brewdiff

A Rust crate that provides diff functionality for Homebrew packages managed through nix-darwin. It compares what Homebrew actually has installed on the system versus what a nix-darwin configuration declares should be installed.

Purpose

This crate is designed to show users what Homebrew changes will occur when they activate a new nix-darwin configuration, similar to how the dix crate shows Nix store diffs.

Features

• Extract Homebrew intent from nix-darwin profiles
• Query current Homebrew state (installed formulae, casks, and taps)
• Compute differences between current state and intended state
• Colorized output with clear add/remove indicators
• Thread-based async processing (mirrors dix pattern)

Example Output

<<< /run/current-system
>>> /nix/var/nix/profiles/system-123-link

ADDED
Formulae
[A] curl
Casks
[A] firefox
[A] visual-studio-code

REMOVED
Formulae
[R] wget
Casks
[R] slack

How It Works

1. Reads the nix-darwin activation script to find the Brewfile path, then parses that Brewfile to extract Homebrew intent
2. Queries Homebrew directly for current state:
   • Uses brew leaves for formulae (only user-installed packages, not dependencies)
   • Uses brew list --cask for casks
   • Uses brew tap for taps
   • Uses mas list for Mac App Store apps
3. Diffs current state with intended state to find additions and removals
4. Formats the diff with colors and clear indicators

API

use brewdiff;
use std::path::Path;

// Primary use case: compare current state with new nix-darwin profile
let new_profile = Path::new("/nix/var/nix/profiles/system-123-link");

// Async diff computation
let handle = brewdiff::spawn_homebrew_diff(new_profile.to_path_buf());
if let Ok(diff_data) = handle.join() {
    let diff = diff_data?;
    println!("Changes: {} additions, {} removals",
             diff.brews.added.len(),
             diff.brews.removed.len());
}

// Or synchronous diff with output
let lines_written = brewdiff::write_homebrew_diffln(
    &mut std::io::stdout(),
    new_profile,
)?;