The point of the crate, the readme you quoted, is

> you can push to the vec even while holding references to elements that have already been pushed.

Expressing this in Rust (without leaking references to a dropped container) means taking &self. It's true that the container struct itself (chunk pointers, len) is mutated, but the compiler has no distinction between this and the pointee data, so you use ..Cell. It does allocate, there's no capacity limit, but it guarantees stable pointers to existing items.