The various {str,String}::from_utf{8,16}() functions should all return a Result<>

[jamesluke]

The functions (note the return types):

collections::str::from_utf8(v: &'a [u8]) -> Option<&'a str>
collections::String::from_utf8(vec: Vec<u8>) -> Result<String, Vec<u8>>
collections::String::from_utf16(v: &[u16]) -> Option<String>

As you can see, String::from_utf8 is an odd one out, returning a Result rather than an Option. I propose that all these functions return a Result, as this more closely matches the semantics of Option and Result. Option is generally used where Some() or None are both valid return values. Result is used where Ok() is a valid value and Err() means the function could not perform as desired. The latter more closely matches common use of from_utf8 (we expect this buffer to be a utf8 string, but need to confirm it before turning it into a str or String, and something has gone wrong if it's not utf8). Running through the rust compiler source code, it's almost exclusively used in this fashion: virtually all cases are followed immediately by a .unwrap(), panicking the task if the buffer was not utf-8.

Exactly what the Result's Err() variant should hold, I do not know. It seems that it should return a proper FromError-style enum with a variant like UtfDecodeError, but I do now know how many different error enums we want in the standard library. I leave it to someone else to decide what's most useful and elegant.

[barosl]

+1 with this. We should unify the interfaces ASAP. It is a bit surprising that even the methods from the same type have the different return types.

[lifthrasiir]

RFC #236 has a strong opinion supporting for this change. Among other reasons, the must_use attribute on Result alone is enough to justify that.

(Aside: While Result is unanimously better for this issue in my opinion, we still don't have a good convention on functions that optionally consume its argument and Result is not desirable. Can't think of any good example, though, so it may be fine.)

[steveklabnik]

This is now true.