code: func Len(t TestingT, object interface{}, length int, msgAndArgs ...interface{}) {
  	if h, ok := t.(tHelper); ok {
  		h.Helper()
  	}
  	if assert.Len(t, object, length, msgAndArgs...) {
  		return
  	}
  	t.FailNow()
  }
docs: // Len asserts that the specified object has specific length.
  // Len also fails if the object has a type that len() not accept.
  //
  //    assert.Len(t, mySlice, 3)