Or put another way, a hypothetical feature that you made up in your head is the thing that requires source access. Editions do not let you change the semantics of types.
To be fair, Rust tooling does tend toward build-from-source. But this is for completely different reasons than the edition system: if you had a way to build a crate and then feed the binary into builds by future compilers, it would require zero additional work to link it into a crate using a different edition.
Exactly, hence why people should stop talking about editions as if they sort out all Rust evolution problems, in your own words it doesn't allow changing type semantics
I think you're too stuck on the current implementation. Work is going into investigating how to evolve the standard library over editions. The "easiest" win would be to have a way to do edition-dependent re-exports of types.
What I am stuck is Rust folks advocating editions as the solution for everything in language evolution, when it clearly isn't.