Use cache if HOMEBREW_LINKAGE_CACHE exists in formula_cellar_checks and formula. Make functions private in LinkageChecker.

Library/Homebrew/extend/os/mac/formula_cellar_checks.rb

@@ -67,7 +67,8 @@ module FormulaCellarChecks
     keg = Keg.new(formula.prefix)
 
     DatabaseCache.new(:linkage) do |database_cache|
-      checker = LinkageChecker.new(keg, database_cache, false, formula)
+      use_cache = !ENV["HOMEBREW_LINKAGE_CACHE"].nil?
+      checker = LinkageChecker.new(keg, database_cache, use_cache, formula)
 
       next unless checker.broken_library_linkage?
       output = <<~EOS

Library/Homebrew/formula.rb

@@ -1528,7 +1528,8 @@ class Formula
     return [] unless keg
 
     DatabaseCache.new(:linkage) do |database_cache|
-      linkage_checker = LinkageChecker.new(keg, database_cache, false, self)
+      use_cache = !ENV["HOMEBREW_LINKAGE_CACHE"].nil?
+      linkage_checker = LinkageChecker.new(keg, database_cache, use_cache, self)
       break linkage_checker.undeclared_deps.map { |n| Dependency.new(n) }
     end
   end

Library/Homebrew/linkage_checker.rb

@@ -10,46 +10,6 @@ class LinkageChecker
     flush_cache_and_check_dylibs unless use_cache
   end
 
-  # 'Hash-type' cache values
-
-  def brewed_dylibs
-    @brewed_dylibs ||= store.fetch_type(:brewed_dylibs)
-  end
-
-  def reverse_links
-    @reverse_links ||= store.fetch_type(:reverse_links)
-  end
-
-  def broken_deps
-    @broken_deps ||= store.fetch_type(:broken_deps)
-  end
-
-  # 'Path-type' cached values
-
-  def system_dylibs
-    @system_dylibs ||= store.fetch_type(:system_dylibs)
-  end
-
-  def broken_dylibs
-    @broken_dylibs ||= store.fetch_type(:broken_dylibs)
-  end
-
-  def variable_dylibs
-    @variable_dylibs ||= store.fetch_type(:variable_dylibs)
-  end
-
-  def undeclared_deps
-    @undeclared_deps ||= store.fetch_type(:undeclared_deps)
-  end
-
-  def indirect_deps
-    @indirect_deps ||= store.fetch_type(:indirect_deps)
-  end
-
-  def unnecessary_deps
-    @unnecessary_deps ||= store.fetch_type(:unnecessary_deps)
-  end
-
   def display_normal_output
     display_items "System libraries", system_dylibs
     display_items "Homebrew libraries", brewed_dylibs
@@ -84,10 +44,50 @@ class LinkageChecker
     !@broken_dylibs.empty? || !@broken_deps.empty?
   end
 
+  def undeclared_deps
+    @undeclared_deps ||= store.fetch_type(:undeclared_deps)
+  end
+
   private
 
   attr_reader :keg, :formula, :store
 
+  # 'Hash-type' cache values
+
+  def brewed_dylibs
+    @brewed_dylibs ||= store.fetch_type(:brewed_dylibs)
+  end
+
+  def reverse_links
+    @reverse_links ||= store.fetch_type(:reverse_links)
+  end
+
+  def broken_deps
+    @broken_deps ||= store.fetch_type(:broken_deps)
+  end
+
+  # 'Path-type' cached values
+
+  def system_dylibs
+    @system_dylibs ||= store.fetch_type(:system_dylibs)
+  end
+
+  def broken_dylibs
+    @broken_dylibs ||= store.fetch_type(:broken_dylibs)
+  end
+
+  def variable_dylibs
+    @variable_dylibs ||= store.fetch_type(:variable_dylibs)
+  end
+
+  def indirect_deps
+    @indirect_deps ||= store.fetch_type(:indirect_deps)
+  end
+
+  def unnecessary_deps
+    @unnecessary_deps ||= store.fetch_type(:unnecessary_deps)
+  end
+
   def dylib_to_dep(dylib)
     dylib =~ %r{#{Regexp.escape(HOMEBREW_PREFIX)}/(opt|Cellar)/([\w+-.@]+)/}
     Regexp.last_match(2)