What causes "only one split can use 'rest'" in zed-industries/zed?

Give all but one split an explicit percentage or absolute count

only one split can use 'rest'

Error message

only one split can use 'rest'

What it means

Bailed while computing split sizes for `ep split`: more than one split spec used the `=rest` size. `rest` means 'whatever remains after all other splits are satisfied', which is only well-defined for a single split, so a second occurrence is rejected before any lines are distributed.

Source

Thrown at crates/edit_prediction_cli/src/split_dataset.rs:187

fn compute_split_counts(specs: &[SplitSpec], total: usize) -> Result<Vec<usize>> {
    let mut counts = vec![0usize; specs.len()];
    let mut remaining = total;
    let mut rest_index: Option<usize> = None;

    for (i, spec) in specs.iter().enumerate() {
        match &spec.size {
            SplitSize::Percentage(pct) => {
                let count = (total as f64 * pct).round() as usize;
                counts[i] = count.min(remaining);
                remaining = remaining.saturating_sub(counts[i]);
            }
            SplitSize::Absolute(count) => {
                counts[i] = (*count).min(remaining);
                remaining = remaining.saturating_sub(counts[i]);
            }
            SplitSize::Rest => {
                if rest_index.is_some() {
                    bail!("only one split can use 'rest'");
                }
                rest_index = Some(i);
            }
        }
    }

    if let Some(idx) = rest_index {
        counts[idx] = remaining;
    }

    Ok(counts)
}

fn write_lines_to_file(path: &Path, lines: &[String]) -> Result<()> {
    if let Some(parent) = path.parent() {
        if !parent.as_os_str().is_empty() {
            std::fs::create_dir_all(parent)
                .with_context(|| format!("failed to create directory '{}'", parent.display()))?;

View on GitHub (pinned to f4178619ac)

Solutions

Give all but one split an explicit percentage or absolute count
Keep `=rest` on the single split that should absorb leftovers

Example fix

# before
ep split data.jsonl train.jsonl=80% dev.jsonl=rest test.jsonl=rest

# after
ep split data.jsonl train.jsonl=80% dev.jsonl=10% test.jsonl=rest

Defensive patterns

Strategy: validation

Validate before calling

let rest_count = specs.iter().filter(|s| s.ends_with("=rest")).count();
if rest_count > 1 {
    // reject before running: only one split may claim the remainder
}

Try / catch

Count `=rest` occurrences across the spec list up front and fail with a clear message naming the duplicate splits.

Prevention

Audit spec lists for duplicate rest before launching split
Convert extra rest specs to explicit percentages or counts when adding splits
Encode the invariant 'at most one rest' in whatever generates the command line

When it happens

Trigger: Running `ep split data.jsonl train.jsonl=80% valid.jsonl=rest test.jsonl=rest` — two specs claiming the remainder.

Common situations: Copy-pasting a `=rest` spec when adding an extra split; adapting a two-way split template to three-way without converting one rest to an explicit size.

Related errors

AI-assisted analysis of zed-industries/zed@f4178619ac (2026-08-20). Data as JSON: /api/errors/6e417c4a56071dbc. Report an issue: GitHub.