Let's not forget about a particularly frustrating kind of "level of abstraction": the bespoke interface to a part of the code that has side effect, and that has

Let's not forget about a particularly frustrating kind of "level of abstraction": the bespoke interface to a part of the code that has side effect, and that has exactly two implementation : one in the production code, and one in the tests. If I were to create a language tomorrow, that's the one aspect where I would try something that I have not yet found elsewhere : can you make it so that you can "plug" test doubles only for test, but keep the production path completely devoid of indirection or late binding. (I'm curious if you know of a language that already does that. I suppose you can hack something in C with #ifdef, of course...)