Re-export features of dependencies

[bikeshedder]

This has been asked before. I can't seam to find the issue right now so I'm writing this down as a reminder.

Right now when using deadpool-postgres and using it together with tokio-postgres + with-uuid-0_8 there is no other way than adding both deadpool-postgres and tokio-postgres (+ with-uuid-0_8) to the dependencies while making sure that the versions match.

It was many times before if deadpool-postgres could just reexport all the feature flags of tokio-postgres. I have no absolute oppionion here and in the end decided against it as it's yet one more thing to maintain.

Just looking at tokio-postgres it is almost a no-brainer to re-export the features. Though when you look at deadpool-sqlite it would require 35 feature flags if I'm counting correctly.

https://github.com/rusqlite/rusqlite/blob/master/Cargo.toml - There are 30 feature flags and 7 optional dependencies. I believe two of the optional dependencies are not feature related (lazy_static and byteorder).

It would be so much easier if Cargo just had a way of enabling features of dependencies of dependencies like that:

deadpool-postgres = { version = "0.10", features = "tokio-postgres/with-uuid-0_8" }

A somewhat related cargo issue is this: rust-lang/cargo#9094

While this is not possible, I wonder if deadpool-* should work around this and re-export the features so the users of the deadpool-* crates don't have to import the backend crates themselves in order to activate features from them.

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Another way to solve this would be to make the backend version explicit. e.g. deadpool-postgres in version 0.10 does have a feature called tokio-postgres-0_7 which enables the deadpool_postgres::tokio_postgres_0_7 module:

deadpool-postgres = { version = "0.10", features = ["tokio-postgres-0_7"] }
tokio-postgres = { version = "0.7", features = ["with-uuid-0_8"] }

This would make the versions 100% obvious to reason about and as a nice side-effect decouple the versions of deadpool-postgres from tokio-postgres. Right now whenever a breaking version of tokio-postgres is released a breaking version of deadpool-postgres must be released as well.

Big downside here would be that users of deadpool-postgres now have to use imports like use deadpool_postgres::tokio_postgres_0_7::Pool as the root of deadpool_postgres now contains no public exports whatsoever.

While it's easy to make this pretty in projects by creating an alias for deadpool_postgres::tokio_postgres_0_7 I'm undecided if that's actually the right path to walk on.

It feels like it adds yet another layer of confusion to an already confusing topic. 🙈

[bikeshedder]

After long consideration I've decided not to implement this.

I don't like the idea of re-exporting feature flags from underlying crates as part of the deadpool-* crates.

Unless someone has a compelling reason why this is a must-have feature I won't be working on this.