cadcraft-cli: --help / -h / help exit 0 and print the usage to stdout (#1526)

A help request is not a failure: --version already goes to stdout with
exit 0, and now --help, -h and help do the same. Bare cadcraft-cli
keeps reporting on stderr with exit 1, like git.

Fixes #1525

apps/cadcraft-cli/src/main.rs

@@ -690,6 +690,10 @@ fn cli() -> ExitCode {
             outln!("cadcraft-cli {}", env!("CARGO_PKG_VERSION"));
             Ok(())
         }
+        Some("--help" | "-h" | "help") => {
+            outln!("{USAGE}");
+            Ok(())
+        }
         _ => Err(USAGE.into()),
     };
     match r {

apps/cadcraft-cli/tests/help.rs

@@ -0,0 +1,23 @@
+//! `--help` is a request, not a failure (#1525): it goes to stdout with exit 0, like `--version`.
+//! Bare `cadcraft-cli` keeps reporting on stderr with exit 1.
+
+use std::process::Command;
+
+#[test]
+fn help_flags_print_usage_to_stdout_and_exit_zero() {
+    for args in [&["--help"][..], &["-h"], &["help"]] {
+        let out = Command::new(env!("CARGO_BIN_EXE_cadcraft-cli")).args(args).output().expect("run cadcraft-cli");
+        assert!(out.status.success(), "{args:?}: {:?}", out.status);
+        let stdout = String::from_utf8_lossy(&out.stdout);
+        assert!(stdout.starts_with("usage:"), "{args:?}: {stdout:?}");
+        assert!(String::from_utf8_lossy(&out.stderr).is_empty(), "{args:?}: stderr should be empty");
+    }
+}
+
+#[test]
+fn bare_invocation_still_reports_on_stderr() {
+    let out = Command::new(env!("CARGO_BIN_EXE_cadcraft-cli")).output().expect("run cadcraft-cli");
+    assert_eq!(out.status.code(), Some(1));
+    assert!(out.stdout.is_empty(), "bare invocation keeps its usage on stderr");
+    assert!(String::from_utf8_lossy(&out.stderr).starts_with("usage:"));
+}