diff --git a/Library/Homebrew/cleanup.rb b/Library/Homebrew/cleanup.rb
index 0cda61fb8bb4d..99d2fa3311f1f 100644
--- a/Library/Homebrew/cleanup.rb
+++ b/Library/Homebrew/cleanup.rb
@@ -555,7 +555,7 @@ def cleanup_cache_db(rack = nil)
     end
 
     def rm_ds_store(dirs = nil)
-      dirs ||= Keg::MUST_EXIST_DIRECTORIES + [
+      dirs ||= Keg.must_exist_directories + [
         HOMEBREW_PREFIX/"Caskroom",
       ]
       dirs.select(&:directory?)
@@ -623,7 +623,7 @@ def prune_prefix_symlinks_and_directories
       dirs = []
       children_count = {}
 
-      Keg::MUST_EXIST_SUBDIRECTORIES.each do |dir|
+      Keg.must_exist_subdirectories.each do |dir|
         next unless dir.directory?
 
         dir.find do |path|
@@ -639,7 +639,7 @@ def prune_prefix_symlinks_and_directories
                 path.unlink
               end
             end
-          elsif path.directory? && Keg::MUST_EXIST_SUBDIRECTORIES.exclude?(path)
+          elsif path.directory? && Keg.must_exist_subdirectories.exclude?(path)
             dirs << path
             children_count[path] = path.children.length if dry_run?
           end
diff --git a/Library/Homebrew/diagnostic.rb b/Library/Homebrew/diagnostic.rb
index 6b63ac93aee05..c88b0b941227a 100644
--- a/Library/Homebrew/diagnostic.rb
+++ b/Library/Homebrew/diagnostic.rb
@@ -315,7 +315,7 @@ def check_for_stray_headers
       def check_for_broken_symlinks
         broken_symlinks = []
 
-        Keg::MUST_EXIST_SUBDIRECTORIES.each do |d|
+        Keg.must_exist_subdirectories.each do |d|
           next unless d.directory?
 
           d.find do |path|
@@ -344,7 +344,7 @@ def check_tmpdir_sticky_bit
       def check_exist_directories
         return if HOMEBREW_PREFIX.writable?
 
-        not_exist_dirs = Keg::MUST_EXIST_DIRECTORIES.reject(&:exist?)
+        not_exist_dirs = Keg.must_exist_directories.reject(&:exist?)
         return if not_exist_dirs.empty?
 
         <<~EOS
@@ -359,8 +359,8 @@ def check_exist_directories
 
       def check_access_directories
         not_writable_dirs =
-          Keg::MUST_BE_WRITABLE_DIRECTORIES.select(&:exist?)
-                                           .reject(&:writable?)
+          Keg.must_be_writable_directories.select(&:exist?)
+             .reject(&:writable?)
         return if not_writable_dirs.empty?
 
         <<~EOS
diff --git a/Library/Homebrew/extend/os/mac/keg.rb b/Library/Homebrew/extend/os/mac/keg.rb
index b71aaee900528..aae4eb6ad3552 100644
--- a/Library/Homebrew/extend/os/mac/keg.rb
+++ b/Library/Homebrew/extend/os/mac/keg.rb
@@ -3,32 +3,38 @@
 
 require "system_command"
 
