I imagine you could do something like "const" in c++. If you're not familiar, const tends to have a lot of behaviors for one keyword, but in this context I mean const methods where basically const means "this method can't modify the state of this object." (Unfortunately it doesn't mean the method is pure, but it's better than nothing)

It does seem like a technique a bit better suited for static compilation though, just in that catching this sort of error at runtime would be slower and harder to figure out.

Even if it was a weak/easily circumvented check it might be useful though, at least as code documentation.