Example.rb

# Example of the implementation

class Template
  module CompileSite; end
  
  if RUBY_VERSION < '1.9'
    def compile(klass, name, source)
      klass.class_eval <<-RUBY
        ::Template::CompileSite.class_eval do
          def #{name}(locals)
            #{source}
          end
        end
      RUBY
    end
  else
    def compile(klass, name, source)
      klass.class_eval <<-RUBY
        def self.definer
        ::Template::CompileSite.send(:define_method, #{name.inspect}) do |locals, &blk|
            (Thread.current[:tilt_blk] ||= []) << blk
            begin
              #{source}
            ensure
              Thread.current[:tilt_blk].pop
            end
          end
        end
        
        definer do |*a, &b|
          if blk = Thread.current[:tilt_blk].last
            blk.call(*a, &b)
          else
            raise LocalJumpError, "no block given"
          end
        end
        
        class << self; remove_method :definer end
      RUBY
    end
  end
end

module Helper
  A = "A"
end

class Scope
  include Template::CompileSite
  include Helper
  B = "B"
end

Template.new.compile(Scope, 'hello', <<-RUBY)
  if locals.nil?
    yield
  else
    [locals, A, B, yield, hello(nil) { 'Y2' }, yield].join(" ")
  end
RUBY

Template::CompileSite.instance_method(:hello)

res = Scope.new.hello("L") { "Y1" }

if res == "L A B Y1 Y2 Y1"
  puts "YOU DID IT!"
else
  puts "Failed:", res
end

Fix.diff

commit 4addd0777245f1618fca8bbc8114194ac8c5fb64
Author: Magnus Holm <judofyr@gmail.com>
Date:   Wed Jul 21 22:47:13 2010 +0200

    Add support for constant access

diff --git a/lib/tilt.rb b/lib/tilt.rb
index e61efe9..c5e5bdf 100644
--- a/lib/tilt.rb
+++ b/lib/tilt.rb
@@ -188,7 +188,7 @@ module Tilt
         if scope.respond_to?(method_name)
           scope.send(method_name, locals, &block)
         else
-          compile_template_method(method_name, locals)
+          compile_template_method(scope.class, method_name, locals)
           scope.send(method_name, locals, &block)
         end
       else
@@ -279,18 +279,109 @@ module Tilt
       digest = Digest::MD5.hexdigest(parts.join(':'))
       "__tilt_#{digest}"
     end
-
-    def compile_template_method(method_name, locals)
-      source, offset = precompiled(locals)
-      offset += 1
-      CompileSite.module_eval <<-RUBY, eval_file, line - offset
-        def #{method_name}(locals)
-          #{source}
-        end
-      RUBY
-
-      ObjectSpace.define_finalizer self,
-        Template.compiled_template_method_remover(CompileSite, method_name)
+    
+    # Compiling the template is surprisingly hard because:
+    # 1) `yield` should work
+    # 2) Constants should be relative to the scope
+    # 
+    # Example:
+    # 
+    #   class Scope
+    #     include Tilt::CompileSite
+    #     Foo = 1
+    #   end
+    #   
+    #   template = Tilt::ERBTemplate.new { '<%= Foo + yield %>' }
+    #   template.render(Scope.new) { 1 }
+    if RUBY_VERSION < '1.9'
+      def compile_template_method(klass, method_name, locals)
+        source, offset = precompiled(locals)
+        offset += 2
+        
+        # In 1.8, #class_eval with a string will change the constant
+        # lookup, while #class_eval with a block will not. Therefore,
+        # all constants in the source will be relative to `klass`.
+        # 
+        # In 1.9 however, both versions of #class_eval changes the
+        # constant lookup, so this won't help here.
+        klass.class_eval <<-RUBY, eval_file, line - offset
+          ::Tilt::CompileSite.class_eval do
+            def #{method_name}(locals)
+              #{source}
+            end
+          end
+        RUBY
+      
+        ObjectSpace.define_finalizer self,
+          Template.compiled_template_method_remover(CompileSite, method_name)
+      end
+    else
+      def compile_template_method(klass, method_name, locals)
+        source, offset = precompiled(locals)
+        definer = "__tilt_definer_#{rand.to_s[2..-1]}"
+        offset += 4
+        
+        # In 1.9, we'll have to do something a little more complex
+        # and ugly. First we'll have to do the #class_eval call
+        # in a separate method in order to not capture the template
+        # object inside the method we're defining. This will result
+        # in a leak, because the method holds a reference to the
+        # template object, and therefore the GC won't release it.
+        # 
+        # We use #class_eval with a string in order to change the
+        # constant lookup, and #define_method in order to not change
+        # it *back* again to Tilt::CompileSite.
+        # 
+        # However, any `yield` inside a block will call an outer method,
+        # and *not* the block passed to the block. Therefore we'll have to
+        # wrap the #define_method inside another method (a definer) which
+        # we call with a custom block. When the template calls `yield`,
+        # this is the actual block it's calling.
+        # 
+        # When the method is called, right before any template-specific
+        # code is ran, we store the block given to #render in a thread
+        # local variable. In our custom block (which is invoked when the
+        # template calls `yield`) we then find the block again and invokes
+        # that instead.
+        # 
+        # Because during the rendering of a template, there is a possibility
+        # that the same template is rendered again, we'll have to make sure
+        # the blocks aren't overwritten in the thread local variables. Here
+        # I'm storing it in an array which is popped when the rendering is
+        # done. Another choice would have been to randomly generate a
+        # variable and use that instead, but only a benchmark would tell
+        # us if it's actually any faster.
+        Template.custom_class_eval klass, <<-RUBY, eval_file, line - offset
+          def self.#{definer}
+          ::Tilt::CompileSite.send(:define_method, #{method_name.inspect}) do |locals, &blk|
+              (Thread.current[:tilt_blk] ||= []) << blk
+              begin
+                #{source}
+              ensure
+                Thread.current[:tilt_blk].pop
+              end
+            end
+          end
+          
+          #{definer} do |*a, &b|
+            if blk = Thread.current[:tilt_blk].last
+              blk.call(*a, &b)
+            else
+              raise LocalJumpError, "no block given"
+            end
+          end
+          
+          class << self; remove_method #{definer.inspect} end
+        RUBY
+      
+        ObjectSpace.define_finalizer self,
+          Template.compiled_template_method_remover(CompileSite, method_name)
+      end
+      
+      # 
+      def self.custom_class_eval(klass, code, file, line)
+        klass.class_eval(code, file, line)
+      end
     end
 
