Replace the build object rather than mutate it

Library/Homebrew/build_options.rb

@@ -1,7 +1,6 @@
 require 'options'
 
 class BuildOptions
-  attr_accessor :args
   attr_accessor :universal
 
   def initialize(args, options)
@@ -15,7 +14,7 @@ class BuildOptions
   end
 
   def include? name
-    args.include? '--' + name
+    @args.include?("--#{name}")
   end
 
   def with? val

Library/Homebrew/dependency.rb

@@ -29,10 +29,9 @@ class Dependency
   end
 
   def to_formula
-    f = Formulary.factory(name)
-    # Add this dependency's options to the formula's build args
-    f.build.args = f.build.args.concat(options)
-    f
+    formula = Formulary.factory(name)
+    formula.build = BuildOptions.new(options, formula.options)
+    formula
   end
 
   def installed?

Library/Homebrew/formula_installer.rb

@@ -303,15 +303,9 @@ class FormulaInstaller
   end
 
   def effective_build_options_for(dependent, inherited_options=[])
-    if dependent == f
-      build = dependent.build.dup
-      build.args |= options
-      build
-    else
-      build = dependent.build.dup
-      build.args |= inherited_options
-      build
-    end
+    args  = dependent.build.used_options
+    args |= dependent == f ? options : inherited_options
+    BuildOptions.new(args, dependent.options)
   end
 
   def inherited_options_for(dep)