I was spelunking some Go internal code today, and came across the definition of testing.TB. See private() at the end:

// TB is the interface common to T, B, and F.
type TB interface {
	Error(args ...any)
	Errorf(format string, args ...any)
	Failed() bool
	Fatal(args ...any)
	Fatalf(format string, args ...any)
	Log(args ...any)
	Logf(format string, args ...any)
	Name() string
	Setenv(key, value string)
	Skip(args ...any)
	Skipf(format string, args ...any)
	Skipped() bool
	TempDir() string

	// A private method to prevent users implementing the
	// interface and so future additions to it will not
	// violate Go 1 compatibility.

The interfaces mixes in a private function to make it impossible for external packages to implement it. This leaves the authors free to add new functions to the interface without worrying about breaking existing user code.

Personally, I hadn’t seen this technique in action before. It’s probably something that most Go APIs could take advantage of.