C#
public record Dto(int Id);

C#
public record Dto(int Id);

C#
public async Task DoSomethingAsync() {
  var dto = await GetSomeDto();
  var my = Build(dto);
  await DoSomethingWith(my);
}
private My Build(Dto someDto) => new MyBuilder().WithId(someDto.Id).Build();

C#
public async Task DoSomethingAsync() {
  var dto = await GetSomeDto();
  var my = Build(dto);
  await DoSomethingWith(my);
}
private My Build(Dto someDto) => new MyBuilder().WithId(someDto.Id).Build();

C#
public struct Example {
  public int First;
  // for simplicity, I have one member. This is source-generated: imagine the example with twenty members and equally many builder extensions
}
public static class ExampleExtensions {
  // I do not want to accept a copy of this instance, but a reference to the instance itself
  public static Example WithFirstByCopy(this Example e, int first) {
    e.First = first;
    // I do not want to return a copy of this instance, but the ref instance I would like to receive
    return e;
  }
  public static ref Example WithFirstByRef(ref this Example e, int first) {
    e.First = first;
    return ref e;
  }
  // This does not compile so it does not suit my needs
  public static Example DoesNotCompile() {
    return new Example().WithFirstByRef(1);
  }
  // This copies 3? times so it does not suit my needs
  public static Example CopiesBuilder() {
    return new Example().WithFirstByCopy(1);
  }
}

C#
public struct Example {
  public int First;
  // for simplicity, I have one member. This is source-generated: imagine the example with twenty members and equally many builder extensions
}
public static class ExampleExtensions {
  // I do not want to accept a copy of this instance, but a reference to the instance itself
  public static Example WithFirstByCopy(this Example e, int first) {
    e.First = first;
    // I do not want to return a copy of this instance, but the ref instance I would like to receive
    return e;
  }
  public static ref Example WithFirstByRef(ref this Example e, int first) {
    e.First = first;
    return ref e;
  }
  // This does not compile so it does not suit my needs
  public static Example DoesNotCompile() {
    return new Example().WithFirstByRef(1);
  }
  // This copies 3? times so it does not suit my needs
  public static Example CopiesBuilder() {
    return new Example().WithFirstByCopy(1);
  }
}