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.