rustfmt is what implements Rust formating. However rustfmt is also distributed via rustup (Rust's toolchain manager). There it is also exposed under the alias cargo-fmt, which makes it work as cargo subcommand `cargo fmt` (a mechanism that's available to any crate). So it's technically not "part of cargo".

Of course the nature of all of this is a bit different, as Rust maintains cargo, rustfmt and a lot more dev tooling as part of it's official teams in comparison to other language ecosystems, where focus is on the compiler only.