     def self.compiled_template_method_remover(site, method_name)

Test.diff

diff --git a/test/tilt_template_test.rb b/test/tilt_template_test.rb
index ec6743f..adf768d 100644
--- a/test/tilt_template_test.rb
+++ b/test/tilt_template_test.rb
@@ -121,8 +121,13 @@ class TiltTemplateTest < Test::Unit::TestCase
     assert inst.prepared?
   end
 
+  module PersonHelper
+    CONSTANT2 = "Joe"
+  end
+  
   class Person
     CONSTANT = "Bob"
+    include PersonHelper
 
     attr_accessor :name
     def initialize(name)
@@ -146,6 +151,11 @@ class TiltTemplateTest < Test::Unit::TestCase
     assert_equal "Hey Bob!", inst.render(Person.new("Joe"))
   end
 
+  test "template which accesses an included constant" do
+    inst = SourceGeneratingMockTemplate.new { |t| 'Hey #{CONSTANT2}!' }
+    assert_equal "Hey Joe!", inst.render(Person.new("Joe"))
+  end
+
   class FastPerson < Person
     include Tilt::CompileSite
   end
@@ -156,4 +166,9 @@ class TiltTemplateTest < Test::Unit::TestCase
   #   inst = SourceGeneratingMockTemplate.new { |t| 'Hey #{CONSTANT}!' }
   #   assert_equal "Hey Bob!", inst.render(FastPerson.new("Joe"))
   # end
+  # 
+  # test "template which accesses an included constant with Tilt::CompileSite" do
+  #   inst = SourceGeneratingMockTemplate.new { |t| 'Hey #{CONSTANT2}!' }
+  #   assert_equal "Hey Joe!", inst.render(FastPerson.new("Joe"))
+  # end
 end