[pigeon] Swift FFI (swift_use_ffi) generates code that doesn't compile for files with event channels or ProxyApis #193881

Found while investigating #193195.

Summary

Pigeon accepts Swift FFI mode for pigeon files that contain event channels or ProxyApis, but the Swift it generates doesn't compile. Users get a wall of Swift errors instead of a clear message at generation time.

Steps to reproduce

1. Write a pigeon file with a @HostApi() and an @EventChannelApi(), or use pigeon's own pigeons/event_channel_tests.dart or pigeons/proxy_api_tests.dart.
2. Generate Swift with FFI enabled (--swift_use_ffi, or SwiftOptions(useFfi: true)).
3. Type-check the generated file against the SDK, the way swiftgen compiles it:

   xcrun swiftc -typecheck -sdk "$(xcrun --show-sdk-path --sdk macosx)" Generated.swift

Expected results

Either pigeon rejects the input with a clear error, or the generated Swift compiles.

Actual results

Pigeon writes the file without complaint, and it fails to type-check:

Cause

• In FFI mode the Swift generator skips import Flutter/import FlutterMacOS (swift_generator.dart L318).
• It uses the FFI codec only when the file has no event channels and no ProxyApis. Otherwise it falls back to the channel codec, which is built on FlutterStandardReader/Writer (L530). The event channel and ProxyApi code also uses Flutter types directly.
• Swift FFI validation checks only @TaskQueue and package dependencies (pigeon_lib_internal.dart L603–620), so nothing stops the combination.
• Adding the Flutter import wouldn't fix it. swiftgen compiles the file with swiftc against the bare SDK, without a Flutter framework search path.

The Kotlin JNI side doesn't have this compile problem: in JNI mode the Kotlin generator imports the Flutter embedding for files with event channels or ProxyApis.

Proposed fix

1. Now: in Swift FFI validation, reject files that contain event channels or ProxyApis, with a message such as: "Event channels and ProxyApis are not supported with Swift FFI yet. Move them to a separate pigeon file that doesn't use swift_use_ffi."
2. Later, as separate designs:
   • event channels over native interop, for example as FlutterApi-style calls from native to Dart;
   • ProxyApis over FFI.

Environment

pigeon 29.0.6, swiftgen 0.2.0, ffigen 22.0.0, Xcode 16.4 (Swift 6.1.2), macOS SDK.