Comment by ameliaquining
9 hours ago
The people working on this are aware that it poses backcompat problems that don't have obvious solutions. They are looking into non-obvious solutions. https://lcnr.de/blog/2025/11/28/implicit-auto-traits-assoc-t... is the most up-to-date one I'm currently aware of.
Niko Matsakis (T-Lang) has since also published a post on only-bounds: https://smallcultfollowing.com/babysteps/blog/2026/06/09/onl.... Sized hierarchy, Move, and Forget/Leak all share the same fundamental compat issues, so we're all pretty motivated to solve the underlying problems in a way that enables all the other features to be built on top.
I had read that post, but IIUC it doesn't address the kind of backcompat issues that I meant to be referring to (the ones that would apply even if there was only ever going to be one new question-mark trait).