Implement feature string_from_utf8_lossy_owned for lossy conversion from Vec<u8> to String methods

[okaneco]

Accepted ACP: rust-lang/libs-team#116
Tracking issue: #129436

Implement feature for lossily converting from Vec<u8> to String

• Add String::from_utf8_lossy_owned
• Add FromUtf8Error::into_utf8_lossy

---

Related to #64727, but unsure whether to mark it "fixed" by this PR.
That issue partly asks for in-place replacement of the original allocation. We fulfill the other half of that request with these functions.

closes #64727

[rustbot]

r? @Noratrieb

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[Noratrieb]

this example isn't great, since it's just String::from_utf8_lossy_owned(input) but with more characters. I can't think of any better one though, so it's fine.

[okaneco]

I agree.

After some thinking, something more illustrative could be matching on the result of String::from_utf8

// some invalid bytes
let input: Vec<u8> = b"Hello \xF0\x90\x80World".into();

let output = match String::from_utf8(input) {
    Ok(s) => s,
    Err(e) => {
        // Log invalid UTF-8 error
        // ...
        
        // Lossily convert the bytes to a `String`
        e.into_utf8_lossy()
    }
};

assert_eq!(String::from("Hello �World"), output);

[Noratrieb]

that sounds better

[Noratrieb]

sorry for the late review, but thanks for the patience and implementation!
@bors r+ rollup

[bors]

📌 Commit 65abcc2 has been approved by Noratrieb

It is now in the queue for this repository.