Yeah. "Why should / shouldn't code be put in the standard library" is a really interesting question that I think people don't think about enough.

I think a lot of the benefit of putting stuff in the standard library is interoperability. It seems obvious but - having a string and list type in std means you can pass strings and lists between packages. I think a lot of standard stuff that acts as "glue" should be in std. For example, in nodejs the standard library includes HTTP request and response types because of how useful they are in their ecosystem.

Notably, unlike swift, rust doesn't have an "inlined string" type in std. There's a lot of crates that implement small strings, but most interfaces that need to pass an actual string buffer use std::String - and thats way less efficient. (Thankfully, &str is more common at interface boundaries). Rust also doesn't have much support for futures in std - which upsets a lot of people, because tokio ends up being included by a lot of programs.

Anyway, when it comes to crates like rand where interoperability isn't important, I think its fine to keep this stuff out of std. Code can evolve much more easily when it lives as a 3rd party library.