Or any time you want to use thread and closure. I know I am joining the threads before the references go away but it's not possible to express that. There's scoped threadpools but sometimes it's at a higher level than scope.

2: Can't get the return value for close(). readdir doesn't return the offset. I ended up wrapping libc myself: https://github.com/kahing/catfs/blob/master/src/catfs/rlibc....

I guess wrt string, it's extra frustrating because on unix, OsStr is really CStr but I get former (from command line) and often need latter for interacting with libc. Also when you learn Rust you first learn about str and String even though in my case I actually want to avoid them. Admittedly that's probably rare. Having every type duplicated (a ref type and a storage type) also increases the complexity.