It's rare that I work on a project I myself started. If I start working on an existing codebase, the warnings might be there already. Then what do I do?

I'm also referring to all the warnings you might get if you use an existing library. If the requirements entail that I use this library, should I just silence them all?

But I'm guessing you might be talking about more specific warnings. Yes I do fix lints specific to my new code before I commit it, but a lot of warnings might still be logged at runtime, and I may have no control over them.