It's not really in Rust (at least, not unless I choose to rewrite it in WASM), but there is one case where a lack of leap-second support has really been a thorn

It's not really in Rust (at least, not unless I choose to rewrite it in WASM), but there is one case where a lack of leap-second support has really been a thorn in my side. I've been working on and off on a JS web application that performs certain high-precision astronomical calculations, and leap seconds are a big challenge w.r.t. future correctness. Ephemerides don't change very much, but leap seconds can. I can embed a big list into my application, but then I have to remember to periodically update it until 2035, or maybe longer if they decide to keep leap seconds after all. So I can try to download a leap-seconds.list from somewhere, except that only a few sources set "Access-Control-Allow-Origin: *", and none promise to keep it set indefinitely. The annoying part is, applications on both Windows (since 10) and Unix-like systems generally have access to up-to-date leap-second information; at worst, browsers can be expected to be updated regularly. But since the JS standards devs want to stick strictly to POSIX time, they provide no means to obtain this information from the inside, through Date, Intl.Locale, or the proposed Temporal. This is all to say, I would really love to always have some way available to get "all leap-second info that the current system knows about, if it does have any such info". For its part, hifitime either downloads from a fixed URL (which doesn't have that Access-Control-Allow-Origin header IIRC), consults a fixed list, or gets the user to locate the info, which isn't nearly as general or foolproof as it could be.