Structs must have at most one 'Packed' annotationDRT-W1559

The analyzer produces this diagnostic when a subclass of Struct has more than one Packed annotation.

For more information about FFI, see C interop using dart:ffi.

Example

The following code produces this diagnostic because the class C, which is a subclass of Struct, has two Packed annotations:

import 'dart:ffi';

@Packed(1)
@Packed(1)
final class C extends Struct {
  external Pointer<Uint8> notEmpty;
}

Common fixes

Remove all but one of the annotations:

import 'dart:ffi';

@Packed(1)
final class C extends Struct {
  external Pointer<Uint8> notEmpty;
}