[piercifani]

Stack 16/21 · base: jni/15-nullable-optionals · previous: #19 · next: #21 (jni/17-closure-results)

Problem

A non-escaping closure whose arguments or result are jextracted types was skipped: func transform(_ item: Item, _ f: (Item) -> Item) -> Item didn't reach Java. The non-escaping closure lowering converts primitives only.

Changes

When the non-escaping lowering can't translate a closure, it now goes through the escaping one. Its @JavaInterface wrapper already converts jextracted types, optionals of them and strings, and the Swift closure it builds can be passed where a non-escaping one is expected.

• Swift: the immediately-called literal that builds the closure spells out its type, { () -> (Item) -> Item in … }(), for a non-escaping parameter. Inferred from that parameter, the returned closure would be non-escaping, and returning it wouldn't compile.
• Java: callers see the same functional interface as before, <Type>.<method>.<parameter>, with jextracted types in its apply.
• Unchanged: closures the non-escaping lowering already handled, such as primitive ones and those matching a JDK functional interface, keep it.

The escaping path costs a global reference per call to hold the Java object for as long as the Swift closure lives, here for the duration of the call.

Kotlin in this PR

ClosuresTest.kt with Closures.swift:

// public func transform(id: String, _ f: (Item) -> Item) -> Item
val out = Notifier.init().transform("a") { item -> Item.init(item.id + "!", item.count + 1) }
assertEquals("a!", out.id)

Why Skip needs it

Skip passes Kotlin lambdas for Swift closure parameters whatever their types, and non-escaping is Swift's default, so closures over the app's own structs are common.

Testing

• Kotlin sample: ./gradlew :Samples:SwiftJavaExtractJNIKotlinSampleApp:test passes (43 tests).
• JNIEscapingClosureJExtractedTypesTests: two new tests check the @JavaInterface wrapper and the spelled-out closure type for a non-escaping (Item) -> Item, and the Java interface.
• JNI sample: ClosuresTest.nonEscapingStructClosure passes a struct-returning lambda to transformStruct.
• Full swift test --disable-experimental-prebuilts --disable-sandbox (675 tests) and ./gradlew :Samples:SwiftJavaExtractJNISampleApp:test (294 tests) pass.
• swift format lint --strict on changed files; git diff --check.

Not in this PR

• Closures as results and closure-typed properties.
• Async closures: upstream jextract/jni: Support async closures (#834) swiftlang/swift-java#913 makes their Java interface return a CompletableFuture that Swift awaits.

🤖 Generated with Claude Code