Unable to open DSM after upgrade to null-safety

[Sunbreak]

In branch: https://github.com/woodemi/twaindsm.dart/tree/repro/nullsafety-pointer-failure

Before null-safety

✅ allocate with empty struct: 0f8504b

Dart-beta: Dart SDK version: 2.12.0-259.16.beta (beta) (Tue Feb 23 00:10:18 2021 +0100) on "windows_x64"

class pTW_IDENTITY extends ffi.Struct {}

var identityPtr = ffi.allocate<pTW_IDENTITY>();
var connect = twainDsm.DSM_Entry(identityPtr,
  nullptr, DG_CONTROL, DAT_PARENT, MSG_OPENDSM, parentPtr.cast<Void>());

✅ allocate with Uint8 pointer: ed97d5e

Dart-beta: Dart SDK version: 2.12.0-259.16.beta (beta) (Tue Feb 23 00:10:18 2021 +0100) on "windows_x64"

...
var identityPtr = ffi.allocate<Uint8>(count: 156);
var connect = twainDsm.DSM_Entry(identityPtr.cast(), ...

After upgrading to null-safety

❌ calloc with opaque: 0ba8788

Dart-beta: Dart SDK version: 2.12.0-259.16.beta (beta) (Tue Feb 23 00:10:18 2021 +0100) on "windows_x64"

This doesn't compile: Opaque cannot...

class pTW_IDENTITY extends ffi.Opaque {}

var identityPtr = ffi.calloc<pTW_IDENTITY>();

❌ calloc with Uint8 pointer again: 84e0b7b

Dart-beta: Dart SDK version: 2.12.0-259.16.beta (beta) (Tue Feb 23 00:10:18 2021 +0100) on "windows_x64"

...
var identityPtr = ffi.allocate<Uint8>(count: 156);
var connect = twainDsm.DSM_Entry(identityPtr.cast(), ...

✅ use empty struct instead of opaque: 1f8168c

Dart-beta: Dart SDK version: 2.12.0-259.16.beta (beta) (Tue Feb 23 00:10:18 2021 +0100) on "windows_x64"

class pTW_IDENTITY extends ffi.Struct {}

var identityPtr = ffi.calloc<pTW_IDENTITY>();
...

❌ use inline-array & nested-struct: 4a151b8

Dart-dev: Dart SDK version: 2.13.0-97.0.dev (dev) (Tue Mar 2 19:57:46 2021 -0800) on "windows_x64"

class TW_VERSION extends ffi.Struct {
  @ffi.Uint16()
  external int MajorNum;

  @ffi.Uint16()
  external int MinorNum;

  @ffi.Uint16()
  external int Language;

  @ffi.Uint16()
  external int Country;
  
  @ffi.Array(34)
  external ffi.Array<ffi.Int8> Info;
}

class pTW_IDENTITY extends ffi.Struct {
  @ffi.Uint32()
  external int Id;

  external TW_VERSION Version;

  @ffi.Uint16()
  external int ProtocolMajor;

  @ffi.Uint16()
  external int ProtocolMinor;

  @ffi.Uint32()
  external int SupportedGroups;

  @ffi.Array(34)
  external ffi.Array<ffi.Int8> Manufacturer;

  @ffi.Array(34)
  external ffi.Array<ffi.Int8> ProductFamily;

  @ffi.Array(34)
  external ffi.Array<ffi.Int8> ProductName;
}