Do this in a saner way

Library/Homebrew/cmd/doctor.rb

@@ -1023,7 +1023,11 @@ def check_for_unlinked_but_not_keg_only
     if not rack.directory?
       true
     elsif not (HOMEBREW_REPOSITORY/"Library/LinkedKegs"/rack.basename).directory?
-      Formula.factory(rack.basename.to_s).keg_only? rescue nil
+      begin
+        Formula.factory(rack.basename.to_s).keg_only?
+      rescue FormulaUnavailableError
+        false
+      end
     else
       true
     end

Library/Homebrew/formula.rb

@@ -350,8 +350,12 @@ class Formula
   end
 
   def self.installed
-    # `rescue nil` is here to skip kegs with no corresponding formulae
-    HOMEBREW_CELLAR.children.map{ |rack| factory(rack.basename.to_s) rescue nil }.compact
+    HOMEBREW_CELLAR.children.map do |rack|
+      begin
+        factory(rack.basename.to_s)
+      rescue FormulaUnavailableError
+      end
+    end.compact
   end
 
   def self.aliases