Crate features

features is a small library that implements runtime feature toggles for your library or program allowing behavior to be changed on boot or dynamically at runtime using the same compiled binary artifact. This is different from cargo's feature support which uses conditional compilation.

At its core, it is a macro (features!) that takes a collection of feature flag names which it uses to generate a module containing a function to enable a feature toggle (::enable()), a function to disable a feature toggle (::disable()) and a function to check if a feature toggle is enabled (::is_enabled()).

Example

Basic example:

#[macro_use]
extern crate bitflags;
#[macro_use]
extern crate features;

features! {
    pub mod feature {
        const Alpha = 0b00000001,
        const Beta = 0b00000010
    }
}

fn main() {
    assert_eq!(false, feature::is_enabled(feature::Alpha));
    assert_eq!(false, feature::is_enabled(feature::Beta));

    feature::enable(feature::Beta);
    assert_eq!(false, feature::is_enabled(feature::Alpha));
    assert_eq!(true, feature::is_enabled(feature::Beta));
}

Multiple feature sets:

#[macro_use]
extern crate bitflags;
#[macro_use]
extern crate features;

features! {
    pub mod ux {
        const JsonOutput = 0b10000000,
        const VerboseOutput = 0b01000000
    }
}

features! {
    pub mod srv {
        const Http2Downloading = 0b10000000,
        const BitTorrentDownloading = 0b01000000
    }
}

fn main() {
    // Parse CLI args, environment, read config file etc...
    srv::enable(srv::BitTorrentDownloading);
    ux::enable(ux::JsonOutput);

    if srv::is_enabled(srv::Http2Downloading) {
        println!("Downloading via http2...");
    } else if srv::is_enabled(srv::BitTorrentDownloading) {
        println!("Downloading via bit torrent...");
    } else {
        println!("Downloading the old fashioned way...");
    }

    if ux::is_enabled(ux::VerboseOutput) {
        println!("COOL");
    }
}

Feature Toggle References

Here are some articles and projects which insipred the implementation of features:

• Feature Toggles (Martin Fowler's blog)
• Using Feature Flags to Ship Changes with Confidence (TravisCI's blog)
• Feature Toggles are one of the worst kinds of Technical Debt (excellent cautionary tale and warning)
• Feature toggle (Wikipedia article)
• Ruby feature gem (nice prior art)

Macros

features

The features! macro generates a module to contain all feature toggling logic.