Why is `RUSTC_BOOTSTRAP` so actively discouraged?

yeah, (a bit off topic but a good example of the issues involved) that's an almost impossible tradeoff, every choice is bad:

• have poor support of in IDE testing for a major IDE where it is generally expected -> poor DevEx for the language
• make (undocumented) use of RUSTC_BOOTSTRAP and have users annoyed about why their machine keeps completely rebuilding the project before and after every test run -> poor DevEx for the language
• clearly document the use of RUSTC_BOOTSTRAP, propagate acceptance and ... see above points made by Ralf -> poor DevEx for the language