In this use case, my instinct tells me that trait specialization is the wrong tool for the job. The author is trying to dispatch different functions based on a flag set in the caller. I can think of two more elegant ways to do this:

* Make the file system itself a trait, with implementations for read-only and read/write. Pass the concrete implementation as a parameter to the function using it.

* Store the capabilities as a variable in the file system object and query it at runtime. This can be done with an if.

Sometimes the simpler approach is better.

isn't the problem that rw is still r, so passes checks for both?

can't you make one rw and the other r(-w)?

The issue is really that `specialization` *has* to deal with lifetimes, even when there is apparently none. The example core with `Read` and `Read + Seek` also suffers from lifetime issues: some type could implement `Seek` depending on some lifetime and this would make your specialization unsound. For this reason `min_specialization` does not accept your specialization.