keg: cache runtime_dependencies.

Library/Homebrew/dev-cmd/bottle.rb

@@ -263,6 +263,7 @@ module Homebrew
 
         changed_files = keg.replace_locations_with_placeholders unless args.skip_relocation?
 
+        Keg.clear_cache
         Tab.clear_cache
         tab = Tab.for_keg(keg)
         original_tab = tab.dup

Library/Homebrew/formula_installer.rb

@@ -649,6 +649,7 @@ class FormulaInstaller
 
     # Update tab with actual runtime dependencies
     tab = Tab.for_keg(keg)
+    Keg.clear_cache
     Tab.clear_cache
     f_runtime_deps = formula.runtime_dependencies(read_from_tab: false)
     tab.runtime_dependencies = Tab.runtime_deps_hash(f_runtime_deps)

Library/Homebrew/keg.rb

@@ -6,6 +6,8 @@ require "lock_file"
 require "ostruct"
 
 class Keg
+  extend Cachable
+
   class AlreadyLinkedError < RuntimeError
     def initialize(keg)
       super <<~EOS
@@ -519,7 +521,8 @@ class Keg
   end
 
   def runtime_dependencies
-    tab.runtime_dependencies
+    Keg.cache[:runtime_dependencies] ||= {}
+    Keg.cache[:runtime_dependencies][path] ||= tab.runtime_dependencies
   end
 
   def aliases

Library/Homebrew/test/spec_helper.rb

@@ -145,6 +145,7 @@ RSpec.configure do |config|
 
     begin
       Homebrew.raise_deprecation_exceptions = true
+      Keg.clear_cache
       Tap.clear_cache
       FormulaInstaller.clear_attempted
 
@@ -177,6 +178,7 @@ RSpec.configure do |config|
       end
 
       Tab.clear_cache
+      Keg.clear_cache
 
       FileUtils.rm_rf [
         TEST_DIRECTORIES.map(&:children),