src/build-remote/build-remote.cc
76:        /* It would be more appropriate to use $XDG_RUNTIME_DIR, since

src/libutil/util.hh
134:/* Return $XDG_CACHE_HOME or $HOME/.cache. */
137:/* Return $XDG_CONFIG_HOME or $HOME/.config. */
143:/* Return $XDG_DATA_HOME or $HOME/.local/share. */

src/libutil/util.cc
510:    auto cacheDir = getEnv("XDG_CACHE_HOME");
517:    auto configDir = getEnv("XDG_CONFIG_HOME");
524:    string configDirs = getEnv("XDG_CONFIG_DIRS").value_or("");
533:    auto dataDir = getEnv("XDG_DATA_HOME");

src/nix-build/nix-build.cc
387:        if (!tmp) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");

doc/manual/command-ref/conf-file.xml
36:    <filename>$XDG_CONFIG_HOME/nix/nix.conf</filename>, or
38:    <envar>XDG_CONFIG_HOME</envar> is not set.</para>
955:      <filename>$XDG_CACHE_HOME/nix/tarballs</filename>.</para>

perl/lib/Nix/Utils.pm
45:    return tempdir("$name.XXXXXX", CLEANUP => 1, DIR => $ENV{"TMPDIR"} // $ENV{"XDG_RUNTIME_DIR"} // "/tmp")

tests/tarball.sh
41:(! TMPDIR=$TEST_ROOT/tmp XDG_RUNTIME_DIR=$TEST_ROOT/tmp nix-env -f file://$(pwd)/bad.tar.xz -qa --out-path)

tests/common.sh.in
24:unset XDG_CACHE_HOME