The goal ATM is simply to allow people to write new drivers in rust, not convert the whole kernel to rust. It will be a very long time, before more core parts would be allowed to be written in rust let alone rewriting any existing core kernel code. Which is all fine as new drivers are a large part where bugs are added - older parts have had a long time for bugs to be found and fixed and so it is far less important to need to rewrite them.
The goal ATM is simply to allow people to write new drivers in rust, not convert the whole kernel to rust. It will be a very long time, before more core parts would be allowed to be written in rust let alone rewriting any existing core kernel code. Which is all fine as new drivers are a large part where bugs are added - older parts have had a long time for bugs to be found and fixed and so it is far less important to need to rewrite them.
Yes there is never old code with bugs that have been sitting there for decades.