Allow for multiple implementations of PgMetadataLookup

[asafigan]

This is inspired by part of #2257. It is simpler and also removes some unsafe code. This will also allow for multiple implementations of Connections for Postgres.

[weiznich]

Thanks for this PR 👍
I've left a few small comments what should be improved/discussed.
Otherwise this should have a entry in our changelog. (Just add a line to the corresponding section of Changelog.md.

[asafigan]

As a side note, I just realized that supporting user defined types with pure async would require a async variant of the HasSqlType trait which would than in turn require an async variant of the dsl too right? or am I being crazy.

[asafigan]

And AST

[weiznich]

As a side note, I just realized that supporting user defined types with pure async would require a async variant of the HasSqlType trait which would than in turn require an async variant of the dsl too right? or am I being crazy.

That's a different topic. Let's discuss one thing at the time. I will try to open a topic at the discussion part of this repo, where we could discuss how to generally deal with designing an async API. (Will update this comment as soon as there is something to link)

[weiznich]

Just as sanity check: These changes should not be required, right?

[asafigan]

Yes, we would have to change this because the new type is dyn PgMetadataLookup. I got a compilation error when just putting dyn PgMetadataLookup in HasSqlType<Array<T>> because the recursive call required 'static and it wasn't for some reason. Changing it to Self::MetadataLookup fixed the error some how. I thinking this is easier to refactor anyway so I changed all the HasSqlType implements for Pg to Self::MetadataLookup.

[weiznich]

That sounds for me like we should require that PgMetadataLookup is always 'static.

[asafigan]

I think it is implied to be 'static as part of the type of the trait (kind of like Sized if you don have ?Sized) but not when in the function signature. I don't know why an error don't occur on the function signature though. I might post something on rust users forum to see if anyone understands what is going on.

[weiznich]

Looks fine from ,my side, at least for the general implementation. The documentation error needs to be fixed + I would like to have a changelog entry for this (Just add a entry into Changelog.md.)

Otherwise I would wait a few days for @Ten0 to have a chance to voice potential problems here

[asafigan]

I made all these types public so third party crates can use them but hid all of them from the docs.

I did find this in the docs of Pg after hiding the new trait.

Notice that PgMetadataLookup appears in the docs but isn't a link.

[weiznich]

A final batch of small stylistic notes, otherwise this is good to go 👍