-class Keg
-  include SystemCommand::Mixin
-
-  # TODO: re-implement these as functions, so that we aren't modifying constants:
-  GENERIC_KEG_LINK_DIRECTORIES = (remove_const :KEG_LINK_DIRECTORIES).freeze
-  KEG_LINK_DIRECTORIES = (GENERIC_KEG_LINK_DIRECTORIES + ["Frameworks"]).freeze
-  GENERIC_MUST_EXIST_SUBDIRECTORIES = (remove_const :MUST_EXIST_SUBDIRECTORIES).freeze
-  MUST_EXIST_SUBDIRECTORIES = (
-    GENERIC_MUST_EXIST_SUBDIRECTORIES +
-    [HOMEBREW_PREFIX/"Frameworks"]
-  ).sort.uniq.freeze
-  GENERIC_MUST_EXIST_DIRECTORIES = (remove_const :MUST_EXIST_DIRECTORIES).freeze
-  MUST_EXIST_DIRECTORIES = (
-    GENERIC_MUST_EXIST_DIRECTORIES +
-    [HOMEBREW_PREFIX/"Frameworks"]
-  ).sort.uniq.freeze
-  GENERIC_MUST_BE_WRITABLE_DIRECTORIES = (remove_const :MUST_BE_WRITABLE_DIRECTORIES).freeze
-  MUST_BE_WRITABLE_DIRECTORIES = (
-    GENERIC_MUST_BE_WRITABLE_DIRECTORIES +
-    [HOMEBREW_PREFIX/"Frameworks"]
-  ).sort.uniq.freeze
-end
-
 module OS
   module Mac
     module Keg
+      include SystemCommand::Mixin
+
+      module ClassMethods
+        def keg_link_directories
+          @keg_link_directories ||= (super + ["Frameworks"]).freeze
+        end
+
+        def must_exist_subdirectories
+          @must_exist_subdirectories ||= (
+            super +
+            [HOMEBREW_PREFIX/"Frameworks"]
+          ).sort.uniq.freeze
+        end
+
+        def must_exist_directories
+          @must_exist_directories = (
+            super +
+            [HOMEBREW_PREFIX/"Frameworks"]
+          ).sort.uniq.freeze
+        end
+
+        def must_be_writable_directories
+          @must_be_writable_directories ||= (
+            super +
+            [HOMEBREW_PREFIX/"Frameworks"]
+          ).sort.uniq.freeze
+        end
+      end
+
       def binary_executable_or_library_files = mach_o_files
 
       def codesign_patched_binary(file)
@@ -121,4 +127,5 @@ def consistent_reproducible_symlink_permissions!
   end
 end
 
+Keg.singleton_class.prepend(OS::Mac::Keg::ClassMethods)
 Keg.prepend(OS::Mac::Keg)
diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index 47bdd56e61e06..15a4df4bf49c9 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -1229,7 +1229,7 @@ def post_install
       sandbox.deny_write_homebrew_repository
       sandbox.allow_write_cellar(formula)
       sandbox.deny_all_network unless formula.network_access_allowed?(:postinstall)
-      Keg::KEG_LINK_DIRECTORIES.each do |dir|
+      Keg.keg_link_directories.each do |dir|
         sandbox.allow_write_path "#{HOMEBREW_PREFIX}/#{dir}"
       end
       sandbox.run(*args)
diff --git a/Library/Homebrew/install.rb b/Library/Homebrew/install.rb
index 795192b3a0a03..34849222f1d8b 100644
--- a/Library/Homebrew/install.rb
+++ b/Library/Homebrew/install.rb
@@ -325,7 +325,7 @@ def check_cc_argv(cc)
       end
 
       def attempt_directory_creation
-        Keg::MUST_EXIST_DIRECTORIES.each do |dir|
+        Keg.must_exist_directories.each do |dir|
           FileUtils.mkdir_p(dir) unless dir.exist?
         rescue
           nil
diff --git a/Library/Homebrew/keg.rb b/Library/Homebrew/keg.rb
index 8cbe45ec603ce..0b0f67d2564cd 100644
--- a/Library/Homebrew/keg.rb
+++ b/Library/Homebrew/keg.rb
@@ -78,38 +78,6 @@ def to_s
   # Locale-specific directories have the form `language[_territory][.codeset][@modifier]`
   LOCALEDIR_RX = %r{(locale|man)/([a-z]{2}|C|POSIX)(_[A-Z]{2})?(\.[a-zA-Z\-0-9]+(@.+)?)?}
   INFOFILE_RX = %r{info/([^.].*?\.info(\.gz)?|dir)$}
-  KEG_LINK_DIRECTORIES = %w[
-    bin etc include lib sbin share var
-  ].freeze
-  MUST_EXIST_SUBDIRECTORIES = (
-    KEG_LINK_DIRECTORIES - %w[var] + %w[
-      opt
-      var/homebrew/linked
-    ]
-  ).map { |dir| HOMEBREW_PREFIX/dir }.sort.uniq.freeze
-
-  # Keep relatively in sync with
-  # {https://github.com/Homebrew/install/blob/HEAD/install.sh}
-  MUST_EXIST_DIRECTORIES = (MUST_EXIST_SUBDIRECTORIES + [
-    HOMEBREW_CELLAR,
-  ].sort.uniq).freeze
-  MUST_BE_WRITABLE_DIRECTORIES = (
-    %w[
-      etc/bash_completion.d lib/pkgconfig
-      share/aclocal share/doc share/info share/locale share/man
-      share/man/man1 share/man/man2 share/man/man3 share/man/man4
-      share/man/man5 share/man/man6 share/man/man7 share/man/man8
-      share/zsh share/zsh/site-functions
-      var/log
-    ].map { |dir| HOMEBREW_PREFIX/dir } + MUST_EXIST_SUBDIRECTORIES + [
-      HOMEBREW_CACHE,
-      HOMEBREW_CELLAR,
-      HOMEBREW_LOCKS,
-      HOMEBREW_LOGS,
-      HOMEBREW_REPOSITORY,
-      Language::Python.homebrew_site_packages,
-    ]
-  ).sort.uniq.freeze
 
