Build fails with 6 month old rust.
And?
Wasn't this a big deal when Rust hit version 1.0 [1]? There's a lot of reasons for not being able to use a bleeding edge compiler version.
They promised improvements in keeping backwards compatibility, i.e. building old code on new rust. No one promised forward compatibility, building new code on old rust.
Yeah ok, makes sense of course. It's still a somewhat strange decision on the project's part. It's basically similar to using C++20 features only six months after the first C++ compilers support C++20. If that's a one-time thing during development of the framework it's probably fine. But if in 5 years the project still reqiures a less than 6-months old compiler version, I'd be worried ;)
Why? The advantage of having a single Rust compiler is that there is almost never any reason to use an old one. There are some cases where I can see that using a pinned compiler version would be important: if you are doing mission-critical embedded or financial programming and need a verified compiler. But none of those apply to game development.