That said, I completely feel what this blog post is saying: strings are _hard_, especially if you're not just doing ASCII.
Each has their own unsized view: str, OsStr, Path, and CStr.
We also provide AsciiExt for those times where you really truly believe you want to be working with a String as Ascii.
That said, we generally try to make it as ergonomic as possible to pass a plain str where a Path/OsStr is expected. This is because utf8 is a subset of wtf8, so it's always fine to convert in that direction blindly (and it's really nice to just be like `File::open("foo.txt")` when hacking something together). This is why so many interfaces are riddled with something like `P: As<Path>`. The differentiation largely exists for the other direction, IMO. Paths and OsStrs aren't guaranteed to be valid UTF8, and shouldn't be provided where a proper utf8 string is expected.
Path is just a convenience wrapper over OsStr that understands the platform's seperator conventions and provides convenient utilities.
1) 40 years of crazy encodings and languages.
2) human languages are wildly diverse and basically any assumption you wish to apply is broken.
For 1, any system that wants to deal with the outside world needs to deal with: operating system encodings (arbitrary bytes on unix, malformed UCS2 on windows), C representation (null-terminated strings), systems that only work with ASCII, systems that only work with utf8, systems that work with arbitrary encodings/languages (HTML). This is arguably unnecessary complexity that exists because of short-sighted decisions in the past.
2 is the necessary complexity; the fact that languages are really complicated.
There are thousands of symbols in writing. Do you try to encode these symbols in a monolithic manner, or in a compositional way? For historical reasons, you can often do both! ë can be a single character, or e with an accent modifier. How do you handle string searching in such a model? Do you match `noel` with `noël`? What's the length of noël? 4 characters? 5 characters? bytes? graphemes? codepoints? Can you correctly reverse noël (do it wrong and you can get leön)?
Different letters which have similar/identical representations but different semantics/origins! Is Ε "capital e" or "capital ε"? How do you upper-case or lower-case these letters? Do you expect to_upper(to_lower(char)) to roundtrip (it won't)? Do you expect capitalization to be doable in-place (it's not)? Do you expect capitalization to be region-specific (it is)?
Are any of these operations even coherent in a language like Japanese? Why are you trying to do them?
God help you if you want to display this text. Are you ready to handle right-to-left text? Are you assuming that your font is monospace (hey there terminal and text editors)? C̢̫a̘̺̯n ̘̜̦̹y̷̫̼̘̩o̶͉u̗̩̻̞ ̻ẹ͡v̴̤͎̹e̶̫̠̤̭̺̤̞n̛̞̹̣̩̲͉̮ ̜͖̪͔̖d̤e̘̯ͅa̺l̟̀ ͚̗̣w̭i̸͇̠̥̣̜̥t̸h̸̻̮̼̙̹ ̗̺̱̣̰̱̙z̟a̺͜l̠̦̖̟̰͍g҉̜͖͓̫ơ̩̹̰͕?̹̳̼̯̘̺̟