   # These paths relative to the keg's share directory should always be real
   # directories in the prefix, never symlinks.
@@ -146,6 +114,51 @@ def self.all
     Formula.racks.flat_map(&:subdirs).map { |d| new(d) }
   end
 
+  def self.keg_link_directories
+    @keg_link_directories ||= %w[
+      bin etc include lib sbin share var
+    ].freeze
+  end
+
+  def self.must_exist_subdirectories
+    @must_exist_subdirectories ||= (
+    keg_link_directories - %w[var] + %w[
+      opt
+      var/homebrew/linked
+    ]
+  ).map { |dir| HOMEBREW_PREFIX/dir }.sort.uniq.freeze
+  end
+
+  # Keep relatively in sync with
+  # {https://github.com/Homebrew/install/blob/HEAD/install.sh}
+  def self.must_exist_directories
+    @must_exist_directories ||= (must_exist_subdirectories + [
+      HOMEBREW_CELLAR,
+    ].sort.uniq).freeze
+  end
+
+  # Keep relatively in sync with
+  # {https://github.com/Homebrew/install/blob/HEAD/install.sh}
+  def self.must_be_writable_directories
+    @must_be_writable_directories ||= (
+    %w[
+      etc/bash_completion.d lib/pkgconfig
+      share/aclocal share/doc share/info share/locale share/man
+      share/man/man1 share/man/man2 share/man/man3 share/man/man4
+      share/man/man5 share/man/man6 share/man/man7 share/man/man8
+      share/zsh share/zsh/site-functions
+      var/log
+    ].map { |dir| HOMEBREW_PREFIX/dir } + must_exist_subdirectories + [
+      HOMEBREW_CACHE,
+      HOMEBREW_CELLAR,
+      HOMEBREW_LOCKS,
+      HOMEBREW_LOGS,
+      HOMEBREW_REPOSITORY,
+      Language::Python.homebrew_site_packages,
+    ]
+  ).sort.uniq.freeze
+  end
+
   attr_reader :path, :name, :linked_keg_record, :opt_record
 
   protected :path
@@ -288,8 +301,8 @@ def unlink(verbose: false, dry_run: false)
 
     dirs = []
 
-    keg_directories = KEG_LINK_DIRECTORIES.map { |d| path/d }
-                                          .select(&:exist?)
+    keg_directories = self.class.keg_link_directories.map { |d| path/d }
+                          .select(&:exist?)
     keg_directories.each do |dir|
       dir.find do |src|
         dst = HOMEBREW_PREFIX + src.relative_path_from(path)
@@ -316,7 +329,7 @@ def unlink(verbose: false, dry_run: false)
     unless dry_run
       remove_old_aliases
       remove_linked_keg_record if linked?
-      (dirs - MUST_EXIST_SUBDIRECTORIES).reverse_each(&:rmdir_if_possible)
+      (dirs - self.class.must_exist_subdirectories).reverse_each(&:rmdir_if_possible)
     end
 
     ObserverPathnameExtension.n
diff --git a/Library/Homebrew/test/spec_helper.rb b/Library/Homebrew/test/spec_helper.rb
index f86636bf2e041..783b522b9664e 100644
--- a/Library/Homebrew/test/spec_helper.rb
+++ b/Library/Homebrew/test/spec_helper.rb
@@ -272,7 +272,7 @@
 
       FileUtils.rm_rf [
         *TEST_DIRECTORIES,
-        *Keg::MUST_EXIST_SUBDIRECTORIES,
+        *Keg.must_exist_subdirectories,
         HOMEBREW_LINKED_KEGS,
         HOMEBREW_PINNED_KEGS,
         HOMEBREW_PREFIX/"var",