BridgeJS: Support non-ConvertibleToJSValue async exported return types

Async exported functions previously required a return type conforming to
ConvertibleToJSValue, because the thunk wrapped the body in JSPromise.async
and lowered the result via .jsValue. Types like @js structs and enums have no
.jsValue, so async functions could not return them.

Settle every async exported return through a new _bjs_makePromise intrinsic
instead: the thunk creates a JS Promise synchronously, then resolves or rejects
it from a Task using generated Promise_resolve_<mangled> / Promise_reject thunks
that lower the value through the regular imported-parameter ABI. This replaces
the JSPromise.async / .jsValue path entirely, unifying async return codegen on
a single path.

Covers all bridged types and their compositions: @js struct, raw-value and case
enums, their Optionals, Arrays, and Dictionaries, alongside the existing
ConvertibleToJSValue types and Void. Stack-using parameters are hoisted and
lifted in the thunk before the deferred Task runs, so the shared bridge stack is
drained synchronously. Async returns of types that cannot be lowered through the
imported-parameter ABI (associated-value enums, protocols, namespace enums) are
diagnosed rather than miscompiled.

The resolve/reject settlers are stashed on the Promise under a Symbol to avoid
clashing with its fields.

Author: krodak

Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift

@@ -90,11 +90,77 @@ public class ExportSwift {
                 decls.append(contentsOf: try renderSingleExportedClass(klass: klass))
             }
         }
+
+        try withSpan("Render Async Promise Helpers") { [self] in
+            let asyncResolveTypes = skeleton.asyncPromiseResolveReturnTypes
+            if !asyncResolveTypes.isEmpty {
+                decls.append(contentsOf: try renderPromiseRejectHelper())
+                for type in asyncResolveTypes {
+                    decls.append(contentsOf: try renderPromiseResolveHelper(type))
+                }
+            }
+        }
         return withSpan("Format Export Glue") {
             return decls.map { $0.description }.joined(separator: "\n\n")
         }
     }
 
+    /// Generates the per-type `Promise_resolve_<mangled>` settlement helper.
+    private func renderPromiseResolveHelper(_ type: BridgeType) throws -> [DeclSyntax] {
+        try renderPromiseSettleHelper(
+            functionName: "Promise_resolve_\(type.mangleTypeName)",
+            externName: "promise_resolve_\(moduleName)_\(type.mangleTypeName)",
+            valueType: type
+        )
+    }
+
+    /// Generates the shared `Promise_reject` settlement helper.
+    private func renderPromiseRejectHelper() throws -> [DeclSyntax] {
+        try renderPromiseSettleHelper(
+            functionName: "Promise_reject",
+            externName: "promise_reject_\(moduleName)",
+            valueType: .jsValue
+        )
+    }
+
+    /// Generates a `@JSFunction func <functionName>(_ promise: JSObject, _ value: T)` and its
+    /// glue, lowering `value` through the standard imported-parameter ABI.
+    private func renderPromiseSettleHelper(
+        functionName: String,
+        externName: String,
+        valueType: BridgeType
+    ) throws -> [DeclSyntax] {
+        let effects = Effects(isAsync: false, isThrows: true)
+        // `Void` can't cross the bridge as a parameter, so the void helper takes only the promise.
+        var parameters = [Parameter(label: nil, name: "promise", type: .jsObject(nil))]
+        if valueType != .void {
+            parameters.append(Parameter(label: nil, name: "value", type: valueType))
+        }
+        let builder = try ImportTS.CallJSEmission(
+            moduleName: "bjs",
+            abiName: externName,
+            effects: effects,
+            returnType: .void,
+            context: .importTS
+        )
+        for parameter in parameters {
+            try builder.lowerParameter(param: parameter)
+        }
+        try builder.call()
+        try builder.liftReturnValue()
+
+        let valueParam = valueType == .void ? "" : ", _ value: \(valueType.swiftType)"
+        let macroDecl: DeclSyntax =
+            "@JSFunction func \(raw: functionName)(_ promise: JSObject\(raw: valueParam)) throws(JSException)"
+        let glueDecl = builder.renderThunkDecl(
+            name: "_$\(functionName)",
+            parameters: parameters,
+            returnType: .void,
+            effects: effects
+        )
+        return [macroDecl, builder.renderImportDecl(), glueDecl]
+    }
+
     class ExportedThunkBuilder {
         var body: [CodeBlockItemSyntax] = []
         var liftedParameterExprs: [ExprSyntax] = []
@@ -104,6 +170,12 @@ public class ExportSwift {
         var externDecls: [DeclSyntax] = []
         let effects: Effects
 
