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Simple, efficient logging for Rust.

fern 0.4.4, 0.5.*, 0.6.* security warning - colored feature + global allocator

One of our downstream dependencies, atty, through colored, has an unsoundness issue:

This shows up in one situation: if you're using colored (the crate, or our feature), and a custom global allocator.

I will be releasing fern 0.7.0, removing colored as a dependency. This may add another color crate, or may just document usage of alternatives (such as owo-colors + enable-ansi-support).

In the meantime, if you're using #[global_allocator], I highly recommend removing the fern/colored feature.

Or, for minimal code changes, you can also enable the colored/no-colors feature:

cargo add colored --features no-color

With the no-color feature, the vulnerable code will still be present, but unless you use any of the following APIs manually, it will never be called:

See #113 for further discussion.

Logging configuration is recursively branched, like a fern: formatting, filters, and output can be applied recursively to match increasingly specific kinds of logging. Fern provides a builder-based configuration backing for rust's standard log crate.

//! With fern, we can:

// Configure logger at runtime
    // Perform allocation-free log formatting
    .format(|out, message, record| {
            "[{} {} {}] {}",
    // Add blanket level filter -
    // - and per-module overrides
    .level_for("hyper", log::LevelFilter::Info)
    // Output to stdout, files, and other Dispatch configurations
    // Apply globally

// and log using log crate macros!
info!("hello, world!");

Examples of all features at the api docs. See fern in use with this example command line program.

Project Status

The fern project is primarily maintained by myself, @daboross on GitHub. It's a hobby project, but one I aim to keep at a high quality.


As this is a hobby project, contributions are very welcome!

The easiest way for you to contribute right now is to use fern in your application, and see where it's lacking. The current library has a solid base, but it lacks features, and I may not anticipate your use cases.

If you have a use case fern does not cover, please file an issue. This is immensely useful to me, to anyone wanting to contribute to the project, and to you as well if the feature is implemented.

If you're interested in helping fix an existing issue, or an issue you just filed, help is appreciated.

See CONTRIBUTING for technical information on contributing.