I'm not defending it (or claiming to really understand anything here), but this is where that came from:

https://rust-lang.github.io/rfcs/1522-conservative-impl-trai...

(OIBIT = auto trait, like Send/Sync)

Two reasons:

- It would have been very annoying if it didn't work like this

- Auto-traits also leak through private fields, so you would have the same issue if you e.g. wrapped the concrete return type in a new type and returned that one instead.