The borrow checker within · baby steps

This is about Rust, but it's a great post and likely relevant to languages with Rust-style ownership. It describes four features planned for Rust to make its borrow checker more expressive and permissive:

• Allow a function to return a reference in one branch of a conditional and not consider it "borrowed" in the other branch (this is supported by the new borrow checker, Polonius, which should be in stable Rust very soon).
• Allow "place" lifetimes; that is, lifetimes that refer to other variables or their fields (e.g. 'foo.bar is a lifetime that, when attached to a reference, means the reference is borrowing foo.bar or some data within it).
• View types and inter-procedural AKA "partial" borrows: define a function that can only borrow specific fields in a structure, so that the function can be called with a structure whose other fields (at the call-site) are mutably borrowed or moved.
• Self-referential structures: structures where one field borrows another owned field (e.g. Foo { foo: String, bar: &'self.foo str }, foo is owned and bar borrows from it). Crates like self_cell and ouroboros provide this feature, but the goal is to have it in safe Rust with a cleaner syntax.