unsupported rust target for file

Error message

unsupported rust target for file {file_name}: unknown libc version

What it means

configure_target derives crosstool-ng configuration from the Rust target triple encoded in the file name. It selects the libc configuration by substring: uclibc, musl, or 'none' (newlib for bare-metal). If the file name contains none of these markers, no libc can be configured and it bails with this error.

Solutions

Pass a Rust target whose triple includes musl (e.g. x86_64-unknown-linux-musl) or uclibc or a bare-metal 'none-eabi' target
Check the target triple for typos (e.g. 'musll' instead of 'musl')
If glibc support is needed, add a configure_glibc branch checking for "gnu" in file_name

Example fix

// before
let target = "x86_64-unknown-linux-gnu";
// after
let target = "x86_64-unknown-linux-musl";

Defensive patterns

Strategy: validation

Validate before calling

fn has_supported_libc(target: &str) -> bool {
    target.contains("uclibc") || target.contains("musl") || target.contains("none")
}
assert!(has_supported_libc(rust_target), "unsupported libc in target: {rust_target}");

Type guard

fn is_cross_target_supported(triple: &str) -> bool {
    ["uclibc", "musl", "none"].iter().any(|m| triple.contains(m))
}

Try / catch

match configure_crosstool(&target) {
    Ok(cfg) => cfg,
    Err(e) if e.to_string().contains("unknown libc version") => {
        eprintln!("target {target} uses an unconfigured libc; use a musl/uclibc/none target");
        std::process::exit(1);
    }
    Err(e) => return Err(e),
}

Prevention

Maintain the target allowlist and cross-check new targets against libc markers
Verify target triples with `rustc --print target-list` before adding them
Note glibc (gnu) targets are intentionally unsupported here — do not add them without code changes

When it happens

Trigger: configure_crosstool is called with a target whose file name has no 'uclibc', 'musl', or 'none' substring — e.g. a gnu/EABI target like 'arm-unknown-linux-gnueabihf' or 'x86_64-pc-windows-msvc' passed into the crosstool pipeline.

Common situations: Adding a new target to the support matrix that uses glibc (gnu) — the script only covers uclibc/musl/newlib; a typo in the target triple; running the cross-compile setup for a host/Windows target.

Related errors

AI-assisted analysis of cross-rs/cross@8c1a8aa4b6 (2026-09-13). Data as JSON: /api/errors/7e25ceedc767d264. Report an issue: GitHub.

Appendix: source

Thrown at xtask/src/crosstool.rs:353

    }
    if ct_gcc.starts_with('\n') {
        ct_gcc.remove(0);
    }
    contents = contents
        .replacen("%CT_GCC_V%", &ct_gcc_v, 1)
        .replacen("%CT_GCC%", &ct_gcc, 1);

    // configure the libc versions
    let (key, libc_v, mut libc, extras) = if file_name.contains("gnu") {
        configure_glibc(glibc_version)?
    } else if file_name.contains("uclibc") {
        configure_uclibc(uclibc_version)?
    } else if file_name.contains("musl") {
        configure_musl(musl_version)?
    } else if file_name.contains("none") {
        configure_newlib(newlib_version)?
    } else {
        eyre::bail!("unsupported rust target for file {file_name}: unknown libc version");
    };
    if libc.starts_with('\n') {
        libc.remove(0);
    }
    contents = contents
        .replacen(&format!("%CT_{key}_V%"), &libc_v, 1)
        .replacen(&format!("%CT_{key}%"), &libc, 1);
    if let Some(extras) = extras {
        contents = contents.replacen(&format!("%CT_{key}_EXTRAS%"), &extras, 1);
    }

    // configure the `CT_LINUX` values
    if file_name.contains("linux") {
        let linux_versions: Vec<&str> = linux_version.split('.').collect();
        if !matches!(linux_versions.len(), 2 | 3) {
            eyre::bail!("invalid linux version, got {linux_version}");
        }
        let linux_major = linux_versions[0].parse::<u32>()?;

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