If the existing code is old-school enough to use thousands of global variables in a thread-unsafe way, seems like changing it enough to compile as safe Rust code would push the "non-trivial" envelope pretty far.

The boundaries within database code are not clear. There are too many interlocking parts to take a nontrivial chunk and rewrite it Rust.

It’s sort of like the inverse of the Matrix when Neo learns kung fu. You realize that you actually don’t know how to program :)