Just copy the [Rust] section from the code below into your snippets file, e.g. in ~/.config/geany/snippets.conf.
[Rust] enum=enum %cursor% {\n\t%cursor%,\n}\n fn=fn %cursor%() {\n\t%cursor%\n}\n for=for elem in %cursor% {\n\t%cursor%\n}\n ife=if %cursor% {\n\t%cursor%\n} else {\n\t%cursor%\n}\n if=if %cursor% {\n\t%cursor%\n}\n loop=loop {\n\t%cursor%\n}\n main=fn main() {\n\t%cursor%\n}\n match=let %cursor% = match %cursor% {\n\tOk(%cursor%) => %cursor%,\n\tErr(_) => {\n\t\tpanic!("Error: {:?}", error)\n\t},\n};\n print=println!("%cursor%{}", %cursor%); std=use std::io%cursor%;\n while=while %cursor% {\n\t%cursor%\n}\n