fix(windows): embed icon in executable

.github/workflows/ci.yml

@@ -99,6 +99,24 @@ jobs:
       - name: Test
         run: cargo xtask test
 
+      - name: Check embedded executable icon
+        shell: pwsh
+        run: |
+          cargo build --release --locked -p disktree-app
+          Add-Type @'
+          using System;
+          using System.Runtime.InteropServices;
+          public static class ExecutableIcons {
+              [DllImport("shell32.dll", CharSet = CharSet.Unicode)]
+              public static extern uint ExtractIconEx(
+                  string file, int index, IntPtr[] large, IntPtr[] small, uint count);
+          }
+          '@
+          $exe = (Resolve-Path target/release/disktree.exe).Path
+          if ([ExecutableIcons]::ExtractIconEx($exe, -1, $null, $null, 0) -lt 1) {
+            throw "disktree.exe has no embedded icon"
+          }
+
   # The other half of each release: the same tests on the architectures
   # the release builds for but the jobs above do not run on.
   other-architectures:

crates/disktree-app/build.rs

@@ -0,0 +1,65 @@
+#[cfg(windows)]
+fn main() {
+    use std::{env, fs, path::PathBuf, process::Command};
+
+    let icon = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap())
+        .join("../../assets/disktree.ico")
+        .canonicalize()
+        .expect("Windows icon asset is missing");
+    let out = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    let rc = out.join("disktree.rc");
+    let res = out.join("disktree.res");
+
+    // rc.exe treats backslashes inside quoted paths as separators, not escapes.
+    let icon_path = icon.to_string_lossy().replace('\\', "/");
+    fs::write(&rc, format!("1 ICON \"{icon_path}\"\n"))
+        .expect("could not write Windows icon resource script");
+
+    // Cargo can run outside a Visual Studio prompt, where the SDK bin is not
+    // on PATH even though the MSVC linker and Windows SDK are installed.
+    let rc_exe = env::var_os("DISKTREE_RC").map_or_else(
+        || {
+            let sdk_bin = env::var_os("WindowsSdkDir")
+                .map(PathBuf::from)
+                .or_else(|| {
+                    env::var_os("ProgramFiles(x86)")
+                        .map(|root| PathBuf::from(root).join("Windows Kits/10"))
+                })
+                .expect("Windows SDK is required to embed the icon")
+                .join("bin");
+            let host = match env::consts::ARCH {
+                "x86_64" => "x64",
+                "aarch64" => "arm64",
+                "x86" => "x86",
+                other => panic!("unsupported Windows build host: {other}"),
+            };
+            let mut versions: Vec<_> = fs::read_dir(&sdk_bin)
+                .expect("Windows SDK bin directory is missing")
+                .filter_map(Result::ok)
+                .map(|entry| entry.path().join(host).join("rc.exe"))
+                .filter(|path| path.is_file())
+                .collect();
+            versions.sort();
+            versions
+                .pop()
+                .expect("rc.exe is missing from the Windows SDK")
+        },
+        PathBuf::from,
+    );
+    let status = Command::new(rc_exe)
+        .arg("/nologo")
+        .arg(format!("/fo{}", res.display()))
+        .arg(&rc)
+        .status()
+        .expect("could not start rc.exe to embed the Windows icon");
+    assert!(
+        status.success(),
+        "rc.exe failed to compile the Windows icon"
+    );
+
+    println!("cargo:rerun-if-changed={}", icon.display());
+    println!("cargo:rustc-link-arg-bin=disktree={}", res.display());
+}
+
+#[cfg(not(windows))]
+fn main() {}