syncthingtray: Fix Nix wrapped path in autostart desktop generation

Fixes #199596.

pkgs/applications/misc/syncthingtray/default.nix

@@ -1,6 +1,7 @@
 { mkDerivation
 , lib
 , fetchFromGitHub
+, substituteAll
 , qtbase
 , qtwebengine
 , qtdeclarative
@@ -31,6 +32,17 @@ mkDerivation rec {
     sha256 = "sha256-uhVRO9aiYJbUmwDp1+LIYF3wNBbVduVpTtVzaS0oUMU=";
   };
 
+  patches = [
+    # Fix Exec= path in runtime-generated
+    # ~/.config/autostart/syncthingtray.desktop file - this is required because
+    # we are wrapping the executable. We can't use `substituteAll` because we
+    # can't use `${placeholder "out"}` because that will produce the $out of
+    # the patch derivation itself, and not of syncthing's "out" placeholder.
+    # Hence we use a C definition with NIX_CFLAGS_COMPILE
+    ./use-nix-path-in-autostart.patch
+  ];
+  NIX_CFLAGS_COMPILE = "-DEXEC_NIX_PATH=\"${placeholder "out"}/bin/syncthingtray\"";
+
   buildInputs = [
     qtbase
     cpp-utilities

pkgs/applications/misc/syncthingtray/use-nix-path-in-autostart.patch

@@ -0,0 +1,13 @@
+diff --git i/widgets/settings/settingsdialog.cpp w/widgets/settings/settingsdialog.cpp
+index 4deff1f..16845b5 100644
+--- i/widgets/settings/settingsdialog.cpp
++++ w/widgets/settings/settingsdialog.cpp
+@@ -802,7 +802,7 @@ bool setAutostartEnabled(bool enabled)
+         desktopFile.write("[Desktop Entry]\n"
+                           "Name=" APP_NAME "\n"
+                           "Exec=\"");
+-        desktopFile.write(qEnvironmentVariable("APPIMAGE", QCoreApplication::applicationFilePath()).toUtf8().data());
++        desktopFile.write(qEnvironmentVariable("APPIMAGE", EXEC_NIX_PATH).toUtf8().data());
+         desktopFile.write("\" qt-widgets-gui --single-instance\nComment=" APP_DESCRIPTION "\n"
+                           "Icon=" PROJECT_NAME "\n"
+                           "Type=Application\n"