TL;DR The primary driver is to remove autoload surprise.
This is related to #12106. (The root cause for that ticket is that
json/add defines Regexp#to_json among others, but here I'll reproduce
the problem without json/add.)
Before:
>> require 'active_support/core_ext/to_json'
=> true
>> //.as_json
NoMethodError: undefined method `as_json' for //:Regexp
from (irb):3
from /Users/godfrey/.rvm/rubies/ruby-2.0.0-p195/bin/irb:16:in `<main>'
>> //.to_json
=> "\"(?-mix:)\""
>> //.as_json
=> "(?-mix:)"
After:
>> require 'active_support/core_ext/to_json'
=> true
>> //.as_json
=> "(?-mix:)"
This is because ActiveSupport::JSON is autoloaded the first time
Object#to_json is called, which causes additional core extentions
(previously defined in active_support/json/encoding.rb) to be loaded.
When someone require 'active_support/core_ext', the expectation is
that it would add certain methods to the core classes NOW. The
previous behaviour causes additional methods to be loaded the first
time you call `to_json`, which could cause nasty surprises and other
unplesant side-effects.
This change moves all core extensions in to core_ext/json. AS::JSON is
still autoloaded on first #to_json call, but since it nolonger
include the core extensions, it should address the aforementioned bug.
*Requiring core_ext/object/to_json now causes a deprecation warnning*
64c88fb5d2