If you want to exhaustively describe the behavior of e.g. Rust 1.25.0, without a formalized model, then the only way to do so it to run it for all kinds of poss

If you want to exhaustively describe the behavior of e.g. Rust 1.25.0, without a formalized model, then the only way to do so it to run it for all kinds of possible (edge-)cases you encounter and document the output. If all you have as a reference is a bunch of test cases without any underlying reasoning as to why input A leads to output B, then that's not really a useful target to implement against. At that point you can just go the direct route, not bother with any standard and just implement directly against the Rust compiler.