[rs] Make file arguments required

rs/src/main.rs

@@ -45,16 +45,13 @@ days! {
 
 #[derive(Parser)]
 struct Args {
+    #[arg(required = true)]
     files: Vec<PathBuf>,
 }
 
 fn main() -> io::Result<()> {
     let args = Args::parse();
 
-    if args.files.is_empty() {
-        eprintln!("No days specified");
-    }
-
     let mut first_day = true;
     for file in args.files {
         if !first_day {