I work on a library used by millions of LOC in a monorepo. For some changes, it is literally impossible to know all the places that could break, so we rely heav

I work on a library used by millions of LOC in a monorepo. For some changes, it is literally impossible to know all the places that could break, so we rely heavily on types and testing to know if a change is safe. I think it's a shared responsibility, the person who wrote the old code needs to have written it in a way that makes it hard to inadvertently break, through testing or otherwise. The person who modifies the code needs to understand what to do if a test fails (modify the test? modify the code?) If we had to guarantee that our changes are never going to break any code downstream, no matter how poorly tested or engineered, we couldn't make changes at all.