Like with everything, it depends. The part that can block you is your dependencies. You might be willing to add annotations to your entire codebase, but not to

Like with everything, it depends. The part that can block you is your dependencies. You might be willing to add annotations to your entire codebase, but not to code you depend on. A way to get around this is to make the annotation optional, but that then means that you're perpetually torn between to diverging ecosystems. TypeScript is an example of how it can be done, but I would also point at C where there's no such thing as a "String". You can include your own String type that avoids memory unsafety through a well thought out API, even without changing anything in the language itself, but at some point you'll have to interact with an existing library that doesn't know about your String.