If I have understood correctly, rust is so fast-moving in terms of changes to the language and therefore annoying in that they typically don't guarantee more than one version backward compatibility, i.e. you typically need 1.63 to build 1.64.
Could we go with a model similar to go, where you can build through several go versions until you reach the most current one? I suppose we could leave the last 5 or so around, so if you missed a few updates, you are still able to catch up by source, but if you missed too many, well, you need to bootstrap again.
-- Jonathan