+        /// When set, the async thunk settles via `_bjs_makePromise` rather than the `.jsValue` path.
+        var asyncResolveReturnType: BridgeType?
+
+        /// Stack-using parameter lifts hoisted ahead of the deferred async closure.
+        var asyncHoistedBindings: [CodeBlockItemSyntax] = []
+
         init(effects: Effects) {
             self.effects = effects
         }
@@ -200,6 +272,9 @@ public class ExportSwift {
             }
 
             if effects.isAsync, returnType != .void {
+                if asyncResolveReturnType != nil {
+                    return CodeBlockItemSyntax(item: .init(StmtSyntax("return \(raw: callExpr)")))
+                }
                 return CodeBlockItemSyntax(item: .init(StmtSyntax("return \(raw: callExpr).jsValue")))
             }
 
@@ -244,6 +319,22 @@ public class ExportSwift {
                 param.type.isStackUsingParameter ? index : nil
             }
 
+            if effects.isAsync {
+                // Drain stack parameters before the deferred `Task` or the shared stack is corrupted.
+                for index in stackParamIndices.reversed() {
+                    let param = parameters[index]
+                    let expr = liftedParameterExprs[index]
+                    let varName = "_tmp_\(param.name)"
+                    var binding: CodeBlockItemSyntax = "let \(raw: varName) = \(expr)"
+                    if !asyncHoistedBindings.isEmpty {
+                        binding = binding.with(\.leadingTrivia, .newline)
+                    }
+                    asyncHoistedBindings.append(binding)
+                    liftedParameterExprs[index] = ExprSyntax(DeclReferenceExprSyntax(baseName: .identifier(varName)))
+                }
+                return
+            }
+
             guard stackParamIndices.count > 1 else { return }
 
             for index in stackParamIndices.reversed() {
@@ -293,8 +384,7 @@ public class ExportSwift {
                 return
             }
             if effects.isAsync {
-                // The return value of async function (T of `(...) async -> T`) is
-                // handled by the JSPromise.async, so we don't need to do anything here.
+                // The async return value is lowered by the generated `Promise_resolve_*` helper.
                 return
             }
 
@@ -328,25 +418,25 @@ public class ExportSwift {
             }
         }
 
+        /// A throwing async body needs an explicit closure type, otherwise Swift infers
+        /// `throws(any Error)` instead of `throws(JSException)`.
+        /// See: https://github.com/swiftlang/swift/issues/76165
+        private func asyncThrowsClosureHead(returnSpelling: String?) -> String {
+            guard effects.isThrows else { return "" }
+            let returns = returnSpelling.map { " -> \($0)" } ?? ""
+            return " () async throws(JSException)\(returns) in"
+        }
+
         func render(abiName: String) -> DeclSyntax {
             let body: CodeBlockItemListSyntax
-            if effects.isAsync {
-                // Explicit closure type annotation needed when throws is present
-                // so Swift infers throws(JSException) instead of throws(any Error)
-                // See: https://github.com/swiftlang/swift/issues/76165
-                let closureHead: String
-                if effects.isThrows {
-                    let hasReturn = self.body.contains { $0.description.contains("return ") }
-                    let ret = hasReturn ? " -> JSValue" : ""
-                    closureHead = " () async throws(JSException)\(ret) in"
-                } else {
-                    closureHead = ""
-                }
+            if effects.isAsync, let resolveType = asyncResolveReturnType {
+                let resolveName = "Promise_resolve_\(resolveType.mangleTypeName)"
+                let closureHead = asyncThrowsClosureHead(returnSpelling: resolveType.swiftType)
                 body = """
-                    let ret = JSPromise.async {\(raw: closureHead)
+                    \(CodeBlockItemListSyntax(asyncHoistedBindings))
+                    return _bjs_makePromise(resolve: \(raw: resolveName), reject: Promise_reject) {\(raw: closureHead)
                         \(CodeBlockItemListSyntax(self.body))
-                    }.jsObject
-                    return ret.bridgeJSLowerReturn()
+                    }
                     """
             } else if effects.isThrows {
                 body = """
@@ -506,8 +596,23 @@ public class ExportSwift {
         return decls
     }
 
