Editions only guarantee that old code builds with new compilers, not vice versa. New features are added to all editions unless they require e.g. new keywords that are available only in new editions.
I like that previous editions get new features when possible, unlike say C++, but I wish they'd gone with semantic versioning for the language version, like python did when both 2 and 3 were active. Oh well, I just need to remember that the edition is like the major version, and compiler version is sort of like minor version, and both are needed to specify forwards compatibility.
2.17
1.17
The issue is that semantic versions are a hierarchy, whereas Rust edition x compiler version are a matrix