Running nixpkgs-update (https://nix-community.org/update-bot/) with UPDATE_INFO: fontconfig 0 -> 1
attrpath: fontconfig
Checking auto update branch...
[version] 
[version] skipping because derivation has updateScript
[rustCrateVersion] 
[rustCrateVersion] No cargoHash found
[golangModuleVersion] 
[golangModuleVersion] Not a buildGoModule package with vendorHash
[npmDepsVersion] 
[npmDepsVersion] No npmDepsHash
[updateScript] 
[updateScript] Success
[updateScript] this derivation will be built:
  /nix/store/pm76s0dj0mdpfc6l3z4jzl99gnf9fj53-packages.json.drv
building '/nix/store/pm76s0dj0mdpfc6l3z4jzl99gnf9fj53-packages.json.drv'...

Going to be running update for following packages:
 - fontconfig-2.17.1

Press Enter key to continue...
Running update for:
Enqueuing group of 1 packages
 - fontconfig-2.17.1: UPDATING ...
 - fontconfig-2.17.1: DONE.

Packages updated!

Diff after rewrites:
diff --git a/pkgs/development/libraries/fontconfig/default.nix b/pkgs/development/libraries/fontconfig/default.nix
index dee1e197dd6e..9e4c1fb36057 100644
--- a/pkgs/development/libraries/fontconfig/default.nix
+++ b/pkgs/development/libraries/fontconfig/default.nix
@@ -17,7 +17,7 @@
 
 stdenv.mkDerivation (finalAttrs: {
   pname = "fontconfig";
-  version = "2.17.1";
+  version = "2.18.1";
 
   outputs = [
     "bin"
@@ -30,7 +30,7 @@ stdenv.mkDerivation (finalAttrs: {
   # ref: https://github.com/NixOS/nixpkgs/pull/401037#discussion_r2055430206
   src = fetchurl {
     url = "https://gitlab.freedesktop.org/api/v4/projects/890/packages/generic/fontconfig/${finalAttrs.version}/fontconfig-${finalAttrs.version}.tar.xz";
-    hash = "sha256-n1yuk/T//B+8Ba6ZzfxwjNYN/WYS/8BRKCcCXAJvpUE=";
+    hash = "sha256-IwDz2/pyU7OkT0/uzbyN+kXd5dws+3H86vMfOUy0EDE=";
   };
 
   nativeBuildInputs = [

No auto update branch exists
Old version 2.17.1" not present in staging derivation file with contents: {
  stdenv,
  lib,
  fetchurl,
  pkg-config,
  python3,
  freetype,
  expat,
  libxslt,
  gperf,
  dejavu_fonts,
  autoreconfHook,
  versionCheckHook,
  testers,
  gitUpdater,
}:

stdenv.mkDerivation (finalAttrs: {
  pname = "fontconfig";
  version = "2.18.1";

  outputs = [
    "bin"
    "dev"
    "lib"
    "out"
  ]; # $out contains all the config

  # GitLab repositrory does not include pre-generated man pages.
  # ref: https://github.com/NixOS/nixpkgs/pull/401037#discussion_r2055430206
  src = fetchurl {
    url = "https://gitlab.freedesktop.org/api/v4/projects/890/packages/generic/fontconfig/${finalAttrs.version}/fontconfig-${finalAttrs.version}.tar.xz";
    hash = "sha256-IwDz2/pyU7OkT0/uzbyN+kXd5dws+3H86vMfOUy0EDE=";
  };

  nativeBuildInputs = [
    autoreconfHook
    gperf
    libxslt
    pkg-config
    python3
  ];

  buildInputs = [
    expat
  ];

  propagatedBuildInputs = [
    freetype
  ];

  postPatch = ''
    # Requires networking.
    sed -i '/check_PROGRAMS += test-crbug1004254/d' test/Makefile.am

    # Test causes error without patch shebangs.
    patchShebangs doc/check-whitespace-in-args.py \
      doc/check-missing-doc.py
  '';

  configureFlags = [
    "--sysconfdir=/etc"
    "--with-arch=${stdenv.hostPlatform.parsed.cpu.name}"
    "--with-cache-dir=/var/cache/fontconfig" # otherwise the fallback is in $out/
    # just <1MB; this is what you get when loading config fails for some reason
    "--with-default-fonts=${dejavu_fonts.minimal}"
  ]
  ++ lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
    "--with-arch=${stdenv.hostPlatform.parsed.cpu.name}"
    "ac_cv_va_copy=C99"
  ];

  enableParallelBuilding = true;

  doCheck = stdenv.buildPlatform.canExecute stdenv.hostPlatform;

  installFlags = [
    # Don't try to write to /var/cache/fontconfig at install time.
    "fc_cachedir=$(TMPDIR)/dummy"
    "RUN_FC_CACHE_TEST=false"
    "sysconfdir=${placeholder "out"}/etc"
  ];

  postInstall = ''
    cd "$out/etc/fonts"
    xsltproc --stringparam fontDirectories "${dejavu_fonts.minimal}" \
      --stringparam includes /etc/fonts/conf.d \
      --path $out/share/xml/fontconfig \
      ${./make-fonts-conf.xsl} $out/etc/fonts/fonts.conf \
      > fonts.conf.tmp
    mv fonts.conf.tmp $out/etc/fonts/fonts.conf
    # We don't keep section 3 of the manpages, as they are quite large and
    # probably not so useful.
    rm -r $bin/share/man/man3
  '';

  nativeInstallCheckInputs = [
    versionCheckHook
  ];
  doInstallCheck = true;
  versionCheckProgram = "${placeholder "bin"}/bin/fc-list";

  installCheckPhase = ''
    runHook preInstallCheck

    [ -d "$bin/share/man/man1" ]
    [ -d "$bin/share/man/man5" ]
    echo "man pages exist"

    runHook postInstallCheck
  '';

  passthru = {
    tests = {
      pkg-config = testers.hasPkgConfigModules {
        package = finalAttrs.finalPackage;
      };
    };

    updateScript = gitUpdater {
      url = "https://gitlab.freedesktop.org/fontconfig/fontconfig.git";
    };
  };

  meta = {
    description = "Library for font customization and configuration";
    homepage = "http://fontconfig.org/";
    license = lib.licenses.bsd2; # custom but very bsd-like
    platforms = lib.platforms.all;
    teams = [ lib.teams.freedesktop ];
    pkgConfigModules = [ "fontconfig" ];
  };
})