Support for flatten(nullable) in sqlx::FromRow

[Niedzwiedzw]

Does your PR solve an issue?

Fixes #2934

Do not just list issue numbers here as they will not be automatically closed on merging this pull request unless prefixed with "fixes" or "closes".

Is this a breaking change?

No, default behaviour is preserved

If yes, this pull request will need to wait for the next major release (0.{x + 1}.0)

Probably would still be worth it

[joeydewaal]

This tries to parse the type which is fragile. What if someone does something like:

type Opt<T> = Option<T>;

and use Opt<...> in their struct?

#3667 uses auto ref based specialization, see #3667 (comment)

[Niedzwiedzw]

Well I thought about it but I think aliasing Option is obscure enough it can be skipped, as the error will be clear and solution is just to move it to a different module without an alias.

[Niedzwiedzw]

@abonander where would be the best place to add tests?
EDIT: figured it out

[Niedzwiedzw]

@joeydewaal after giving it some thought you're right it's better to not parse the option, I pushed an update which uses associated type to extract inner type