cargo-whys

A cargo subcommand that explains why dependencies are in your tree.

Features

• Reverse Dependency Lookup: Find out which packages depend on a specific crate
• Version Information: See all versions of a dependency in your tree
• Usage Scanning: Scan your codebase to find where dependencies are actually used
• Workspace Support: Distinguishes between workspace members and external dependencies
• JSON Output: Machine-readable output for integration with other tools
• Fast & Accurate: Uses cargo metadata for precise dependency resolution

Installation

cargo install cargo-whys

Or build from source:

git clone https://github.com/polysystems/cargo-whys
cd cargo-whys
cargo install --path .

Usage

Basic Usage

Find out why a dependency is in your tree:

cargo why serde

Output:

Analyzing dependency: serde

  Version: 1.0.210
    Used by:
      • axum 0.7.0 [external]
      • config 0.13.0 [external]
      • my-app 0.1.0 [workspace]

  Total imports found: 32 in codebase

  Tip: Run with --where-used to see where it's used

Show Usage Locations

See where in your codebase the dependency is imported:

cargo why serde --where-used

Output:

Analyzing dependency: serde

  Version: 1.0.210
    Used by:
      • axum 0.7.0 [external]
      • my-app 0.1.0 [workspace]

  Total imports found: 32 in codebase

  Usage locations:
    • src/main.rs:3
      use serde::{Serialize, Deserialize};
    • src/models/user.rs:1
      use serde::Serialize;
    • src/config.rs:2
      use serde_json;

Show All Versions

If multiple versions of a dependency exist in your tree:

cargo why serde --all-versions

JSON Output

For programmatic use:

cargo why serde --json

Output:

{
  "dependency_name": "serde",
  "found": true,
  "versions": [
    {
      "version": "1.0.210",
      "used_by": [
        {
          "name": "my-app",
          "version": "0.1.0",
          "is_workspace_member": true
        },
        {
          "name": "axum",
          "version": "0.7.0",
          "is_workspace_member": false
        }
      ]
    }
  ],
  "total_usage_count": 32,
  "usage_locations": null
}

Custom Manifest Path

Analyze a project in a different directory:

cargo why serde --manifest-path /path/to/project

Command-Line Options

cargo why <DEPENDENCY> [OPTIONS]

Arguments:
  <DEPENDENCY>  The dependency name to analyze

Options:
      --where-used           Show where in the codebase the dependency is used
      --manifest-path <PATH> Path to the Cargo project directory [default: .]
      --all-versions         Show all versions of the dependency in the tree
      --json                 Output in JSON format
  -h, --help                 Print help
  -V, --version              Print version

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)