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.
replyI 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)
AIUI, the main limitation with &Cell<T> is (1) no provision for concurrency, obviously - though the various Atomic* types work similarly via interior mutability; and (2) data has to be read, written or modified as a single operation - any provision for enabling subobject access or the like is quite ad hoc, and has to be implemented for the specific type you're working with. GhostCell (and QCell, LCell etc.) generalizes this by tracking the use of shared references at compile time, such that writes will not overlap with each other or with reads, without enforcing a "get/set in a single step" workflow.
reply> any provision for enabling subobject access or the like is quite ad hoc, and has to be implemented for the specific type you're working with
replyThere is ongoing work on a language feature ("field projection") that could alleviate this.