Users of the binary package do **not** need to have Cargo (the package manager
of the Rust programming language).  Installing the source package, however,
uses Cargo to compile Rust source code.  The "cargo" package on CRAN
facilitates finding and running Cargo on the system.  If a suitable Cargo
installation is not available, a static library is instead downloaded for
select operating systems and CPU architectures.  

See https://rustup.rs to set up Cargo.  The default options are sufficient in
most situations.  On Windows, however, please do the following:

   - Download rustup-init.exe from https://win.rustup.rs
   - In the terminal, run:  rustup-init.exe -y --default-host x86_64-pc-windows-gnu
   - Restart the terminal (so the new terminal picks up changes to your PATH).
   - In a new terminal, run:  rustup target add i686-pc-windows-gnu