[steveklabnik]

Libraries can declare "I require semantic x or semantic y." Applications can as well. If an application says it needs semantic x, and a library says it requires semantic y, you get a compile error.

Most libraries do not depend on a particular semantic, in my experience.

[Groxx]

I assume library authors can specialize code to either semantic as well, and carefully target both in a single library? E.g. with cfg attributes/macros.

[steveklabnik]

I actually don't think so, but I'm also not sure what use case would require you to do this. Generally, you're agnostic, and only in very specific circumstances would you require unwind. I'm not sure when a library would require abort.

[Groxx]

I can't come up with a case for requiring abort, but perhaps there are some performance optimizations possible when not unwinding? I assume the compiler does plenty already, but e.g. there would be no need to hang on to any data for making better error messages from a panic, since you can't capture it.