Config is the second most popular crate in the configuration category. It's a layered configuration system for Rust applications, which allows to read configuration values from a variety of file formats (JSON, TOML, YAML etc), from environment variables and from user-defined sources such as command-line parameters.
Are you interested in helping maintaining it? It's more than a year that there has been little to no activity, but as discussed here the project owner is open to add new maintainers to the project.