alt Hacker News> Just to clarify, Valen's borrow checker can express shared-xor-mutable, its value proposition is that it can _also_ express mutable aliasing (with `in`).
Just to clarify, Rust's borrow checker can express mutable aliasing via interior mutability (i.e. UnsafeCell<T> and the various Cell<T> types). A &Cell<T> reference is essentially a mutably aliased reference. The goal is exactly that people "only reach for mutable aliasing when it will benefit them". Of course, any ergonomic improvements around the Cell types are quite welcome, especially if they help C/C++ interop - provided that they're proven to be as sound as the existing borrowck.
Good catch, and well said. Another good example I like citing is GhostCell, which adds mutable aliasing to Rust in a way that's even more flexible than Cell.
I would say Valen's real benefit here is in making a more ergonomic way for functions to work with an arbitrary number of GhostTokens / brands, and to track the relationships between them. (But I admit, I'm no expert with GhostCell, happy to be corrected by someone here)