Why is `RUSTC_BOOTSTRAP` so actively discouraged?

MusicalNinjaDad:

I can trust that stable will not accidentally break something given the wonderful nightly-beta-stable release process that's in place. That means my only risk is of a deliberate breaking change in an unstable feature I've chosen to use. That's a risk I can better estimate and weigh against the value of using that feature.

This isn't really true for unstable features, they're commonly undertested and have bugs which will reach a stable compiler, and as already mentioned you then need to wait 6+ weeks till the next release to see if it fixes it. Whereas if you were using a pinned nightly you can report the bug and maybe it'll be fixed sooner and you could update your pin.

But really the main reason is that you must take all responsibility of anything that breaks if you use RUSTC_BOOTSTRAP, if it becomes normalized to use it then people that don't take that responsibility will start using it and complaining when stuff breaks. Increasing the workload on contributors for no real gain.