Confusing errors when using Self variants with lifetimes

[clarfonthey]

Minimal example, imagine that we have the following type:

enum MyCow<'a> {
    Owned(String),
    Borrowed(&'a str),
}

If the user returns Self variants, this fails:

impl<'a> MyCow<'a> {
    fn borrow(&self) -> MyCow<'_> {
        match self {
            Self::Owned(s) => Self::Borrowed(s),
            Self::Borrowed(s) => Self::Borrowed(s),
        }
    }
    fn own(&self) -> MyCow<'static> {
        match self {
            Self::Owned(s) => Self::Owned(s.clone()),
            Self::Borrowed(s) => Self::Owned(s.to_string()),
        }
    }
}

But the same code works when you change Self to MyCow!

impl<'a> MyCow<'a> {
    fn borrow(&self) -> MyCow<'_> {
        match self {
            Self::Owned(s) => MyCow::Borrowed(s),
            Self::Borrowed(s) => MyCow::Borrowed(s),
        }
    }
    fn own(&self) -> MyCow<'static> {
        match self {
            Self::Owned(s) => MyCow::Owned(s.clone()),
            Self::Borrowed(s) => MyCow::Owned(s.to_string()),
        }
    }
}

The real error here is that Self really represents MyCow<'a> and not MyCow<'_> or MyCow<'static>-, but the errors don't really relay this. Potentially linked issue: #30904 (has been fixed).

[clarfonthey]

FWIW this is still an issue on the latest stable: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c6da051f190e76b03a995df938a86f2c

Also potentially related to #86555, even though the two are not technically the same. We could potentially just allow the version that uses Self here.