Implement Decode, Encode and Type for Box, Arc, Cow and Rc

[joeydewaal]

fixes #3100

Implements Decode, Encode and Type for Box<T>, Arc<T>, Cow<'_,T> and Rc<T>. I left out the Decode impl for Rc<T> because of the Send trait bounds in QueryAs and QueryScalar (which makes it impossible to use Rc).

[abonander]

I left out the Decode impl for Rc<T> because of the Send trait bounds in QueryAs and QueryScalar (which makes it impossible to use Rc).

@joeydewaal technically it's still possible to use by getting the Row and then calling get(). Someone's gonna end up asking why it doesn't exist, and justifying that would be more work than just carrying the impl.

We can relax the Send bound on QueryAs/QueryScalar in the future when we get rid of BoxStream in the fetch() method: https://github.com/launchbadge/sqlx/blob/277dd36c7868acb10eae20f50418e273b71c8499/sqlx-core/src/query_as.rs#L111

[abonander]

Previous comment.

[joeydewaal]

Makes sense, I added the impl for Rc

[abonander]

CI should be fixed if you rebase.

[abonander]

If you wouldn't mind, let's just commit to making the Decode for Cow impl return Cow::Owned.

[joeydewaal]

Right, that should be reflected in the latest changes.