Klippy is currently Google-internal, not for any ideological reason, but because the tooling that powers most of its checks hasn't been open sourced yet. We're working to open source that this quarter, and hopefully then we will open source the Klippy checks as well.
Klippy is a set of optional suggestions to simplify your code, often using Guava APIs. For example, it will tell you to rewrite a loop like:
int x = 0;
for (T t : elements) {
x++;
}
to:
int x = Iterables.size(elements);
Klippy's goal is to make code more readable, and often to improve performance as well. Hence why we moved PreconditionsExpensiveString there.