How to tell what "features" are available per crate?

10
votes

Is there a standard way to determine what features are available for a given crate?

I'm trying to read Postgres timezones, and this says to use the crate postgres = "0.17.0-alpha.1" crate's with-time or with-chrono features.

When I try this in my Cargo.toml:

[dependencies]
postgres = { version = "0.17.0-alpha.1", features = ["with-time"] }

I get this error:

error: failed to select a version for `postgres`.
    ... required by package `mypackage v0.1.0 (/Users/me/repos/mypackage)`
versions that meet the requirements `^0.17.0-alpha.1` are: 0.17.0, 0.17.0-alpha.2, 0.17.0-alpha.1

the package `mypackage` depends on `postgres`, with features: `with-time` but `postgres` does not have these features.

Furthermore, the crate page for postgres 0.17.0 says nothing about these features, so I don't even know if they're supposed to be supported or not.

It seems like there would be something on docs.rs about it?

1
rustrust-cargorust-crates

1 Answers

15
votes

The only guaranteed way to see what features are available is to look at the Cargo.toml for the crate. This generally means that you need to navigate to the project's repository, find the correct file for the version you are interested in, and read it.

You are primarily looking for the [features] section, but also for any dependencies that are marked as optional = true, as optional dependencies count as an implicit feature flag.

Unfortunately, there's no requirement that the crate author puts any documentation about what each feature flag does. Good crates will document their feature flags in one or more locations:

  • As comments in Cargo.toml
  • Their README
  • Their documentation

In addition to looking at the Cargo.toml, crates uploaded to docs.rs will show what feature flags exist.

crates.io issue #465 suggests placing the feature list on the crate's page as well.

See also:

For the postgres crate, we can start at crates.io, then click "repository" to go to the repository. We then find the right tag (postgres-v0.17.0), then read the Cargo.toml:

[features]
with-bit-vec-0_6 = ["tokio-postgres/with-bit-vec-0_6"]
with-chrono-0_4 = ["tokio-postgres/with-chrono-0_4"]
with-eui48-0_4 = ["tokio-postgres/with-eui48-0_4"]
with-geo-types-0_4 = ["tokio-postgres/with-geo-types-0_4"]
with-serde_json-1 = ["tokio-postgres/with-serde_json-1"]
with-uuid-0_8 = ["tokio-postgres/with-uuid-0_8"]