefaultImplementationIsUnique() throws {
+    var config = config
+    config.enableJavaCallbacks = false
+
+    try assertOutput(
+      input: protocolDefaultImplementationSource,
+      config: config,
+      .jni,
+      .swift,
+      detectChunkByInitialLines: 1,
+      expectedChunks: [
+        """
+        @_cdecl("Java_com_example_swift_TestBox__00024action_1__JJ")
+        public func Java_com_example_swift_TestBox__00024action_1__JJ(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass, selfPointer: jlong, selfTypePointer: jlong) {
+          ...
+        }
+        """
+      ],
+      expectedOccurrences: [
+        "@_cdecl(\"Java_com_example_swift_TestBox__00024action_1__JJ\")": 1
+      ]
+    )
+  }
+
   @Test
   func existentialBoxPropertyAccessors_swift() throws {
     try assertOutput(