How to set the dependency on SQLx for different runtime features

[photino]

We have defined two features in a lib:

[features]
default = ["async-std-rt"]
async-std-rt = ["async-std"]
tokio-rt = ["tokio"]

[dependencies]
async-std = { version = "1.10.0", features = ["attributes"], optional = true }
tokio = { version = "1.11.0", features = ["full"], optional = true }
sqlx = { version = "0.5.7", features = ["runtime-async-std-native-tls", "postgres", "uuid", "chrono", "json"] }

However, it only works when the async-std-rt feature is enabled. How can we set the dependency on SQLx for different runtime features?

[abonander]

You would do something like this:

[features]
default = ["async-std-rt"]
async-std-rt = ["async-std", "sqlx/runtime-async-std-native-tls"]
tokio-rt = ["tokio", "sqlx/runtime-tokio-native-tls"]

[dependencies]
async-std = { version = "1.10.0", features = ["attributes"], optional = true }
tokio = { version = "1.11.0", features = ["full"], optional = true }
sqlx = { version = "0.5.7", features = ["postgres", "uuid", "chrono", "json"] }

[photino]

It returns an 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

[abonander]

If you want to enable the tokio-rt feature of your package you need to disable the async-std-rt default feature.

[photino]

It seems that it is impossible to set mutually exclusive features.

[abonander]

Yeah, Cargo does not have a concept of mutually exclusive features. It's generally considered an anti-pattern but it's really difficult to code runtime-agnostic async crates that need a lot of runtime support like SQLx without something like this. We enforce it with #[cfg] here: https://github.com/launchbadge/sqlx/blob/master/sqlx-rt/src/lib.rs#L1-L25