FWIW, I don't think it's nice that rustup fetches and installs new versions without prompting, but I suppose that other users like it or get used to it. Fortunately most Rust projects work on any recently stable version.
I don't think it's true. rustup installs new version only when you run `rustup update`. What parent is talking about is pinning a particular rustc version in Cargo.toml, which allows rustup to download that version of rustc to build that particular project/crate.