+    private func configureAsyncResolve(
+        _ builder: ExportedThunkBuilder,
+        returnType: BridgeType,
+        effects: Effects
+    ) throws {
+        guard effects.isAsync else { return }
+        guard returnType.isAsyncResolvable else {
+            throw BridgeJSCoreError(
+                "Returning '\(returnType.swiftType)' from an async exported function is not yet supported"
+            )
+        }
+        builder.asyncResolveReturnType = returnType
+    }
+
     func renderSingleExportedFunction(function: ExportedFunction) throws -> DeclSyntax {
         let builder = ExportedThunkBuilder(effects: function.effects)
+        try configureAsyncResolve(builder, returnType: function.returnType, effects: function.effects)
         for param in function.parameters {
             try builder.liftParameter(param: param)
         }
@@ -551,6 +656,7 @@ public class ExportSwift {
         instanceSelfType: BridgeType
     ) throws -> DeclSyntax {
         let builder = ExportedThunkBuilder(effects: method.effects)
+        try configureAsyncResolve(builder, returnType: method.returnType, effects: method.effects)
         if !method.effects.isStatic {
             try builder.liftParameter(param: Parameter(label: nil, name: "_self", type: instanceSelfType))
         }

Plugins/BridgeJS/Sources/BridgeJSLink/BridgeJSLink.swift

@@ -356,6 +356,40 @@ public struct BridgeJSLink {
         ]
     }
 
+    /// JS const (in the import glue scope) holding the `Symbol` under which a promise's
+    /// resolve/reject settlers are stashed.
+    private static let promiseSettlersSymbol = "__bjs_promiseSettlers"
+
+    /// Renders a `bjs[...]` settlement handler that lifts `(promise, value)` and calls the
+    /// promise's stashed `resolve` / `reject` settler.
+    private func renderPromiseSettleHandler(
+        externName: String,
+        valueType: BridgeType,
+        settle: String,
+        into printer: CodeFragmentPrinter
+    ) throws {
+        let builder = ImportedThunkBuilder(
+            effects: Effects(isAsync: false, isThrows: true),
+            returnType: .void,
+            intrinsicRegistry: intrinsicRegistry
+        )
+        try builder.liftParameter(param: Parameter(label: nil, name: "promise", type: .jsObject(nil)))
+        // `Void` can't cross the bridge as a parameter, so the void resolve settles with `undefined`.
+        let valueArg: String
+        if valueType == .void {
+            valueArg = ""
+        } else {
+            try builder.liftParameter(param: Parameter(label: nil, name: "value", type: valueType))
+            valueArg = builder.parameterForwardings[1]
+        }
+        builder.body.write(
+            "\(builder.parameterForwardings[0])[\(Self.promiseSettlersSymbol)].\(settle)(\(valueArg));"
+        )
+        var lines = builder.renderFunction(name: nil)
+        lines[0] = "bjs[\"\(externName)\"] = \(lines[0])"
+        printer.write(lines: lines)
+    }
+
     private func generateAddImports(needsImportsObject: Bool) throws -> CodeFragmentPrinter {
         let printer = CodeFragmentPrinter()
         let allStructs = skeletons.compactMap { $0.exported?.structs }.flatMap { $0 }
@@ -526,6 +560,39 @@ public struct BridgeJSLink {
                     }
                 }
 
