output of --print= missing when learning about…

Error message

output of --print={request} missing when learning about target-specific information from rustc
{err_info}

What it means

Cargo invokes `rustc --print=<request>` (e.g. `file-names`, `sysroot`, `target-list`) to gather target-specific data, and the expected output line was entirely absent. `error_missing_print_output` formats the command and its stdout/stderr for diagnosis.

Solutions

Inspect `err_info` in the message: run the printed `command was:` line manually to see rustc's actual response.
Update the toolchain: `rustup update stable`.
Remove `RUSTC_WRAPPER`/`RUSTC_WORKSPACE_WRAPPER` and retry.
Validate custom target JSON against the schema; fall back to a built-in target to confirm.

Example fix

// before
RUSTC_WRAPPER=cache-build cargo build

// after
unset RUSTC_WRAPPER
cargo build
# then run the failing --print command by hand:
# rustc --print=file-names --target <triple>

Defensive patterns

Strategy: validation

Validate before calling

fn rustc_supports_print(request: &str) -> bool {
    let out = std::process::Command::new("rustc").arg(format!("--print={request}")).output();
    matches!(out, Ok(o) if o.status.success() && !o.stdout.is_empty())
}

Type guard

null

Try / catch

match learn_target_info(request) {
    Err(e) if e.to_string().contains("output of --print") => fallback_builtin_target_info()?,
    r => r?,
}

Prevention

Sanity-check rustc with `rustc --print=sysroot` in CI boot.
Avoid `RUSTC_WRAPPER` shims that swallow stdout.
Keep custom target specs minimal and conform to the schema.

When it happens

Trigger: Any `rustc --print=...` invocation that returns no matching output: missing `--print` support in an old/custom rustc, a malformed custom target JSON, or a wrapper that strips rustc stdout during probing.

Common situations: Using a heavily patched or pre-1.0 rustc; custom target specs (`--target foo.json`) that don't expose required keys; CI images where rustc is a shim; `RUSTC_WRAPPER` interfering with `--print` queries.

Related errors

AI-assisted analysis of rust-lang/cargo@98a09e7e7d (2026-08-11). Data as JSON: /api/errors/6795c1022263c1d0. Report an issue: GitHub.

Appendix: source

Thrown at src/compiler/build_context/target_info.rs:733

    };
    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,
    stdout: &str,
    stderr: &str,
) -> CargoResult<T> {
    let err_info = output_err_info(cmd, stdout, stderr);
    anyhow::bail!(
        "output of --print={request} missing when learning about \
     target-specific information from rustc\n{err_info}",
    )
}

/// Helper for creating an error message when parsing rustc output fails.
fn output_err_info(cmd: &ProcessBuilder, stdout: &str, stderr: &str) -> String {
    let mut result = format!("command was: {}\n", cmd);
    if !stdout.is_empty() {
        result.push_str("\n--- stdout\n");
        result.push_str(stdout);
    }
    if !stderr.is_empty() {
        result.push_str("\n--- stderr\n");
        result.push_str(stderr);
    }
    if stdout.is_empty() && stderr.is_empty() {
        result.push_str("(no output received)");

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