Add .cloned() method to Result?

[novacrazy]

It caught me off-guard that there isn't a .cloned() method for Result<&T, E>, yet there is one for Option<&T>.

I don't think this is a big enough feature to have a whole RFC for, even though I can do that if necessary, but it would be nice to have a .cloned() method like this:

impl<'a, T, E> Result<&'a T, E>
where
    T: Clone,
{
    #[inline]
    pub fn cloned(self) -> Result<T, E> {
        self.map(Clone::clone)
    }
}

and the equivalent mutable reference version.

However, if there is a reason why this is not already in place, I can understand. It simply feels a little awkward having to manually map Clone::clone in otherwise idiomatic code.

[scottmcm]

A bunch of questions from #50267 come up here too, like should cloned() clone both T and E, or should there be cloned_ok() and cloned_err()?

And, in general, if figuring out these answers better than just .map(Clone::clone) 🤣

[novacrazy]

I think it would make sense to follow the pattern of .map and .map_err by having .cloned() and .cloned_err(), as they would just call .map and .map_err internally.

I've seen a few times where functions return references in a Result, but I've never seen a Result with a reference in the Err value. The likelihood of having both the Ok and Err values be references is probably pretty small.

[novacrazy]

Added in #63166