So? Use the external library then. One thing does not preclude the other.
> No, they cannot. Many users are stuck in older windows versions, linux versions, LTS linux versions, etc. because of their organization or their clients requirements.
I work in such an organization and no, we cannot use third-party packages. The same way we cannot update our toolchain. So in most cases the point is moot.
> These updates would only apply to newer Rust toolchains, that many users cannot use. Unless you are suggesting the release of patch versions for the soon to be 100 old Rust toolchains in existence every time the unicode standard is updated.
You can provide standard Unicode handling that is good enough for 99% software out there. If you need to be on the bleeding edge, then use the bleeding edge library or rustc.
It is pretty simple, actually!