query_as! macros do not use FromRow

[icewind1991]

instead the query_as! macro's directly build the target struct, while it might be nice not to have to derive FromRow for the struct, this also means that you can't customize the behaviour with a custom FromRow implementation.

[abonander]

We unfortunately have no way to do this right now while keeping the guarantee that output columns and types match the field names and types in the struct. We could possibly do some type-level magic with FromRow to get this information in a way that the macros can use it but leaning too heavily on typechecking produces some really obtuse compiler errors.

I think with the const_if_match and const_panic features we can make this work, although we would also need to be able to loop through a list which isn't currently possible in a const context either, but I think we can work around that.

[jplatte]

would also need to be able to loop through a list which isn't currently possible in a const context either

const_loop also got stabilized for 1.46, same as const_if_match. So that shouldn't be a problem :)

[abonander]

Ah, thanks. const_panic is still a necessary feature for this, though, otherwise we can't control the error message. There's been some promising discussion on the tracking issue so I think I could push it through the stabilization process if I can find the time: rust-lang/rust#51999

[Waelwindows]

Could we at least document this currently for now? As It was not clear for me.

[abonander]

Could we at least document this currently for now? As It was not clear for me.

This is already stated in the documentation: https://github.com/launchbadge/sqlx/blob/master/src/macros.rs#L379

This is also stated in the documentation for 0.3.5, it's just missing the bold formatting: https://github.com/launchbadge/sqlx/blob/v0.3.5/src/macros.rs#L247

If that isn't clear enough then we'd love suggestions for better phrasing.

[Waelwindows]

The beta version is okay as it is, but I think it would be better if it directly mentions the FromRow as it's currently vague by which "trait implementation" it refers to, perhaps update FromRow's docs since they indicate that it's required for query_as, and shouldn't it link to this issue as well?

[abonander]

The documentation also goes on to explain, pretty clearly I think, how the macro maps rows to the struct.

However, "no trait implementations are required" can be misleading because Decode is still required for the individual field types of the struct so the wording should be adjusted anyway.

FromRow is indeed required for the function version of query_as which is why it's mentioned in its docs, which also link to the function to make it clear what it's referring to. In general if the docs are referring to a macro they will include the bang (!) in the item name to disambiguate.