emacs: make gc mark trace buffer configurable, disable by default (#448201)

doc/release-notes/rl-2511.section.md

@@ -282,6 +282,8 @@
   recommendation](https://clickhouse.com/docs/faq/operations/production). Users
   can continue to use the `clickhouse-lts` package if desired.
 
+- `emacs` now disables the GC mark trace buffer by default. This improves GC performance by 5%, but can make GC issues harder to debug. This is configurable with `withGcMarkTrace`.
+
 - `buildPythonPackage` and `buildPythonApplication` now default to `nix-update-script` as their default `updateScript`. This should improve automated updates, since nix-update is better maintained than the in-tree update script and has more robust fetcher support.
 
 ## Nixpkgs Library {#sec-nixpkgs-release-25.11-lib}

pkgs/applications/editors/emacs/make-emacs.nix

@@ -74,6 +74,7 @@
   withCairo ? withX,
   withCsrc ? true,
   withDbus ? stdenv.hostPlatform.isLinux,
+  withGcMarkTrace ? false, # increase gc performance for reduced debugability
   withGTK3 ? withPgtk && !noGui,
   withGlibNetworking ? withPgtk || withGTK3 || (withX && withXwidgets),
   withGpm ? stdenv.hostPlatform.isLinux,
@@ -408,6 +409,7 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.withFeature withNS "ns")
   ]
   ++ [
+    (lib.enableFeature withGcMarkTrace "gc-mark-trace")
     (lib.withFeature withCompressInstall "compress-install")
     (lib.withFeature withToolkitScrollBars "toolkit-scroll-bars")
     (lib.withFeature withNativeCompilation "native-compilation")