Handle world Cargo help locally before the host request

userland/cargo/src/lib.rs

@@ -23,8 +23,17 @@ struct Executable {
 fn error(value: impl std::fmt::Debug) -> io::Error {
     io::Error::other(format!("{value:?}"))
 }
+fn local_help(argv: &[String]) -> Option<&'static str> {
+    matches!(argv.first().map(String::as_str), Some("--help" | "-h" | "help")).then_some(
+        "Usage: cargo <COMMAND> [OPTIONS]\n\nWorld commands: build, check, clippy, test, run, fmt, fetch, add, remove\n",
+    )
+}
 pub fn run() -> io::Result<i32> {
     let argv: Vec<String> = std::env::args().skip(1).collect();
+    if let Some(help) = local_help(&argv) {
+        io::stdout().write_all(help.as_bytes())?;
+        return Ok(0);
+    }
     let data = serde_json::to_vec(
         &serde_json::json!({"argv": argv, "cwd": std::env::current_dir()?.to_string_lossy()}),
     )?;
@@ -108,6 +117,17 @@ pub fn run() -> io::Result<i32> {
     }
     Ok(0)
 }
+#[cfg(test)]
+#[test]
+fn help_is_local_and_lists_world_commands() {
+    for flag in ["--help", "-h", "help"] {
+        let help = local_help(&[flag.into()]).expect("local help");
+        assert!(help.starts_with("Usage: cargo <COMMAND> [OPTIONS]\n"));
+        assert!(help.contains("build, check, clippy, test, run, fmt, fetch, add, remove"));
+    }
+    assert!(local_help(&["build".into()]).is_none());
+    assert!(local_help(&[]).is_none());
+}
 fn execution_args(argv: &[String]) -> Vec<String> {
     let split = argv.iter().position(|a| a == "--").unwrap_or(argv.len());
     let mut args = argv.get(split + 1..).unwrap_or_default().to_vec();