prototype.go

type Assertion interface { 
	// ... redacted for brevity
}

func With(t *testing.T) Assertion {
	return &assertImpl{wrappedT: t}
}

type assertImpl struct {
	wrappedT *testing.T
	stopOnFailure bool
	failedMessage string
}

func (a *assertImpl) Must() Assertion {
	a.stopOnFailure = true
	return a
}

func (a *assertImpl) Message(msg string) Assertion {
	a.failedMessage = msg
	return a
}

func (a *assertImpl) Messagef(msg string, args ...interface{}) Assertion {
	a.failedMessage = fmt.Sprintf(msg, args...)
	return a
}

func (a *assertImpl) Equal(sideA, sideB interface{}) {
	// Equal logic using "github.com/google/go-cmp/cmp"
  // But this can be substituted by the custom equal logic found in testify.
	if !cmp.Equal(sideA, sideB) {
		diff := cmp.Diff(sideA, sideB)
		a.wrappedT.Errorf("Test failed: %s\n%s", a.failedMessage, diff)
		if a.stopOnFailure {
			a.wrappedT.FailNow()
		}
	}
}