Dependency injection in Go: constructor injection, wiring in main, avoiding global state, and when frameworks (wire, fx, dig) earn their complexity. Use when: "dependency injection", "wire up dependencies", "inject this", "remove global state", "singleton in Go", "use google/wire", "uber fx", "make this testable". Not for: interface design (go-interface-design), directory structure (go-project-layout), test doubles and mocks (go-test-quality).
