Just what I want, a C requirement for a Rust project
Sounds like it's not your Rust project.
But cmake is required by Rustls (or more precisely, aws-lc-rs).
Even more precisely would be that it's a requirement for the default rustls backed on some platforms for some features. aws-lc-rs is a pain, but fortunately ring is just a feature flag away.
Sorry, it's not the point of what you were saying. You can tell I've been stung supporting users with this.