About Cargo.toml

Each crate from Crates.io comes with a file called Cargo.toml (there are a few exception that are probably old). As I understand this file is generated from the Cargo.toml that is in the repository which is included as Cargo.toml.orig.

The fields are described as the Manifest Format.

MSRV - Minimum Supported Rust Version

There are two fields that help with the version requirements. The older package/edition field and the newer package/rust-version field. See the MSRV report.