Machine readable output.

Cargo can output information about project and build in JSON format.

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 ]

Compiler errors

If you supply --message-format json to commands like cargo build, Cargo reports compilation errors and warnings in JSON format. Messages go to the standard output. Each message occupies exactly one line and does not contain internal \n symbols, so it is possible to process messages one by one without waiting for the whole build to finish.

The message format looks like this:

  // Type of the message.
  "reason": "compiler-message",

  // Unique opaque identifier of compiled package.
  "package_id": PackageId,

  // Unique specification of a particular target within the package.
  "target": Target,

  // The error message from the compiler in JSON format.
  "message": {...}

Package and target specification are the same that cargo metadata uses.