A function signature describes the paths it modifies.

The problem with this is that it imposes a cost on abstraction. Zero-cost abstraction is one of the most fundamental design principles behind both C++ and Rust.

A field-path into a struct/enum fundamentally depends on its concrete implementation. If you hide the fields of a struct and use getters/setters, you break the ability to talk about the "paths modified" by a function which uses getters/setters.

Aside: it really annoyed me that I had to read halfway through this article to find the first attempt at defining "group borrowing". The whole first half of the article is basically fluff.