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The compat issue has always been associated types on std traits. For example, should Iterator::Item be Move or ?Move If you leave it as Move, you can't create
by Dagonfly 2mo ago
The compat issue has always been associated types on std traits.
For example, should Iterator::Item be Move or ?Move
If you leave it as Move, you can't create any iterators over !Move types. If you change it to ?Move, then functions using generic iterators can't assume that the elements of an Iterator are always moveable. Which is a breaking change compared to now.
The most critical trait is probably Deref. Using !Move types without `Deref::Target: ?Move` is painful, because calling any method on boxed types relies on Deref.
- simonask 2mo agoI'm very probably missing something, but as a user I would definitely expect `Iterator::Item: Move`, but then also that `&{mut} T: Move where T: ?Move`. But yeah I can see how these bounds are somewhat viral. Thanks!
- Dagonfly 2mo agoHere is an example of the problem: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=c4e2a17964a92c19db33e1169e2806b8 https://play.rust-lang.org/?version=stable&mode=debug&editio... Imagine if MyTrait comes from core/std. Adding an opt-out bound like ?Sized (or ?Move) is a breaking change for any generic code that relies on Sized/Move. But you want some traits from std to be open for !Move types.
- ameliaquining 2mo agoThe 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-types.html https://lcnr.de/blog/2025/11/28/implicit-auto-traits-assoc-t... is the most up-to-date one I'm currently aware of.
- yoshuaw 2mo agoNiko Matsakis (T-Lang) has since also published a post on only-bounds: https://smallcultfollowing.com/babysteps/blog/2026/06/09/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.
- ameliaquining 2mo agoI 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).