How do you implement dependency injection in Go without a framework?

// Define dependencies as interfaces (for testability)
type UserRepository interface {
    FindByID(ctx context.Context, id int) (*User, error)
    Save(ctx context.Context, user *User) error
}

type EmailSender interface {
    Send(ctx context.Context, to, subject, body string) error
}

// Service accepts interfaces — can be mocked in tests
type UserService struct {
    repo  UserRepository
    email EmailSender
    logger *slog.Logger
}

// Constructor injection — explicit, no magic
func NewUserService(
    repo  UserRepository,
    email EmailSender,
    logger *slog.Logger,
) *UserService {
    return &UserService{repo: repo, email: email, logger: logger}
}

func (s *UserService) Register(ctx context.Context, req RegisterRequest) (*User, error) {
    user := &User{Name: req.Name, Email: req.Email}
    if err := s.repo.Save(ctx, user); err != nil {
        return nil, fmt.Errorf("UserService.Register: %w", err)
    }
    s.email.Send(ctx, user.Email, "Welcome!", "Thanks for joining.")
    return user, nil
}

// Wire everything together in main()
func main() {
    db, _ := sql.Open("postgres", os.Getenv("DATABASE_URL"))
    logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))

    repo  := postgres.NewUserRepository(db)
    email := smtp.NewEmailSender(os.Getenv("SMTP_HOST"))
    svc   := NewUserService(repo, email, logger)

    h := handlers.NewUserHandler(svc)
    mux := http.NewServeMux()
    mux.Handle("/users/", h)
    http.ListenAndServe(":8080", mux)
}