This crate provides various "frozen" collections.
These are append-only collections where references to entries can be held on to even across insertions. This is safe because these collections only support storing data that's present behind some indirection -- i.e. String, Vec<T>, Box<T>, etc, and they only yield references to the data behind the allocation (&str, &[T], and &T respectively)
The typical use case is having a global cache of strings or other data which the rest of the program borrows from.