golint-sl

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mockverify

About 323 words

Ensures mock implementations have compile-time interface verification.

Category

Testability

What It Checks

This analyzer detects mock implementations that don't verify they implement their interface at compile time.

It checks the package-level struct types whose name contains Mock, Fake or Stub as a whole word (MockStorage, StorageFake, but not Stubborn), declared in a mock/ directory or in a file named *_mock.go or mock_*.go. A blank-identifier declaration anywhere in the package whose value has the mock's type, or a pointer to it, is the verification: &MockStorage{}, MockStorage{}, new(MockStorage) and (*MockStorage)(nil) all count, as do instances of a generic mock such as &MockStorage[int]{}.

Why It Matters

Without compile-time verification, interface changes don't cause compilation errors in mocks. Tests pass with incomplete mocks, then fail mysteriously at runtime.

Examples

Bad: No Verification

type MockStorage struct {
    GetFunc func(key string) (string, error)
}

func (m *MockStorage) Get(key string) (string, error) {
    return m.GetFunc(key)
}

// If Storage interface changes, this still compiles!

Good: Compile-Time Verification

type MockStorage struct {
    GetFunc    func(key string) (string, error)
    SetFunc    func(key, value string) error
    DeleteFunc func(key string) error
}

// Compile-time check - fails if interface changes
var _ Storage = (*MockStorage)(nil)

func (m *MockStorage) Get(key string) (string, error) {
    return m.GetFunc(key)
}

func (m *MockStorage) Set(key, value string) error {
    return m.SetFunc(key, value)
}

func (m *MockStorage) Delete(key string) error {
    return m.DeleteFunc(key)
}

The Verification Pattern

var _ InterfaceName = (*MockTypeName)(nil)

This:

  1. Creates a nil pointer of the mock type
  2. Assigns it to the interface type
  3. Fails compilation if the mock doesn't implement the interface

Configuration

# .golint-sl.yaml
analyzers:
  mockverify: true  # enabled by default

When to Disable

  • Using mock generation tools that handle this automatically
analyzers:
  mockverify: false