Editing Developing macOS Applications in Rust

For training AI models, I use an Ubuntu server with 16 cores alongside a MacBook Pro M4 Max. During training, I frequently monitor CPU and memory usage to ensure all cores are utilized efficiently and to estimate memory consumption.
However, I missed having on macOS a system monitor that provides a visualization similar to what I’m used to on Ubuntu. So, I built one — a simple System Monitor application that displays CPU and memory usage.

Although it’s easy to develop such a Rust-based application, I wanted to go a step further and create a fully native macOS app, complete with an icon, bundle metadata, and Launchpad integration. Fortunately, this only takes a few additional steps once your Rust application is working.

Step 1: Install cargo-bundle
cargo install cargo-bundle
This tool packages your Rust application into a proper .app bundle for macOS.

Step 2: Prepare an App Icon
Create or download a 1024×1024 icon. In my case, I generated one using ChatGPT.
Save it as

Step 3: Add Bundle Metadata to Cargo.toml
[package.metadata.bundle]
name = "SysMonitor"
identifier = "xx.yyy.sysmonitor"
icon = ["assets/SysMonitor.icns"]       
#resources = ["assets/"]

Here you define the app name, bundle identifier, and icon set. Optional: you can later add a resources section for assets.

Step 4: Generate the .icns Icon Set
Use the following shell script (make_icns.sh) to convert your 1024×1024 PNG into an Apple .icns bundle.
#!/usr/bin/env bash
set -euo pipefail

# make_icns.sh
# Usage: ./make_icns.sh <input-image> <output-dir>
# Example: ./make_icns.sh SysMonitor_1024.png ./assets

if [[ $# -ne 2 ]]; then
  echo "Usage: $0 <input-image> <output-dir>"
  exit 1
fi

INPUT="$1"
OUTDIR="$2"

# ---- Preflight checks -------------------------------------------------------
if [[ ! -f "$INPUT" ]]; then
  echo "Error: input file not found: $INPUT" >&2
  exit 1
fi

if ! command -v sips >/dev/null 2>&1; then
  echo "Error: 'sips' not found (macOS only). Install Xcode command line tools." >&2
  exit 1
fi

if ! command -v iconutil >/dev/null 2>&1; then
  echo "Error: 'iconutil' not found. Install Xcode or its command line tools." >&2
  exit 1
fi

mkdir -p "$OUTDIR"

# Derive names
BASENAME="$(basename "$INPUT")"
STEM="${BASENAME%.*}"
ICONSET_DIR="$OUTDIR/${STEM}.iconset"
ICNS_PATH="$OUTDIR/$STEM.icns"

# ---- Validate input dimensions ---------------------------------------------
# We want at least 1024x1024 for best results.
WIDTH=$(sips -g pixelWidth "$INPUT" 2>/dev/null | awk '/pixelWidth:/ {print $2}')
HEIGHT=$(sips -g pixelHeight "$INPUT" 2>/dev/null | awk '/pixelHeight:/ {print $2}')

if [[ -z "${WIDTH:-}" || -z "${HEIGHT:-}" ]]; then
  echo "Error: couldn't read image dimensions from: $INPUT" >&2
  exit 1
fi

if [[ "$WIDTH" -ne "$HEIGHT" ]]; then
  echo "Error: input image must be square. Got ${WIDTH}x${HEIGHT}." >&2
  exit 1
fi

if [[ "$WIDTH" -lt 1024 ]]; then
  echo "Error: input image must be at least 1024x1024. Got ${WIDTH}x${HEIGHT}." >&2
  exit 1
fi

# Create a 1024x1024 working copy (downscale if larger, keep quality)
TMPDIR="$(mktemp -d)"
trap 'rm -rf "$TMPDIR"' EXIT
WORK_1024="$TMPDIR/icon_1024.png"
sips -s format png -z 1024 1024 "$INPUT" --out "$WORK_1024" >/dev/null

# ---- Build iconset ----------------------------------------------------------
rm -rf "$ICONSET_DIR"
mkdir -p "$ICONSET_DIR"

sips -z 16 16     "$WORK_1024" --out "$ICONSET_DIR/icon_16x16.png"      >/dev/null
sips -z 32 32     "$WORK_1024" --out "$ICONSET_DIR/icon_16x16@2x.png"   >/dev/null
sips -z 32 32     "$WORK_1024" --out "$ICONSET_DIR/icon_32x32.png"      >/dev/null
sips -z 64 64     "$WORK_1024" --out "$ICONSET_DIR/icon_32x32@2x.png"   >/dev/null
sips -z 128 128   "$WORK_1024" --out "$ICONSET_DIR/icon_128x128.png"    >/dev/null
sips -z 256 256   "$WORK_1024" --out "$ICONSET_DIR/icon_128x128@2x.png" >/dev/null
sips -z 256 256   "$WORK_1024" --out "$ICONSET_DIR/icon_256x256.png"    >/dev/null
sips -z 512 512   "$WORK_1024" --out "$ICONSET_DIR/icon_256x256@2x.png" >/dev/null
sips -z 512 512   "$WORK_1024" --out "$ICONSET_DIR/icon_512x512.png"    >/dev/null
cp "$WORK_1024"         "$ICONSET_DIR/icon_512x512@2x.png"  # 1024x1024

