Interestingly, Java had exactly that in mind when introduced `assert` keyword in very early versions (2002). It's equivalent to an `if` that can throw a runtime AssertionException, but only if the program is launched with `-enableassertions` flag.

The idea is that you use `-enableassertions` only during development, and your program should never rely on it. But it helps IDEs and static analyzers to have a contract about code.

D has a keyword prefix `debug`. So, one can write things like:

    ...
    debug if (x) printf("x: %d\n", x);
    ...

    debug assert(lookMa);

Another interesting property of `debug` is it turns off the safety checks in the target statement that follows it. Because, sometimes you just gotta do some unsafe tricks to figure out where your code went wrong.