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

These three terms are often used interchangeably, but they describe different test double patterns with different purposes. Go's implicit interfaces make all three easy to implement without a framework.

// 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))
    }
}

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