cargo metadata

The cargo metadata command outputs a machine-readable JSON document describing the workspace structure, package manifests, compilation targets, resolved dependency graph, and active features. IDEs (such as rust-analyzer), build wrappers, security scanners, and custom cargo xtask tools invoke cargo metadata --format-version 1 instead of parsing Cargo.toml files by hand.

Cargo.toml + Cargo.lock + .cargo/config.toml
                     |
                     v  `cargo metadata --format-version 1 --no-deps`
         [ JSON Document on stdout ]
         ├── "packages": [ { "name", "version", "targets", "features", ... } ]
         ├── "workspace_members": [ ... ]
         ├── "workspace_root": "/path/to/workspace"
         ├── "target_directory": "/path/to/workspace/target"
         └── "resolve": { "nodes": [ ... ], "root": ... }

Key Flags for cargo metadata

Always pass --format-version 1 when calling cargo metadata so Cargo guarantees a stable JSON schema:

Top-Level Fields in the JSON Schema

{
  "packages": [
    {
      "name": "my_service",
      "version": "0.1.0",
      "id": "path+file:///Users/dev/my_service#0.1.0",
      "edition": "2024",
      "rust_version": "1.85",
      "manifest_path": "/Users/dev/my_service/Cargo.toml",
      "targets": [
        {
          "kind": ["bin"],
          "crate_types": ["bin"],
          "name": "my_service",
          "src_path": "/Users/dev/my_service/src/main.rs",
          "edition": "2024"
        }
      ],
      "features": {
        "default": ["std"],
        "std": []
      }
    }
  ],
  "workspace_members": [
    "path+file:///Users/dev/my_service#0.1.0"
  ],
  "workspace_root": "/Users/dev/my_service",
  "target_directory": "/Users/dev/my_service/target",
  "version": 1
}

Invoking cargo metadata from a Rust Tool

In cargo xtask or custom CLI tools, you can run cargo metadata --format-version 1 --no-deps via std::process::Command (or via the cargo_metadata crate when serde is available) to discover workspace roots and member paths:

use std::process::Command;

fn extract_json_string_field<'a>(json: &'a str, key: &str) -> Option<&'a str> {
    let pattern = format!("\"{key}\":\"");
    let start = json.find(&pattern)? + pattern.len();
    let rest = &json[start..];
    let end = rest.find('"')?;
    Some(&rest[..end])
}

fn main() {
    let sample_json = r#"{"workspace_root":"/Users/dev/workspace","target_directory":"/Users/dev/workspace/target","version":1}"#;

    let root = extract_json_string_field(sample_json, "workspace_root").unwrap();
    let target = extract_json_string_field(sample_json, "target_directory").unwrap();

    // Also verify `cargo --version` is callable via Command
    let cargo_ok = Command::new("cargo")
        .arg("--version")
        .output()
        .map(|o| o.status.success())
        .unwrap_or(false);

    println!("Workspace root:   {root}");
    println!("Target directory: {target}");
    println!("Cargo available:  {cargo_ok}");
}

Workspace root:   /Users/dev/workspace
Target directory: /Users/dev/workspace/target
Cargo available:  true