Cannot compile a workspace when using different runtimes with SQLx

[dsferruzza]

Hi!

I have a project that is organized with a Cargo workspace which contains 2 inner crates:

1. an Actix Web app
2. a worker, that uses the async-std runtime

Both apps depends on SQLx 0.4.1 with default features disabled and different runtime features:

1. runtime-actix-rustls
2. runtime-async-std-rustls

Both apps compile and work just fine from their own sub-directory, but compiling at the workspace level gives the following error:

error: only one of ['runtime-actix-native-tls', 'runtime-async-std-native-tls', 'runtime-tokio-native-tls', 'runtime-actix-rustls', 'runtime-async-std-rustls', 'runtime-tokio-rustls'] can be enabled
  --> C:\Users\D\.cargo\registry\src\github.com-1ecc6299db9ec823\sqlx-rt-0.2.0\src\lib.rs:21:1
   |
21 | / compile_error!(
22 | |     "only one of ['runtime-actix-native-tls', 'runtime-async-std-native-tls', \
23 | |      'runtime-tokio-native-tls', 'runtime-actix-rustls', 'runtime-async-std-rustls', \
24 | |      'runtime-tokio-rustls'] can be enabled"
25 | | );
   | |__^

Is there any way to fix this and allow compiling the whole workspace at once?

[dsferruzza]

Also, I believe this prevents rust-analyzer to complete its work when the whole workspace is opened in VSCode. Maybe there is a solution for this particular issue, but I have not found it.

[abonander]

There's unfortunately no easy fix here, because Cargo itself has no knowledge of mutually exclusive features and thus it's trying to build SQLx with both runtime-actix-rustls and runtime-async-std-rustls on. Mutually exclusive features are actually considered an anti-pattern and we've got some ideas on how to address it in 0.5, but for now the best solution IMO would be to switch both to using the same runtime.

Fortunately, unless you're using SQLite, you don't technically need to use runtime-actix-rustls and instead you can use runtime-tokio-rustls (the only effective difference is which spawn_blocking() call is used, which is only needed by the SQLite driver for the background worker). Then if you modify your worker crate to use Tokio, you can also use the runtime-tokio-rustls feature there. This also means you don't have to build two separate runtimes when building your workspace which should improve compile times and reduce the space used by the target/ directory.

If switching runtimes is a non-starter, you might have some success switching one of the crates to use SQLx as a Git dependency, which should make Cargo build it twice instead of once with both runtimes turned on. I hesitate to recommend this since it'll make your build times even worse than they are now, and there's no way to get automatic updates unless you pin to master, which may receive commits that break the build or make breaking changes to the API, but it is an option nonetheless:

# switch the SQLx dependency in *one* of your two crates to this instead:
[dependencies.sqlx]
git = "https://github.com/launchbadge/sqlx"
tag = "v0.4.0" # it seems we didn't tag 0.4.1, unfortunately; comment this out to pin to `master` (not really recommended for reasons stated above)
features = [...]

[dsferruzza]

Thanks a lot for this detailed answer!
I decided to switch the runtime as I am using exclusively PostgreSQL; also I needed reqwest to work here and I realized it was easier with tokio...
This solution seems to work!

I saw you added this issue to a milestone so I'll let you close it whenever you want.

[mehcode]

The plan in the next big version is to parameterize the runtime. This would close this issue.

struct PgConnection<Rt = DefaultRuntime> { ... }

[abonander]

This has been fixed since 0.7; runtime features are now additive and SQLx can be built without any enabled.