# ---- Create .icns -----------------------------------------------------------
iconutil -c icns "$ICONSET_DIR" -o "$ICNS_PATH"

echo "Iconset: $ICONSET_DIR"
echo "ICNS:    $ICNS_PATH"
echo

Then, generate the icon set:
./icons/mkicon.sh ./icons/SysMonitor.png ./assets

Step 5: Build the App Bundle
cargo bundle --release
APP=./target/release/bundle/osx/SysMonitor.app

Step 6: Test the Application
open $APP   

Step 7: Troubleshooting

If you run into issues, here are some quick diagnostics:
# Quick way to edit/check the plist
/usr/libexec/PlistBuddy -c "Print" $APP/Contents/Info.plist

# Check that the .icns is inside the bundle
ls -l $APP/Contents/Resources

# Verify Info.plist points to the icon
/usr/libexec/PlistBuddy -c "Print :CFBundleIconFile" $APP/Contents/Info.plist
# If it prints AppIcon.icns, it's good.

If needed, extract the .icns to inspect it:
# Validate the .icns itself (most common cause)
mkdir -p tmp/SM.iconset
iconutil -c iconset $APP/Contents/Resources/SysMonitor.icns -o tmp/SM.iconset
ls -1 tmp/SM.iconset

Step 8: Install the App
You can copy the app bundle to either your system or user Applications folder:
cp -R target/release/bundle/osx/SysMonitor.app /Applications/
# or
cp -R target/release/bundle/osx/SysMonitor.app ~/Applications/

Now it appears in Launchpad, Spotlight search, and you can pin it to the Dock.

Step 9: Adding Resources
If your app uses additional resources (icons, images, fonts, configuration files, etc.), add them in Cargo.toml:
[package.metadata.bundle]
...
resources = ["assets", "images/**/*.png", "data/config_defaults.json"]

These files will be copied into your bundle at SysMonitor.app/Contents/Resources/...

Typical examples include:
* 		Images, icons, cursors
* 		Fonts (for custom UI frameworks like Iced)
* 		Shaders, CSV/JSON, templates, localization files
* 		Default configuration files
* 		LICENSE, README, etc.

All resources are read-only inside the bundle.
Accessing Resources at Runtime (Rust)
When running from the bundle, resources live under .../Contents/Resources. When running with cargo run, they’re in your project directory.

A small helper function can make this transparent:
use std::{
    env,
    path::{Path, PathBuf},
};

#[allow(dead_code)]
fn resource_path(rel: impl AsRef<Path>) -> PathBuf {
    // Path to the running executable
    let exe = env::current_exe().unwrap_or_else(|_| PathBuf::from("."));
    // If we’re inside My.app/Contents/MacOS, hop to Contents/Resources
    if let Some(macos_dir) = exe.parent() {
        if macos_dir.ends_with("MacOS") {
            if let Some(contents_dir) = macos_dir.parent() {
                let res = contents_dir.join("Resources");
                return res.join(rel.as_ref());
            }
        }
    }
    // Fallback for `cargo run`: use repo-relative path
    PathBuf::from(rel.as_ref())
}

Usage example:
let icon_path = resource_path("assets/SysMonitor.icns");
let out=format!("IconPath: {:?}", icon_path);
let _ =std::fs::write("sys_monitor/output.txt", out);

When you run the bundle:
open $APP
# in output.txt:
# IconPath: "xxxxx/target/release/bundle/osx/SysMonitor.app/Contents/Resources/assets/SysMonitor.icns"

And when you run via Cargo:
cargo run --release
# in output.txt:
# IconPath: "assets/SysMonitor.icns"

Notes on Writable Files
Only include read-only assets inside the .app bundle. For writable files like logs or user settings, use a per-user directory such as:
~/Library/Application Support/SysMonitor/
You can conveniently locate this path via the directories crate.

Summary
Building a native macOS app in Rust is straightforward with cargo-bundle. Once your application logic is ready, adding a proper icon, bundle metadata, and resource management takes just a few steps — resulting in a polished, fully integrated macOS experience.