field_must_be_external_in_struct

Fields of 'Struct' and 'Union' subclasses must be marked external.

The analyzer produces this diagnostic when a field in a subclass of either Struct or Union isn't marked as being external.

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

The following code produces this diagnostic because the field a isn't marked as being external:

import 'dart:ffi';

final class C extends Struct {
  @Int16()
  int a;
}

Add the required external modifier:

import 'dart:ffi';

final class C extends Struct {
  @Int16()
  external int a;
}

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