Comment by xyzzyz
1 day ago
The way Rust handles this is perfectly fine. String type promises its contents are valid UTF-8. When you create it from array of bytes, you have three options: 1) ::from_utf8, which will force you to handle invalid UTF-8 error, 2) ::from_utf8_lossy, which will replace invalid code points with replacement character code point, and 3) from_utf8_unchecked, which will not do the validity check and is explicitly marked as unsafe.
But there's no option to just construct the string with the invalid bytes. 3) is not for this purpose; it is for when you already know that it is valid.
If you use 3) to create a &str/String from invalid bytes, you can't safely use that string as the standard library is unfortunately designed around the assumption that only valid UTF-8 is stored.
https://doc.rust-lang.org/std/primitive.str.html#invariant
> Constructing a non-UTF-8 string slice is not immediate undefined behavior, but any function called on a string slice may assume that it is valid UTF-8, which means that a non-UTF-8 string slice can lead to undefined behavior down the road.
How could any library function work with completely random bytes? Like, how would it iterate over code points? It may want to assume utf8's standard rules and e.g. know that after this byte prefix, the next byte is also part of the same code point (excuse me if I'm using wrong terminology), but now you need complex error handling at every single line, which would be unnecessary if you just made your type represent only valid instances.
Again, this is the same simplistic, vs just the right abstraction, this just smudges the complexity over a much larger surface area.
If you have a byte array that is not utf-8 encoded, then just... use a byte array.
There are a lot of operations that are valid and well-defined on binary strings, such as sorting them, hashing them, writing them to files, measuring their lengths, indexing a trie with them, splitting them on delimiter bytes or substrings, concatenating them, substring-searching them, posting them to ZMQ as messages, subscribing to them as ZMQ prefixes, using them as keys or values in LevelDB, and so on. For binary strings that don't contain null bytes, we can add passing them as command-line arguments and using them as filenames.
The entire point of UTF-8 (designed, by the way, by the group that designed Go) is to encode Unicode in such a way that these byte string operations perform the corresponding Unicode operations, precisely so that you don't have to care whether your string is Unicode or just plain ASCII, so you don't need any error handling, except for the rare case where you want to do something related to the text that the string semantically represents. The only operation that doesn't really map is measuring the length.
15 replies →
I don’t understand this complaint. (3) sounds like exactly what you are asking for. And yes, doing unsafe thing is unsafe.
> I don’t understand this complaint. (3) sounds like exactly what you are asking for. And yes, doing unsafe thing is unsafe
You're meant to use `unsafe` as a way of limiting the scope of reasoning about safety.
Once you construct a `&str` using `from_utf8_unchecked`, you can't safely pass it to any other function without looking at its code and reasoning about whether it's still safe.
Also see the actual documentation: https://doc.rust-lang.org/std/primitive.str.html#method.from...
> Safety: The bytes passed in must be valid UTF-8.
> If you use 3) to create a &str/String from invalid bytes, you can't safely use that string as the standard library is unfortunately designed around the assumption that only valid UTF-8 is stored.
Yes, and that's a good thing. It allows every code that gets &str/String to assume that the input is valid UTF-8. The alternative would be that every single time you write a function that takes a string as an argument, you have to analyze your code, consider what would happen if the argument was not valid UTF-8, and handle that appropriately. You'd also have to redo the whole analysis every time you modify the function. That's a horrible waste of time: it's much better to:
1) Convert things to String early, and assume validity later, and
2) Make functions that explicitly don't care about validity take &[u8] instead.
This is, of course, exactly what Rust does: I am not aware of a single thing that &str allows you to do that you cannot do with &[u8], except things that do require you to assume it's valid UTF-8.
> This is, of course, exactly what Rust does: I am not aware of a single thing that &str allows you to do that you cannot do with &[u8], except things that do require you to assume it's valid UTF-8.
Doesn't this demonstrate my point? If you can do everything with &[u8], what's the point in validating UTF-8? It's just a less universal string type, and your program wastes CPU cycles doing unnecessary validation.
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