rust-lang/cargo · warning

just inserted

Error message

just inserted

What it means

This panic is in rename_table() immediately after the parent.remove(old)/parent.insert(new) sequence. On line 558 it inserts the renamed key into the TOML table, then on line 559 calls parent.key_mut(new).expect("just inserted") to copy decoration from the old key to the new key. The invariant is that inserting a key makes it immediately retrievable via key_mut.

Source

Thrown at src/ops/cargo_fix/mod.rs:559

    fixes
}

fn rename_target_fields_2024(target: &mut dyn toml_edit::TableLike) -> usize {
    let mut fixes = 0;
    fixes += rename_table(target, "crate_type", "crate-type");
    fixes += rename_table(target, "proc_macro", "proc-macro");
    fixes
}

fn rename_table(parent: &mut dyn toml_edit::TableLike, old: &str, new: &str) -> usize {
    let Some(old_key) = parent.key(old).cloned() else {
        return 0;
    };

    let project = parent.remove(old).expect("returned early");
    if !parent.contains_key(new) {
        parent.insert(new, project);
        let mut new_key = parent.key_mut(new).expect("just inserted");
        *new_key.dotted_decor_mut() = old_key.dotted_decor().clone();
        *new_key.leaf_decor_mut() = old_key.leaf_decor().clone();
    }
    1
}

fn check_resolver_change<'gctx>(
    ws: &Workspace<'gctx>,
    target_data: &mut RustcTargetData<'gctx>,
    opts: &FixOptions,
) -> CargoResult<()> {
    let root = ws.root_maybe();
    match root {
        MaybePackage::Package(root_pkg) => {
            if root_pkg.manifest().resolve_behavior().is_some() {
                // If explicitly specified by the user, no need to check.
                return Ok(());
            }

View on GitHub (pinned to 0e07a15537)

Solutions

1. Manually rename the deprecated fields in Cargo.toml before running cargo fix.
2. Update cargo to the latest stable release to get a compatible toml_edit version.
3. If embedding cargo, ensure the toml_edit version matches what cargo expects.

// Same as 298 — pre-validate manifest field names before cargo fix
let content = std::fs::read_to_string("Cargo.toml")?;
let doc: toml_edit::DocumentMut = content.parse()?;
// Check for deprecated fields
for table_name in ["lib", "bin", "crate_type", "proc_macro"] {
    if doc.get(table_name).is_some() && table_name.contains('_') {
        eprintln!("rename `{}` to `{}`", table_name, table_name.replace('_', "-"));
    }
}

When it happens

Trigger: Inserting a key into a toml_edit table and then not finding it via key_mut — this would require a toml_edit internal bug where insert doesn't register the key, or the new key string is empty/whitespace and toml_edit silently rejects it while reporting success.

Common situations: Running cargo fix --edition on a manifest with deprecated fields; using a cargo version built against a buggy or incompatible toml_edit release; essentially unreachable in normal operation.

Related errors

• returned early
• able to invoke rustc
• bin is an array of tables
• package is a table
• artifact-dir was not locked

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/16163e6180604b6e.json. Report an issue: GitHub.