External tools

One of the goals of Cargo is simple integration with third-party tools, like IDEs and other build systems. To make integration easier, Cargo has several facilities:

  • a cargo metadata command, which outputs project structure and dependencies information in JSON,

  • a --message-format flag, which outputs information about a particular build, and

  • support for custom subcommands.

Information about project structure

You can use cargo metadata command to get information about project structure and dependencies. The output of the command looks like this:

  // Integer version number of the format.
  "version": integer,

  // List of packages for this workspace, including dependencies.
  "packages": [
      // Opaque package identifier.
      "id": PackageId,

      "name": string,

      "version": string,

      "source": SourceId,

      // A list of declared dependencies, see `resolve` field for actual dependencies.
      "dependencies": [ Dependency ],

      "targets: [ Target ],

      // Path to Cargo.toml
      "manifest_path": string,

  "workspace_members": [ PackageId ],

  // Dependencies graph.
  "resolve": {
     "nodes": [
         "id": PackageId,
         "dependencies": [ PackageId ]

The format is stable and versioned. When calling cargo metadata, you should pass --format-version flag explicitly to avoid forward incompatibility hazard.

If you are using Rust, there is cargo_metadata crate.

Information about build

When passing --message-format=json, Cargo will output the following information during the build:

  • compiler errors and warnings,

  • produced artifacts,

  • results of the build scripts (for example, native dependencies).

The output goes to stdout in the JSON object per line format. The reason field distinguishes different kinds of messages.

Information about dependencies in the Makefile-compatible format is stored in the .d files alongside the artifacts.

Custom subcommands

Cargo is designed to be extensible with new subcommands without having to modify Cargo itself. This is achieved by translating a cargo invocation of the form cargo (?<command>[^ ]+) into an invocation of an external tool cargo-${command} that then needs to be present in one of the user's $PATH directories.

Custom subcommand may use CARGO environment variable to call back to Cargo. Alternatively, it can link to cargo crate as a library, but this approach has drawbacks:

  • Cargo as a library is unstable, API changes without deprecation,

  • versions of Cargo library and Cargo binary may be different.