Fun fact: in order to be valid, Rust code in an `unsafe` block must uphold all of Rust's invariants. So (correct) Unsafe Rust is the only code written in Rust that's explicitly safe.
Idiomatically the unsafe block should have a comment explaining why this is actually fine, and if it's an unsafe public API it should have a doc-comment explaining how it can be used safely by other unsafe code.
If you're using unsafe functions to flag something other than Rust's safety considerations (e.g. Rust's core concept doesn't care that this flag bit disables the interrupt controller, and thus if you get it wrong now the product doesn't work, but you probably do so let's mark that "unsafe") the same likely applies for that too.
One of the things I like in Jon Gjengset's live coding Youtube videos is that he takes the time to write such comments, which means both the final code and the live session explain why he thinks this is safe, and once in a while there's a realisation while doing this - aha, this is the wrong way to do it, we need to change other things.