rust-lang/cargo · error

malformed output when learning about crate-type {} informati

Error message

malformed output when learning about crate-type {} information
{}

What it means

In parse_crate_type (target_info.rs:664-692), Cargo runs `rustc --print=...` and reads crate-type information line by line. After confirming the crate type is supported (no 'unsupported crate type' in stderr), it expects at least one more line of output; if `lines.next()` is None, the rustc output is malformed/truncated and Cargo bails naming the crate-type and the captured output.

Source

Thrown at src/compiler/build_context/target_info.rs:679

///
/// This function can not handle more than one file per type (with wasm32-unknown-emscripten, there
/// are two files for bin (`.wasm` and `.js`)).
fn parse_crate_type(
    crate_type: &CrateType,
    cmd: &ProcessBuilder,
    output: &str,
    error: &str,
    lines: &mut str::Lines<'_>,
) -> CargoResult<Option<(String, String)>> {
    let not_supported = error.lines().any(|line| {
        (line.contains("unsupported crate type") || line.contains("unknown crate type"))
            && line.contains(&format!("crate type `{}`", crate_type))
    });
    if not_supported {
        return Ok(None);
    }
    let Some(line) = lines.next() else {
        anyhow::bail!(
            "malformed output when learning about crate-type {} information\n{}",
            crate_type,
            output_err_info(cmd, output, error)
        )
    };
    let mut parts = line.trim().split("___");
    let prefix = parts.next().unwrap();
    let Some(suffix) = parts.next() else {
        return error_missing_print_output("file-names", cmd, output, error);
    };

    Ok(Some((prefix.to_string(), suffix.to_string())))
}

/// Helper for creating an error message for missing output from a certain `--print` request.
fn error_missing_print_output<T>(
    request: &str,
    cmd: &ProcessBuilder,

View on GitHub (pinned to 0e07a15537)

Solutions

1. Inspect the full error (Cargo prints rustc stdout/stderr) to see what rustc actually returned.
2. Disable rustc wrappers temporarily (`unset RUSTC_WRAPPER RUSTC_WORKSPACE_WRAPPER`) to isolate the cause.
3. Reinstall/repair the toolchain with `rustup toolchain install`.
4. If using a custom target JSON, ensure the crate-type list is valid.

Example fix

// before
$ RUSTC_WRAPPER=./broken-wrapper cargo build
error: malformed output when learning about crate-type cdylib information

// after
$ unset RUSTC_WRAPPER && cargo build

// Run a smoke test of rustc --print in CI; surface wrapper problems early
match std::process::Command::new(rustc)
    .args(["--print", "crate-type", "--", "-"])
    .output()
{
    Ok(o) if !o.stdout.is_empty() => { /* ok */ }
    _ => eprintln!("rustc --print output looks broken; check wrappers/install"),
}

When it happens

Trigger: A custom rustc (or rustc wrapper) that returns success for `--print` but emits fewer lines than Cargo expects; a broken/raced rustc installation; a misbehaving `RUSTC_WRAPPER`/`RUSTC` override; partial rustc output due to a signal/timeout.

Common situations: Using sccache or a custom rustc wrapper that mangles --print output; corrupted toolchain install; concurrent builds racing on a shared target dir; very old or patched rustc.

Related errors

• output of --print={request} missing when learning about targ
• could not find specification for target `{target}`. Depend
• {}: {:?}
• rustc load ok
• able to invoke rustc

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/ca977566f105ed6e.json. Report an issue: GitHub.