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Timeline: 2014 - now
Language: Rust
fern is a logging library backend favoring runtime configurability and fast logging-path execution. Notably, it can run full formatting and filtering with zero allocations on the logging path.
The sell is simple configuration. Since it's all Rust code, changing logging based on command line parameters or config files is easy. The biggest downside, compared to projects like log4rs, is that that configuration still all has to be written in Rust code.