> But because the stdlib isn't versioned

I honestly feel like that's one of Rust's biggest failings. In my ideal world libstd would be versioned, and done in such a way that different dependencies could call different versions of libstd, and all (sound/secure) versions would always be provided. E.g. reserve the "std" module prefix (and "core", and "alloc"), have `cargo new` default to adding the current std version in `cargo.toml`, have the prelude import that current std version, and make the module name explicitly versioned a la `std1::fs::File`, `std2::fs::File`. Then you'd be able to type `use std1::fs::File` like normal, but if you wanted a different version you could explicitly qualify it or add a different `use` statement. And older libraries would be using older versions, so no conflicts.

I'm afraid it won't work. The point of std lib is to be universal connection for all the libraries. But with versioned std I just can't see how can you have DateTime in std1, DateTime in std2 and use them interchangeably, for example being able to pass std2::DateTime to library depending on std1 etc. Maybe conversion methods, but it get really complicated really quickly