Features in Cargo

In the last section, we defined the hex crate as an external dependency that we could use within our project with a simple name and version of the crate. However, there are other important factors to consider than declaring a version.

In some cases, such as the case in Substrate runtime development, the std library may not be available. This is due to the constraints of running in a Wasm runtime versus a native binary.

There are also other cases where std may not be available. For this reason, some crates offer the option of a no-std implementation which does not use the Rust Standard Library and can run in these resource-constrained environments.

Specifying features

A feature in the context of Rust crates refers to the ability to conditionally/optionally include (or not include) certain sub-dependencies.

To have the hex crate be compatible in a no-std environment, default-features must be declared:

[dependencies]
hex = { version = "0.4", default-features = false }

This is now more akin to a JSON object, with curly brackets encasing the keys and values within hex. Setting the default features of a crate to false will force the crate not to use std, as that was the default.

Keep in mind that not all crates support no-std environments.