I just added an impl that I thought should be valid now with rust-lang/rfcs#2451, but I got a compiler error. So I found the RFC and copied the feature name re_rebalancing_coherence from it. However, I still got the compile error, so I spent a while trying to figure out if the impl was really valid. Eventually, I tried commenting out the impl, and then (thanks to #52644), the compiler told me the feature was unknown (it has been renamed to re_rebalance_coherence). The impl was valid using the correct feature name. I didn't get the error saying the feature name was invalid because of the error caused by the feature name being invalid.

I think the compiler should report unknown feature names even if there are other errors. For example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=e46fde7bf4076220df3005d702df9cbb doesn't report the feature name is invalid. With box syntax, it suggests "help: add #![feature(box_syntax)] to the crate attributes to enable" but the coherence error didn't.

Feel free to close this issue if you don't think it is worth fixing.