heaptrack: enable rustc-demangle (#481593)

pkgs/by-name/he/heaptrack/package.nix

@@ -11,6 +11,7 @@
   sparsehash,
   zstd,
   kdePackages,
+  rustc-demangle,
 }:
 
 stdenv.mkDerivation {
@@ -43,6 +44,7 @@ stdenv.mkDerivation {
     libunwind
     sparsehash
     zstd
+    rustc-demangle
   ]
   ++ (with kdePackages; [
     qtbase
@@ -58,6 +60,11 @@ stdenv.mkDerivation {
     elfutils
   ];
 
+  postPatch = ''
+    substituteInPlace src/interpret/demangler.cpp \
+      --replace-fail "librustc_demangle.so" "${rustc-demangle}/lib/librustc_demangle.so"
+  '';
+
   postInstall = lib.optionalString stdenv.hostPlatform.isDarwin ''
     makeWrapper \
       $out/Applications/KDE/heaptrack_gui.app/Contents/MacOS/heaptrack_gui \