`String::from_utf16_lossy` signature differs from `from_utf8_lossy`

[VoidStarKat]

String::from_utf8_lossy returns a Cow<'a, str> while String::from_utf16_lossy returns simply String. It would be useful for their signature to both match. String::from_utf16_lossy should return Cow<'a, str> as well, though this is obviously a breaking, but easily solved, change.

[bluss]

The conversion from utf-16 can never result in a &str, since the representation of the data changes from using u16 units to u8.

[nagisa]

Moreover you can trivially convert an owned datatype to Cow with into:

let x: Cow<str> = String::from_utf16_lossy(&[]).into();

[alexcrichton]

Yeah unfortunately these signatures can't change (as they're stable), but they're both intended to return "string-like types" in whichever way is most suitable, but perhaps the docs can be bolstered up to emphasize that!

[Mark-Simulacrum]

Relabeling as a docs issue since we can't change these APIs. However, I'm not entirely sure what needs to be changed here. Specifically, we should mention that conversion from UTF-16 can never be done without allocating, so we don't return a Cow. I'm personally not sure this is a good idea, but I can see the benefits, since it explains the API inconsistency.

[frewsxcv]

Opened a PR for this: #44572