What causes "Deprecated dependency sections are unsupported: {}" in rust-lang/cargo?

Run `cargo fix --edition` or manually update the manifest to use modern dependency section syntax

rust-lang/cargo · error · anyhow::Error

Deprecated dependency sections are unsupported

Error message

Deprecated dependency sections are unsupported: {}

What it means

Thrown by the cargo add operation when the target manifest contains deprecated dependency section syntax that cargo add cannot safely modify. The get_legacy_sections() check identifies old-style sections that predate current manifest conventions.

Solutions

Run `cargo fix --edition` or manually update the manifest to use modern dependency section syntax
Inspect the manifest for sections listed in the error and convert them to standard [dependencies] table format
Use `cargo migrate` or equivalent tooling to modernize the manifest before running cargo add

Defensive patterns

Strategy: validation

Validate before calling

// Check for legacy sections before running cargo add
fn check_legacy_sections(manifest_path: &Path) -> Result<(), String> {
    let content = std::fs::read_to_string(manifest_path).map_err(|e| e.to_string())?;
    let toml_val: toml::Value = toml::from_str(&content).map_err(|e| e.to_string())?;
    // Check for known deprecated section patterns
    if let Some(table) = toml_val.as_table() {
        for key in table.keys() {
            if key.contains("_dependencies") && !is_standard_dep_section(key) {
                return Err(format!("deprecated section found: {key}"));
            }
        }
    }
    Ok(())
}

Prevention

Run cargo fix --edition to modernize old manifests before using cargo add
Keep manifests in modern syntax by upgrading editions proactively
Avoid manually creating non-standard dependency section names

When it happens

Trigger: Running `cargo add <dep>` against a Cargo.toml that contains deprecated dependency section formats detected by LocalManifest::get_legacy_sections().

Common situations: Very old manifests with pre-2018 edition syntax. Manually edited manifests with non-standard section names. Manifests migrated through tooling that left legacy artifacts.

Related errors

AI-assisted analysis of rust-lang/cargo@98a09e7e7d (2026-08-11). Data as JSON: /api/errors/7f982abfdb26bffc. Report an issue: GitHub.

Appendix: source

Thrown at src/ops/cargo_add/mod.rs:88

    /// Whether the minimum supported Rust version should be considered during resolution
    pub honor_rust_version: Option<bool>,
}

/// Add dependencies to a manifest
pub fn add(workspace: &Workspace<'_>, options: &AddOptions<'_>) -> CargoResult<()> {
    let dep_table = options
        .section
        .to_table()
        .into_iter()
        .map(String::from)
        .collect::<Vec<_>>();

    let manifest_path = options.spec.manifest_path().to_path_buf();
    let mut manifest = LocalManifest::try_new(&manifest_path)?;
    let original_raw_manifest = manifest.to_string();
    let legacy = manifest.get_legacy_sections();
    if !legacy.is_empty() {
        anyhow::bail!(
            "Deprecated dependency sections are unsupported: {}",
            legacy.join(", ")
        );
    }

    let mut registry = workspace.package_registry()?;

    let deps = {
        let _lock = options
            .gctx
            .acquire_package_cache_lock(CacheLockMode::DownloadExclusive)?;
        registry.lock_patches();
        options
            .dependencies
            .iter()
            .map(|raw| {
                resolve_dependency(
                    &manifest,

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