Rust is basically in the same place as C++, i.e. provenance rules are currently ad-hoc/conventional, meaning that pointer tagging is a grey area.

Nope. Rust stabilized strict provenance over a year ago. Some details about aliasing aren't tied down, but so long as you can obey the strict provenance rules you're golden today in Rust to hide flags in pointers etc.

https://blog.rust-lang.org/2025/01/09/Rust-1.84.0/#strict-pr...