> The blurb toward the end about Rent/return makes me a bit nervous though. They say they’re not going full borrow checker, but rent at least sounds an awful lot like borrow to me. Details were basically non-existent though.

Since they state outright that they're not going for a borrow checker, I would assume they're going for "second class references": the borrow checker is both powerful and complicated because references are first-class types: you can pass a reference as parameter, you can return a reference, and you can store a reference.

You can get a lot of the benefits (though also lose a fair amount of expressive power) if you drop the last two and only allow borrows downwards, and that is way easier to track.

Graydon Hoare's original conception of rust used second-class references (https://graydon2.dreamwidth.org/307291.html#:~:text=First-cl...). The Val language uses second-class references. Hylo (formerly Val) uses second-class references under the name of mutable value semantics (https://www.jot.fm/issues/issue_2022_02/article2.pdf).

Although the rent/return case doesn't even seem like a references concern, instead it's affine types which is orthogonal: after you `Return` an array to the pool, you want the array to become inaccessible to the caller (you could make the value linear, but as the text explains missing a `Return` is a safe leak it doesn't look like that's in scope). Rust mutable references are affine but you don't need references to do this...