defines Module#qualified_const_(defined?|get|set) and String#deconstantize

This commit also implements a faster version of #demodulize I was unable
to isolate with git add --patch.

Not a big fan of the name #deconstantize. It complements #demodulize
getting rid of the rightmost constant, hence the name, but it is
unrelated to the well-known #constantize. So unsure. Could not come
with anything better, please feel free to rename.

5 files changed, 112 insertions, 13 deletions

diff --git a/activesupport/lib/active_support/core_ext/module.rb b/activesupport/lib/active_support/core_ext/module.rb
index 9fed346b7c..f399fce410 100644
--- a/activesupport/lib/active_support/core_ext/module.rb
+++ b/activesupport/lib/active_support/core_ext/module.rb
@@ -8,4 +8,5 @@ require 'active_support/core_ext/module/delegation'
 require 'active_support/core_ext/module/synchronization'
 require 'active_support/core_ext/module/deprecation'
 require 'active_support/core_ext/module/remove_method'
-require 'active_support/core_ext/module/method_names'
\ No newline at end of file
+require 'active_support/core_ext/module/method_names'
+require 'active_support/core_ext/module/qualified_const'
\ No newline at end of file
diff --git a/activesupport/lib/active_support/core_ext/module/qualified_const.rb b/activesupport/lib/active_support/core_ext/module/qualified_const.rb
new file mode 100644
index 0000000000..d1a0ee2f83
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/module/qualified_const.rb
@@ -0,0 +1,64 @@
+require 'active_support/core_ext/string/inflections'
+
+#--
+# Allows code reuse in the methods below without polluting Module.
+#++
+module QualifiedConstUtils
+  def self.raise_if_absolute(path)
+    raise NameError, "wrong constant name #$&" if path =~ /\A::[^:]+/
+  end
+
+  def self.names(path)
+    path.split('::')
+  end
+end
+
+##
+# Extends the API for constants to be able to deal with qualified names. Arguments
+# are assumed to be relative to the receiver.
+#
+#--
+# Qualified names are required to be relative because we are extending existing
+# methods that expect constant names, ie, relative paths of length 1. For example,
+# Object.const_get("::String") raises NameError and so does qualified_const_get.
+#++
+class Module
+  if method(:const_defined?).arity == 1
+    def qualified_const_defined?(path)
+      QualifiedConstUtils.raise_if_absolute(path)
+
+      QualifiedConstUtils.names(path).inject(self) do |mod, name|
+        return unless mod.const_defined?(name)
+        mod.const_get(name)
+      end
+      return true
+    end
+  else
+    def qualified_const_defined?(path, search_parents=true)
+      QualifiedConstUtils.raise_if_absolute(path)
+
+      QualifiedConstUtils.names(path).inject(self) do |mod, name|
+        return unless mod.const_defined?(name, search_parents)
+        mod.const_get(name)
+      end
+      return true
+    end
+  end
+
+  def qualified_const_get(path)
+    QualifiedConstUtils.raise_if_absolute(path)
+
+    QualifiedConstUtils.names(path).inject(self) do |mod, name|
+      mod.const_get(name)
+    end
+  end
+
+  def qualified_const_set(path, value)
+    QualifiedConstUtils.raise_if_absolute(path)
+
+    const_name = path.demodulize
+    mod_name = path.deconstantize
+    mod = mod_name.empty? ? self : qualified_const_get(mod_name)
+    mod.const_set(const_name, value)
+  end
+end
diff --git a/activesupport/lib/active_support/core_ext/string/inflections.rb b/activesupport/lib/active_support/core_ext/string/inflections.rb
index fd91b3cacb..9356940650 100644
--- a/activesupport/lib/active_support/core_ext/string/inflections.rb
+++ b/activesupport/lib/active_support/core_ext/string/inflections.rb
@@ -117,10 +117,25 @@ class String
   #
   #   "ActiveRecord::CoreExtensions::String::Inflections".demodulize # => "Inflections"
   #   "Inflections".demodulize                                       # => "Inflections"
+  #
+  # See also +deconstatize+.
   def demodulize
     ActiveSupport::Inflector.demodulize(self)
   end
 
+  # Removes the rightmost segment from the constant expression in the string.
+  #
+  #   "Net::HTTP".deconstantize   # => "Net"
+  #   "::Net::HTTP".deconstantize # => "::Net"
+  #   "String".deconstantize      # => ""
+  #   "::String".deconstantize    # => ""
+  #   "".deconstantize            # => ""
+  #
+  # See also +demodulize+.
+  def deconstantize
+    ActiveSupport::Inflector.deconstantize(self)
+  end
+
   # Replaces special characters in a string so that it may be used as part of a 'pretty' URL.
   #
   # ==== Examples
diff --git a/activesupport/lib/active_support/dependencies.rb b/activesupport/lib/active_support/dependencies.rb
index c072a8e646..b3ac271778 100644
--- a/activesupport/lib/active_support/dependencies.rb
+++ b/activesupport/lib/active_support/dependencies.rb
@@ -5,6 +5,7 @@ require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/module/attribute_accessors'
 require 'active_support/core_ext/module/introspection'
 require 'active_support/core_ext/module/anonymous'
+require 'active_support/core_ext/module/qualified_const'
 require 'active_support/core_ext/object/blank'
 require 'active_support/core_ext/load_error'
 require 'active_support/core_ext/name_error'
@@ -357,12 +358,13 @@ module ActiveSupport #:nodoc:
     end
 
     # Is the provided constant path defined?
-    def qualified_const_defined?(path)
-      names = path.sub(/^::/, '').to_s.split('::')
-
-      names.inject(Object) do |mod, name|
-        return false unless local_const_defined?(mod, name)
-        mod.const_get name
+    if Module.method(:const_defined?).arity == 1
+      def qualified_const_defined?(path)
+        Object.qualified_const_defined?(path.sub(/^::/, ''))
+      end
+    else
+      def qualified_const_defined?(path)
+        Object.qualified_const_defined?(path.sub(/^::/, ''), false)
       end
     end
 
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 454637191e..e76ee60dd7 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -160,15 +160,32 @@ module ActiveSupport
       underscored_word.gsub(/_/, '-')
     end
 
-    # Removes the module part from the expression in the string.
+    # Removes the module part from the expression in the string:
     #
-    # Examples:
     #   "ActiveRecord::CoreExtensions::String::Inflections".demodulize # => "Inflections"
     #   "Inflections".demodulize                                       # => "Inflections"
-    def demodulize(class_name_in_module)
-      # If you remove the module part of an empty string, you get an empty string.
-      # That's why the regexp uses the * quantifier.
-      class_name_in_module.to_s[/[^:]*\z/]
+    #
+    # See also +deconstantize+.
+    def demodulize(path)
+      path = path.to_s
+      if i = path.rindex('::')
+        path[(i+2)..-1]
+      else
+        path
+      end
+    end
+
+    # Removes the rightmost segment from the constant expression in the string:
+    #
+    #   "Net::HTTP".deconstantize   # => "Net"
+    #   "::Net::HTTP".deconstantize # => "::Net"
+    #   "String".deconstantize      # => ""
+    #   "::String".deconstantize    # => ""
+    #   "".deconstantize            # => ""
+    #
+    # See also +demodulize+.
+    def deconstantize(path)
+      path.to_s[0...(path.rindex('::') || 0)] # implementation based on the one in facets' Module#spacename
     end
 
     # Creates a foreign key name from a class name.