rust-lang/cargo · error

validation ensures this is a table

Error message

validation ensures this is a table

What it means

Invariant in lockfile encoding: `meta.as_table().expect("validation ensures this is a table")`. When emitting `[metadata]` in `Cargo.lock`, cargo assumes the TOML value passed validation as a table. Lockfile metadata is validated upstream, so a non-table value here means validation was bypassed.

Source

Thrown at src/ops/lockfile.rs:188

        emit_package(dep, &mut out);
    }

    if let Some(patch) = toml.get("patch") {
        let list = patch["unused"].as_array().unwrap();
        for entry in list {
            out.push_str("[[patch.unused]]\n");
            emit_package(entry.as_table().unwrap(), &mut out);
            out.push('\n');
        }
    }

    if let Some(meta) = toml.get("metadata") {
        // 1. We need to ensure we print the entire tree, not just the direct members of `metadata`
        //    (which `toml_edit::Table::to_string` only shows)
        // 2. We need to ensure all children tables have `metadata.` prefix
        let meta_table = meta
            .as_table()
            .expect("validation ensures this is a table")
            .clone();
        let mut meta_doc = toml::Table::new();
        meta_doc.insert("metadata".to_owned(), toml::Value::Table(meta_table));

        out.push_str(&meta_doc.to_string());
    }

    // Historical versions of Cargo in the old format accidentally left trailing
    // blank newlines at the end of files, so we just leave that as-is. For all
    // encodings going forward, though, we want to be sure that our encoded lock
    // file doesn't contain any trailing newlines so trim out the extra if
    // necessary.
    if resolve.version() >= ResolveVersion::V2 {
        while out.ends_with("\n\n") {
            out.pop();
        }
    }
    out

View on GitHub (pinned to 0e07a15537)

Solutions

1. Delete the `[metadata]` block from `Cargo.lock` (and regenerate with `cargo generate-lockfile` if needed).
2. Inspect `Cargo.lock` for a `[metadata]` that is not a table and fix its shape.
3. Report a cargo bug if no manual edit was made to the lockfile.

Example fix

// before
let meta_table = meta.as_table().expect("validation ensures this is a table").clone();

// after
let meta_table = meta.as_table()
    .ok_or_else(|| anyhow::format_err!("[metadata] in Cargo.lock is not a table"))?
    .clone();

// Before lockfile ops, assert [metadata] is a table.
if let Some(meta) = lockfile_toml.get("metadata") {
    if !meta.is_table() {
        return Err(anyhow!("Cargo.lock [metadata] is not a table"));
    }
}

fn metadata_is_table(v: &toml::Value) -> bool { matches!(v, toml::Value::Table(_)) }

When it happens

Trigger: Fires only if `Cargo.lock`'s `[metadata]` section parses to a non-table TOML value (array, string, etc.) that slipped past validation. Requires either a hand-edited lockfile with a malformed `[metadata]` or a cargo bug in the encode/decode round-trip.

Common situations: Hand-editing `Cargo.lock` and replacing `[metadata]` with a scalar/array; a cargo regression in lockfile metadata handling; a tool that rewrites `Cargo.lock` with an invalid metadata shape.

Related errors

• env var was not array
• len() == 1 above
• dupes must be in sync
• source ID should have valid URLs
• artifact-dir was not locked

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/32c1a7881c5ae079.json. Report an issue: GitHub.