>> It is completely different of old sane codebase not compiling anymore due to syntactic sugar change in the language.

That is not very common with Rust.

If you just download some project off of Github (or using some dependency) from 2015, it is very very likely to "just work" even with a new compiler, due to how editions work.

A more likely way to encounter issues is by pasting some code from Stackoverflow into a Rust project that is using a different edition. Yes, that can happen, but it's not anywhere close to as big of a problem.