[vm/ffi] Introduce SizedNativeType

[dcharkes]

Split off from discussion on:

• [vm/ffi] Pointer.asArray or Array.fromPointer #54526

TODO:

• Use in sizeOf and AllocatorAlloc.call.
• Use in Struct/Union documentation and error messages.

[rainyl]

I am wondering whether common operations or properties like Pointer<Uint8>.value, Pointer<Uint8> operator + (int offset) can also be added to SizedNativeType? This will simplify the usage like:

num getValue(int i0, int i1) {
  final ffi.Pointer<ffi.Uint8> pdata = dataPtr.$1;
  final (int, int, int) step = this.step;
  final MatType type = this.type;

  // https://github.com/opencv/opencv/blob/71d3237a093b60a27601c20e9ee6c3e52154e8b1/modules/core/include/opencv2/core/mat.inl.hpp#L894
  final ptr = pdata + i0 * step.$1;
  return switch (type.depth) {
    MatType.CV_8U => (ptr.cast<ffi.Uint8>() + i1).value,
    MatType.CV_8S => (ptr.cast<ffi.Int8>() + i1).value,
    MatType.CV_16U => (ptr.cast<ffi.Uint16>() + i1).value,
    MatType.CV_16S => (ptr.cast<ffi.Int16>() + i1).value,
    MatType.CV_32S => (ptr.cast<ffi.Int>() + i1).value,
    MatType.CV_32F => (ptr.cast<ffi.Float>() + i1).value,
    MatType.CV_64F => (ptr.cast<ffi.Double>() + i1).value,
    MatType.CV_16F => float16((ptr.cast<ffi.Uint16>() + i1).value),
    _ => throw UnsupportedError("Unsupported type: $type")
  };
}

As you can see, without common operations/fields defined in SizedNativeType, I have to test the desired pointer type to cast, but if they were defined, the above code can be simplified to:

num getValue<T extends ffi.SizedNativeType>(int i0, int i1) {
  final pdata = dataPtr.$1;
  final step = this.step;
  // https://github.com/opencv/opencv/blob/71d3237a093b60a27601c20e9ee6c3e52154e8b1/modules/core/include/opencv2/core/mat.inl.hpp#L894
  final ptr = pdata + i0 * step.$1;
  return (ptr.cast<T>() + i1).value;
}

[dcharkes]

I am wondering whether common operations or properties like Pointer<Uint8>.value, Pointer<Uint8> operator + (int offset) can also be added to SizedNativeType? This will simplify the usage like:

That will unfortunately not work. We cannot branch on the runtime type argument of Pointer. The type argument is always Never at runtime, because our goal is that a Pointer is just an int in the Dart runtime:

• [vm/ffi] Unbox Pointer #50777

[rainyl]

get it, thx~