Feature gated struct fields

[jayvdb]

Closes #150

IGNORE PR ATM - it can only be used to show the problem by running

cargo test -p butane --no-default-features --features sqlite,async,async-adapter

[jayvdb]

This is causing the CI failure when running with the datetime feature enabled.

As this macro is being run with the cfg's not evaluated and thus fields removed, the .table files contain all of the fields, causing a mismatch with the ORM objects.

I think we need to extract the cfgs, and evaluate them explicitly.

[jayvdb]

It looks like there isnt any built in Rust support for this. c.f. rust-lang/rust#90765

Maybe a crate which can help...

[jayvdb]

https://github.com/EmbarkStudios/cfg-expr looks like it can help.

[jayvdb]

CARGO_CFG_FEATURE is only available in build.rs , so this approach would require that there is a build.rs, and it saves CARGO_CFG_FEATURE somewhere so that the migration code can know which features will be active during compilation.

[jayvdb]

An alternative is to add a "cfg attributes" member to AColumn , so that the .table files describe the fact that some columns are conditional based on features.

This sounds more sensible, as then the .table file doesnt change based on which features are enabled.

However this will have some strange effects on migrations.

If a migration is created with a feature gated field enabled, and then compile without that feature, butane will be wanting to create a new migration to remove the field. While migration creation is an explicit user action, it may have been months since they created the feature gated field, and may not realise why the migration is trying to delete a field.

I am thinking we might want to put the active feature list on disk under .butane, and then warn users if they try to generate a migration using a different feature set.