Rust is getting a new `only` bounds syntax to replace the limited `?Sized` opt-out mechanism. The current `Sized`/`?Sized` binary doesn't scale to a richer hierarchy of sizedness traits (`Sized`, `MetadataSized`, `MaybeSized`), which is needed for ARM Scalable Vector Extensions and extern types. `only` bounds let you specify exactly which level of a trait hierarchy you require (e.g., `T: only MetadataSized`) while disabling higher default bounds, and they compose cleanly with other bounds. The post also explores a speculative second family of default traits around value access/movement/destruction (`Move`, `Destruct`, `Forget`, `Leak`, `Access`), showing how `only` bounds would work across multiple families. Examples include how `Option::map` would use `only Move`, `Option::or` would need `only Move + Destruct`, and `Rc` would use `only MaybeSized + only Leak`. The feature is being driven by the ARM team as part of the Sized Hierarchy and Scalable Vector Extensions project goal.
