rust-lang/cargo · error · anyhow::Error

expected rustc or `@path` as first argument

Error message

expected rustc or `@path` as first argument

What it means

Thrown in FixArgs::from_args (src/ops/cargo_fix/mod.rs:1224-1229) when cargo is running as the rustc proxy during `cargo fix` and argv has no second element (no rustc/`@path` after the program name). This is an internal contract: cargo-fix invokes itself with the rustc executable as the first real argument.

Source

Thrown at src/ops/cargo_fix/mod.rs:1229

    other: Vec<OsString>,
    /// Path to the `rustc` executable.
    rustc: PathBuf,
    /// Path to host sysroot.
    sysroot: Option<PathBuf>,
}

impl FixArgs {
    fn get() -> CargoResult<FixArgs> {
        Self::from_args(env::args_os())
    }

    // This is a separate function so that we can use it in tests.
    fn from_args(argv: impl IntoIterator<Item = OsString>) -> CargoResult<Self> {
        let mut argv = argv.into_iter();
        let mut rustc = argv
            .nth(1)
            .map(PathBuf::from)
            .ok_or_else(|| anyhow::anyhow!("expected rustc or `@path` as first argument"))?;
        let mut file = None;
        let mut enabled_edition = None;
        let mut other = Vec::new();

        let mut handle_arg = |arg: OsString| -> CargoResult<()> {
            let path = PathBuf::from(arg);
            if path.extension().and_then(|s| s.to_str()) == Some("rs") && path.exists() {
                file = Some(path);
                return Ok(());
            }
            if let Some(s) = path.to_str() {
                if let Some(edition) = s.strip_prefix("--edition=") {
                    enabled_edition = Some(edition.parse()?);
                    return Ok(());
                }
            }
            other.push(path.into());
            Ok(())

View on GitHub (pinned to 0e07a15537)

Solutions

1. Re-run `cargo fix` normally (without a custom proxy wrapper) so cargo assembles the correct argv.
2. Update/reinstall cargo to rule out a corrupted toolchain.
3. If you are invoking the proxy manually, pass the rustc executable path as the first argument (or `@argfile`).
4. Reduce command-line length so the @path argfile mechanism is not triggered incorrectly.

Example fix

# before: manual proxy call missing rustc arg
/home/user/.cargo/bin/cargo-fix

# after: let cargo invoke the proxy
cargo fix --edition

// Internal proxy contract: argv must contain at least program + rustc path.
fn proxy_argv_ok(argv: &[std::ffi::OsString]) -> bool {
    argv.len() >= 2
}
// assert!(proxy_argv_ok(&env::args_os().collect::<Vec<_>>()));

fn has_rustc_or_argfile(argv: &[std::ffi::OsString]) -> bool {
    argv.get(1).is_some()
}

// FixArgs::from_args is internal; surface a clear message if it fails.
match FixArgs::from_args(env::args_os()) {
    Err(e) if e.to_string().contains("expected rustc") => {
        eprintln!("proxy invoked without a rustc argument; re-run `cargo fix` normally");
        return Err(e);
    }
    r => r,
}

When it happens

Trigger: The proxy entry point receives fewer than two arguments — e.g. the FIX-related env drove cargo into proxy mode but the invoking command lacked the rustc path. Essentially an internal/invocation bug, not a user manifest error.

Common situations: A wrapper or custom tool calling the cargo-fix proxy binary with no rustc argument. Corruption of the proxy command line (e.g. very long arg lists causing the @path argfile path to be dropped). Broken cargo installation or shim that strips arguments.

Related errors

• expected rustc as first argument
• could not find .rs file in rustc args
• able to invoke rustc
• malformed output when learning about crate-type {} informati
• output of --print={request} missing when learning about targ

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/02be171baa02a191.json. Report an issue: GitHub.