retval(1): Rewrite in Rust

retval/Makefile

@@ -1,2 +1,4 @@
-retval: retval.c
-	$(CC) -o retval retval.c
+RUSTC = rustc
+
+retval: retval.rs
+	$(RUSTC) $(RUSTCFLAGS) -o retval retval.rs

retval/retval.rs

@@ -0,0 +1,18 @@
+use std::{env::args, process::ExitCode};
+
+fn usage(s: &str) -> ExitCode {
+    eprintln!("Usage: {} [status]", s);
+    ExitCode::from(64 /* NetBSD sysexits(3) EX_USAGE */)
+}
+
+fn main() -> ExitCode {
+    let argv = args().collect::<Vec<String>>();
+
+    match argv.len() {
+        2 => match argv[1].parse::<u8>() {
+            Ok(e) => ExitCode::from(e),
+            _ => usage(&argv[0]),
+        },
+        _ => usage(&argv[0]),
+    }
+}