Avoid unnecessary allocation in <Value as From<Cow<'a, str>>>::from()

[H2CO3]

The current implementation of From<Cow<'a, str>>::from() is:

fn from(f: Cow<'a, str>) -> Self {
    Value::String(f.to_string())
}

Now, Cow<'a, str>::to_string() unconditionally allocates and copies (and it has to, because it takes &self by pointer, as dictated by the ToString trait). But since From::from takes its argument by value, I believe it would be possible to implement it like this instead:

fn from(f: Cow<'a, str>) -> Self {
    match f {
        Cow::Borrowed(slice) => Value::String(slice.to_owned()),
        Cow::Owned(buf) => Value::String(buf),
    }
}

What do you think? If it's possible and worth implementing, I'd be happy to open a PR.

[dtolnay]

Seems reasonable! Please send a PR.

[dtolnay]

Value::String(f.into_owned()) should work.

[H2CO3]

Oh, indeed, good catch!