Hehhhh, wondering if diagnostics could be improved for interior mutability in some scenarios. E.g. if T:Read, but you're trying to get &T:Read when say using an Arc, diagnostics should mention that maybe add &T:Read to bounds. If I can create a repro I might just file an issue!
The problem here is that if Arc doesn't work the way you want it to it can be tricky to figure out exactly what is causing it because the compiler doesn't say much. This would be something that could help with that.