This is a definition of fields in Cargo.toml files for serde. It allows reading of Cargo.toml data, and serializing it using TOML or other formats. It's used by the lib.rs site to extract information about crates.
To get started, see Manifest::from_slice.
Additionally, this crate supports post-processing of the data to emulate Cargo's workspace inheritance and autobins features. It supports files on disk as well as other non-disk data sources.