But you can't just say that the flag is stable while the output may change.

Third party tools might rely on the output. This is okay for things read by humans (say the error messages) but fails for things that tools are parsing, like data needed for code coverage tools. Stabilization of instrument-coverage, as proposed, is a bad idea in my opinion. I'm sad that the concerns have not been addressed: https://github.com/rust-lang/rust/pull/90132#issuecomment-94...