Rust doesn't have the concept of package. (Cargo does, but Cargo is a different thing from Rust, and it's entirely possible to use Rust without Cargo).

Rust has the concept of _crate_, which is very close to the concept of compilation unit in C++. You build a crate by invoking `rustc` with a particular set of arguments, just as you build a compilation unit by invoking `g++` or `clang++` with a particular set of arguments.

One of these arguments defines the edition, for Rust, just like it could for C++.

That only works for C++ code using C++20 modules (i.e. for approximately nothing). With textual includes, you need to be able to switch back and forth the edition within a single compilation unit.