Explain the difference between mocks, stubs, and fakes in Go testing. When do you use each?

// Interface to test against
type UserRepository interface {
    FindByID(ctx context.Context, id int) (*User, error)
    Save(ctx context.Context, user *User) error
}

// STUB: returns fixed values, no logic
type stubUserRepo struct {
    user *User
    err  error
}
func (s *stubUserRepo) FindByID(_ context.Context, _ int) (*User, error) {
    return s.user, s.err
}
func (s *stubUserRepo) Save(_ context.Context, _ *User) error { return nil }

// FAKE: in-memory map — works like a real repo but without a DB
type fakeUserRepo struct {
    mu    sync.Mutex
    users map[int]*User
    nextID int
}
func (f *fakeUserRepo) FindByID(_ context.Context, id int) (*User, error) {
    f.mu.Lock(); defer f.mu.Unlock()
    u, ok := f.users[id]
    if !ok { return nil, ErrNotFound }
    return u, nil
}
func (f *fakeUserRepo) Save(_ context.Context, u *User) error {
    f.mu.Lock(); defer f.mu.Unlock()
    f.nextID++; u.ID = f.nextID
    f.users[u.ID] = u
    return nil
}

// MOCK: records calls for assertion
type mockUserRepo struct {
    FindByIDCalls []int
    SaveCalls     []*User
    stub          stubUserRepo
}
func (m *mockUserRepo) FindByID(ctx context.Context, id int) (*User, error) {
    m.FindByIDCalls = append(m.FindByIDCalls, id)
    return m.stub.FindByID(ctx, id)
}
func (m *mockUserRepo) Save(ctx context.Context, u *User) error {
    m.SaveCalls = append(m.SaveCalls, u)
    return m.stub.Save(ctx, u)
}

// Test using mock
func TestService_Register(t *testing.T) {
    mock := &mockUserRepo{stub: stubUserRepo{}}
    svc := NewUserService(mock)
    svc.Register(context.Background(), "alice@example.com")
    if len(mock.SaveCalls) != 1 {
        t.Errorf("expected 1 Save call, got %d", len(mock.SaveCalls))
    }
}