What causes "rustfmt failed with {exit_status}" in rust-lang/rust?

Run the exact rustfmt invocation from the error's context manually to see the real diagnostic.

rust-lang/rust · error · FormatDiffError

rustfmt failed with {exit_status}

Error message

rustfmt failed with {exit_status}

What it means

Returned by rustfmt's format-diff tool when the spawned rustfmt process exits with a non-success status. The exit status is interpolated into the message and wrapped as ErrorKind::Other (FormatDiffError::IoError). format-diff feeds git-diff ranges to rustfmt via --file-lines; a non-zero exit means rustfmt rejected the input or hit an internal error.

Source

Thrown at src/tools/rustfmt/src/format-diff/main.rs:112

    let ranges_as_json = json::to_string(ranges).unwrap();

    debug!("Files: {:?}", files);
    debug!("Ranges: {:?}", ranges);

    let rustfmt_var = env::var_os("RUSTFMT");
    let rustfmt = match &rustfmt_var {
        Some(rustfmt) => rustfmt,
        None => OsStr::new("rustfmt"),
    };
    let exit_status = process::Command::new(rustfmt)
        .args(files)
        .arg("--file-lines")
        .arg(ranges_as_json)
        .status()?;

    if !exit_status.success() {
        return Err(FormatDiffError::IoError(io::Error::new(
            io::ErrorKind::Other,
            format!("rustfmt failed with {exit_status}"),
        )));
    }
    Ok(())
}

/// Scans a diff from `from`, and returns the set of files found, and the ranges
/// in those files.
fn scan_diff<R>(
    from: R,
    skip_prefix: u32,
    file_filter: &str,
) -> Result<(HashSet<String>, Vec<Range>), FormatDiffError>
where
    R: io::Read,
{
    let diff_pattern = format!(r"^\+\+\+\s(?:.*?/){{{skip_prefix}}}(\S*)");

View on GitHub (pinned to 7088e4b63a)

Solutions

Run the exact rustfmt invocation from the error's context manually to see the real diagnostic.
Ensure the rustfmt version matches what format-diff expects (nightly for --file-lines).
Fix syntax errors in the files included in the diff ranges.
Validate the --file-lines JSON structure against the expected schema.

Example fix

# before: rustfmt --file-lines fails opaquely

# after: reproduce and read the real error
rustfmt --file-lines file_lines.json --edition 2021 src/lib.rs
echo $?   # inspect rustfmt's own stderr for the cause

Defensive patterns

Strategy: try-catch

Validate before calling

fn file_lines_json_valid(json: &str) -> bool {
    serde_json::from_str::<Vec<serde_json::Value>>(json).is_ok()
}
// Before invoking rustfmt --file-lines, assert file_lines_json_valid(&ranges_as_json).

Try / catch

let status = Command::new(rustfmt).args(files).arg("--file-lines").arg(json).status()?;
if !status.success() {
    // capture rustfmt's own stderr to report the real diagnostic
    return Err(format!("rustfmt failed with {status}").into());
}
Ok(())

Prevention

Reproduce the exact rustfmt invocation manually to read its real stderr.
Use a rustfmt version that supports --file-lines (typically nightly).
Validate the file-lines JSON and fix syntax errors in diffed files before formatting.

When it happens

Trigger: Running `rustfmt --file-lines <json>` (typically via git diff integration / format-diff) where rustfmt exits non-zero - syntax error in the ranged file, invalid --file-lines JSON, rustfmt config error, or an internal rustfmt panic. format-diff checks exit_status.success() and raises this error.

Common situations: Unparsable Rust in the diff range; mismatched rustfmt version that does not support --file-lines; nightly-only config option used on stable rustfmt; malformed file-lines JSON; partial edits that confuse the range formatter.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/e3be8e65f98e4716. Report an issue: GitHub.