Installing the source package (i.e., "compiling the package") uses Cargo
(the package manager of the Rust programming language) if it is available.
Cargo is available on the "Tier 1" platforms listed here:

https://forge.rust-lang.org/release/platform-support.html

If Cargo is not available, the necessary static library is downloaded for
select operating systems (namely Linux, macOS, and Windows running on x86_64).
In either case, Cargo is an install-time dependency only.  Users of the binary
package do **not** need to have Cargo (or Rust) installed.

The "cargo" package on CRAN facilitates the installation of Cargo. Just run
"cargo::install()".  Or, below are manual installation notes for some
platforms.

On Linux and macOS,

   - Run the following in the terminal:

         curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

   OR, to use your operating system's package manager, run one of the following:

   - sudo apt install cargo (Debian/Ubuntu)
   - sudo dnf install cargo (Fedora/CentOS)
   - brew install rust (macOS)

On Windows,

   - 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
   - Restart R (so it picks up changes to your PATH) and install this package.

