darwin: show homebrew diff if available

CHANGELOG.md

@@ -16,6 +16,13 @@ functionality, under the "Removed" section.
 
 ## Unreleased
 
+### Added
+
+- `nh darwin` now displays Homebrew package changes alongside Nix package
+  changes when using nix-darwin configurations with Homebrew management enabled.
+  This shows formulae, casks, taps, and Mac App Store apps that will be
+  added or removed during a configuration switch.
+
 ### Changed
 
 - Nh checks are now more robust in the sense that unnecessary features will not

Cargo.toml

@@ -59,6 +59,7 @@ tracing-subscriber = { features = [ "env-filter", "registry", "std" ], version =
 which = "8.0.0"
 
 [target.'cfg(target_os="macos")'.dependencies]
+brewdiff             = "0.2.0"
 system-configuration = "0.6.1"
 
 [dev-dependencies]

src/darwin.rs

@@ -17,7 +17,7 @@ use crate::{
   },
   nixos::toplevel_for,
   update::update,
-  util::{get_hostname, print_dix_diff},
+  util::{get_hostname, print_dix_diff, print_homebrew_diff},
 };
 
 const SYSTEM_PROFILE: &str = "/nix/var/nix/profiles/system";
@@ -142,6 +142,8 @@ impl DarwinRebuildArgs {
         target_profile.display()
       );
       let _ = print_dix_diff(&PathBuf::from(CURRENT_PROFILE), &target_profile);
+      let _ =
+        print_homebrew_diff(&PathBuf::from(CURRENT_PROFILE), &target_profile);
     }
 
     if self.common.ask && !self.common.dry && !matches!(variant, Build) {

src/util.rs

@@ -30,6 +30,7 @@ impl<W: io::Write> fmt::Write for WriteFmt<W> {
     self.0.write_all(string.as_bytes()).map_err(|_| fmt::Error)
   }
 }
+
 /// Get the Nix variant (cached)
 pub fn get_nix_variant() -> &'static NixVariant {
   NIX_VARIANT.get_or_init(|| {
@@ -336,3 +337,85 @@ pub fn print_dix_diff(
   }
   Ok(())
 }
+
+/// Prints the difference between Homebrew packages in darwin generations.
+///
+/// # Arguments
+///
+/// * `old_generation` - A reference to the path of the old/current generation.
+/// * `new_generation` - A reference to the path of the new generation.
+///
+/// # Returns
+///
+/// Returns `Ok(())` if the operation completed successfully, or an error
+/// wrapped in `eyre::Result` if something went wrong. Silently returns Ok(())
+/// if Homebrew is not available or not configured.
+#[cfg(target_os = "macos")]
+pub fn print_homebrew_diff(
+  _old_generation: &Path,
+  new_generation: &Path,
+) -> Result<()> {
+  if !homebrew_available() {
+    debug!("Homebrew not found, skipping Homebrew diff");
+    return Ok(());
+  }
+
+  // Try to extract the nix-darwin Homebrew intent from the new profile
+  // If this fails, it likely means Homebrew isn't configured in the profile
+  let nix_intent = match brewdiff::extract_nix_darwin_intent(new_generation) {
+    Ok(intent) => intent,
+    Err(e) => {
+      debug!("Could not extract Homebrew intent from profile: {}", e);
+      return Ok(());
+    },
+  };
+
+  if !nix_intent.has_packages() {
+    debug!("No Homebrew packages configured in profile, skipping diff");
+    return Ok(());
+  }
+
+  let mut out = WriteFmt(io::stdout());
+
+  let diff_handle = brewdiff::spawn_homebrew_diff(new_generation.to_path_buf());
+  let diff_data = match diff_handle.join() {
+    Ok(Ok(data)) => data,
+    Ok(Err(e)) => {
+      debug!("Failed to compute Homebrew diff: {}", e);
+      return Ok(());
+    },
+    Err(_) => {
+      debug!("Homebrew diff thread panicked");
+      return Ok(());
+    },
+  };
+
+  if diff_data.has_changes() {
+    // Separator from dix' output
+    println!();
+
+    // Print statistics first to make it clear the details below belong to
+    // Homebrew
+    brewdiff::write_homebrew_stats(&mut out, &diff_data)?;
+    brewdiff::display::write_diff(&mut out, &diff_data)?;
+  } else {
+    info!("No Homebrew package changes.");
+  }
+
+  Ok(())
+}
+
+/// Checks if Homebrew is available on the system
+#[cfg(target_os = "macos")]
+fn homebrew_available() -> bool {
+  which::which("brew").is_ok()
+}
+
+/// Stub for non-macOS platforms
+#[cfg(not(target_os = "macos"))]
+pub fn print_homebrew_diff(
+  _old_generation: &Path,
+  _new_generation: &Path,
+) -> Result<()> {
+  Ok(())
+}