What causes "packages downloaded" in rust-lang/cargo?

Run cargo fetch to ensure all packages are downloaded: cargo fetch.

packages downloaded

Error message

packages downloaded

What it means

This panic is in build_unit_features() during compilation. It calls package_set.get_one(dep_id).expect("packages downloaded") to check if a dependency is a proc-macro. The invariant is that by the time feature resolution runs, all packages in the dependency graph have already been downloaded and are available in the PackageSet. If get_one() returns an error (package not found), the expect panics.

Source

Thrown at src/ops/cargo_compile/mod.rs:1228

        .activated_features(package_id, FeaturesFor::NormalOrDev)
        .iter()
        .map(|s| s.to_string())
        .collect();

    // Include features enabled for use by dependencies so targets can also use them with the
    // required-features field when deciding whether to be built or skipped.
    let filtered_deps = PackageSet::filter_deps(
        package_id,
        resolve_with_overrides,
        has_dev_units,
        requested_kinds,
        target_data,
        force_all_targets,
    );
    for (dep_id, deps) in filtered_deps {
        let is_proc_macro = package_set
            .get_one(dep_id)
            .expect("packages downloaded")
            .proc_macro();
        for dep in deps {
            let features_for = FeaturesFor::from_for_host(is_proc_macro || dep.is_build());
            for feature in resolved_features
                .activated_features_unverified(dep_id, features_for)
                .unwrap_or_default()
            {
                features.insert(format!("{}/{}", dep.name_in_toml(), feature));
            }
        }
    }

    features
}

View on GitHub (pinned to 0e07a15537)

Solutions

Run cargo fetch to ensure all packages are downloaded: cargo fetch.
Clear the cargo registry cache and re-download: rm -rf ~/.cargo/registry && cargo fetch.
Regenerate Cargo.lock: rm Cargo.lock && cargo generate-lockfile.
Check network connectivity to the registry and retry the build.

Defensive patterns

Strategy: retry

Validate before calling

// Before compiling, ensure all packages are downloaded
cargo fetch
// Verify registry cache is populated
let registry_cache = dirs::home_dir().unwrap().join(".cargo/registry");
if !registry_cache.exists() {
    eprintln!("registry cache missing; run cargo fetch");
}

Prevention

Run cargo fetch before building in CI or offline environments.
Clear and re-download the registry cache if corrupted: rm -rf ~/.cargo/registry && cargo fetch.
Regenerate Cargo.lock if dependency metadata is inconsistent.
Check network connectivity to the registry before building.

When it happens

Trigger: Compiling a project where a dependency in the resolved graph was never downloaded — this could happen if the download step was skipped, if the PackageSet was constructed incompletely, or if a dependency was added to the resolution after the download phase.

Common situations: Corrupted or incomplete cargo cache (~/.cargo/registry); a cargo internal bug where feature resolution runs before package downloads complete; a network interruption during cargo fetch that was not properly detected; using cargo as a library with an incompletely populated PackageSet.

Related errors

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/117f5d4e7922a2ce.json. Report an issue: GitHub.