+                // Always provided: the runtime's `_bjs_makePromise` imports it unconditionally.
+                // The settlers are stored under a Symbol to avoid clashing with promise fields.
+                printer.write("const \(Self.promiseSettlersSymbol) = Symbol(\"JavaScriptKit.promiseSettlers\");")
+                printer.write("bjs[\"swift_js_make_promise\"] = function() {")
+                printer.indent {
+                    printer.write("let resolve, reject;")
+                    printer.write("const promise = new Promise((res, rej) => { resolve = res; reject = rej; });")
+                    printer.write("promise[\(Self.promiseSettlersSymbol)] = { resolve, reject };")
+                    printer.write(
+                        "return \(JSGlueVariableScope.reservedSwift).\(JSGlueVariableScope.reservedMemory).retain(promise);"
+                    )
+                }
+                printer.write("}")
+                for skeleton in skeletons {
+                    guard let exported = skeleton.exported else { continue }
+                    let asyncResolveTypes = exported.asyncPromiseResolveReturnTypes
+                    guard !asyncResolveTypes.isEmpty else { continue }
+                    for type in asyncResolveTypes {
+                        try renderPromiseSettleHandler(
+                            externName: "promise_resolve_\(skeleton.moduleName)_\(type.mangleTypeName)",
+                            valueType: type,
+                            settle: "resolve",
+                            into: printer
+                        )
+                    }
+                    try renderPromiseSettleHandler(
+                        externName: "promise_reject_\(skeleton.moduleName)",
+                        valueType: .jsValue,
+                        settle: "reject",
+                        into: printer
+                    )
+                }
+
                 printer.write("bjs[\"swift_js_return_optional_bool\"] = function(isSome, value) {")
                 printer.indent {
                     printer.write("if (isSome === 0) {")

Plugins/BridgeJS/Sources/BridgeJSSkeleton/BridgeJSSkeleton.swift

@@ -1027,6 +1027,30 @@ public struct ExportedSkeleton: Codable {
     public var isEmpty: Bool {
         functions.isEmpty && classes.isEmpty && enums.isEmpty && structs.isEmpty && protocols.isEmpty
     }
+
+    /// Distinct `async` return types needing a `Promise_resolve_<mangled>` helper, deduplicated
+    /// by mangled name. Shared by the Swift codegen and JS link.
+    public var asyncPromiseResolveReturnTypes: [BridgeType] {
+        var seen = Set<String>()
+        var result: [BridgeType] = []
+        func consider(_ returnType: BridgeType, _ effects: Effects) {
+            guard effects.isAsync, returnType.isAsyncResolvable,
+                seen.insert(returnType.mangleTypeName).inserted
+            else { return }
+            result.append(returnType)
+        }
+        for function in functions { consider(function.returnType, function.effects) }
+        for klass in classes {
+            for method in klass.methods { consider(method.returnType, method.effects) }
+        }
+        for structDef in structs {
+            for method in structDef.methods { consider(method.returnType, method.effects) }
+        }
+        for enumDef in enums {
+            for method in enumDef.staticMethods { consider(method.returnType, method.effects) }
+        }
+        return result
+    }
 }
 
 // MARK: - Imported Skeleton
@@ -1584,6 +1608,25 @@ extension BridgeType {
         return false
     }
 
+    /// Whether a value of this type can be passed to a generated `Promise_resolve_<mangled>`
+    /// settlement helper, i.e. lowered through the imported-parameter ABI. Every `async`
+    /// exported return settles through `_bjs_makePromise`; the few types that cannot be lowered
+    /// (associated-value enums, protocols, namespace enums, and their compositions) are diagnosed.
+    public var isAsyncResolvable: Bool {
+        switch self {
+        case .associatedValueEnum, .swiftProtocol, .namespaceEnum:
+            return false
+        case .nullable(let wrapped, _):
+            return wrapped.isAsyncResolvable
+        case .array(let element):
+            return element.isAsyncResolvable
+        case .dictionary(let value):
+            return value.isAsyncResolvable
+        default:
+            return true
+        }
+    }
+
     /// Simplified Swift ABI-style mangled name
     /// https://github.com/swiftlang/swift/blob/main/docs/ABI/Mangling.rst#types
     public var mangleTypeName: String {