The crates.io package index
The registry index that Cargo, Rust's package manager, reads to resolve crate versions and their dependencies, available both as a Git repository and as an HTTP sparse-protocol index.
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The crates.io index holds the metadata that Cargo needs to download Rust crates and resolve their dependencies. It is published both as a Git repository, in the format this repository represents, and as an HTTP API using the sparse index protocol.
The exact structure of the index, including how crate metadata and dependency information are encoded, is documented in the Cargo Book. The index is maintained by the crates.io team as part of the Rust package registry infrastructure that every Cargo installation relies on.