Conditional type annotation

[Sunbreak]

Background

I'm working on Dart wrapper via dart:ffi for libusb: https://github.com/woodemi/libusb.dart

Problem

libusb_linux.dart is identical to libusb_windows.dart while libusb_mac.dart is a little different

• Windows/Linux

class timeval extends ffi.Struct {
  // ...
  @ffi.Int64()
  int tv_usec;
}

• macOS

class timeval extends ffi.Struct {
  // ...
  @ffi.Int32()
  int tv_usec;
}

Calling the APIs requires import 'packages:libusb/libusb_windows.dart for Windows, libusb_macos.dart for macOS and libusb_linux.dart for Linux

https://github.com/woodemi/libusb.dart/blob/afabb0fb3deeef240b6a893cf79a92cf1aa878f8/example/listdevs.dart#L7-L10

// git update-index --assume-unchanged example/listdevs.dart
// import 'package:libusb/libusb_windows.dart';
// import 'package:libusb/libusb_macos.dart';
// import 'package:libusb/libusb_linux.dart';

It's quite cumbersome and error prone

Feature Request

Could we support something like

class timeval extends ffi.Struct {
  // ...
  @ffi.Int64() if (Platforms.isMac) @ffi.Int32()
  int tv_usec;
}

[Sunbreak]

BTW, Conditional Import via Platforms.isWindows/isMacOS/isLinux would work as well

import 'timeval64.dart if (Platforms.isMac) timeval32.dart';

However, Conditional Import is only available for Dart SDK library, i.e. dart.library.io/dart.library.js/dart.library.html/dart.library.ffi as far as I know

[dcharkes]

Yes, we are tracking this here: dart-lang/sdk#42563.

[dcharkes]

I'll go ahead and close this issue, as this is not a package:ffi issue, but rather a dart:ffi feature, and we're already tracking it.