Thanks @toretore.
https://github.com/rails/rails/commit/9e4c41c903e8e58721f2c41776a8c60ddba7a0a9#commitcomment-2134706

Match classes without to_s conversions.

There is no need in `value_under_case.class.to_s` conversion since case already matches classes with `Class === value_under_case`.

Test Hash#diff deprecation, also prevent deprecation message in AS tests

Removing warning : ambiguous first argument

Two threads may be in method_missing at the same time. If so, they might
both try to define the same delegator method.

Such a situation probably wouldn't result in a particularly spectacular
bug as one method would probably just be overridden by an identical
method, but it could cause warnings to pop up. (It could be worse if
method definition is non-atomic in a particular implementation.)

(We will also need this mutex shortly anyway, see #8127.)

Removing warning : assigned but unused variable

Remove unwanted transaction when has one association is built

Keep the code related to serialization in Serialization module.

We should not need any `serialized_attributes` checks outside `ActiveRecord::AttributeMethods::Serialization` module.

Trivial documentation fix for ActiveModel::Naming comment [ci skip]

get is the most common usage, and match without an explicit verb
was disallowed in 56cdc81c.

[ci skip]

Most apps upgrading from 3.x will have options for mass assigment in
their application.rb and environments/*.rb config files. Rather than
just raising a NoMethodError when copying the config, this commit
adds a warning message until either the protected_attributes gem
is installed or the relevant config options are removed.

Deprecate Hash#diff.

It's no longer used in Rails any more.

See https://github.com/rails/rails/pull/8142\#issuecomment-10227297 for more

These were removed with ActiveRecord::Model in
https://github.com/rails/rails/commit/9e4c41c903e8e58721f2c41776a8c60ddba7a0a9#L15L156

Rather than just changing it and hoping for the best.

Requested by @jeremy:

https://github.com/rails/rails/commit/ba1544d71628abff2777c9c514142d7e9a159111#commitcomment-2106059

Removing duplication in callback normalization.

These two things were 100% identical.

:counter_cache option for  to support custom named counter caches

So that the scope may be a NullRelation and return a result without
executing a query.

Fixes #7928

Fixes #8102.

I couldn't find a nicer way to deal with this than delegate the call to
 #scope, which will be a NullRelation when we want it to be.

This allows us to avoid hacks like the "return 0 if owner.new_record?"
in #count (which this commit removes).

Also, the relevant foreign key may actually be present even on a new
owner record, in which case we *don't* want a null relation. This logic
is encapsulated in the #null_scope? method.

We also need to make sure that the CollectionProxy is not 'infected'
with the NullRelation module, or else the methods from there will
override the definitions in CollectionProxy, leading to incorrect
results. Hence the nullify: false option to CollectionAssociation#scope.
(This feels a bit nasty but I can't think of a better way.)

delegate PathSet's enumerator methods

Fix message about include_blank option. [ci skip]

For example, the following should not run any query on the database:

Post.new.comments.where(body: 'omg').to_a # => []

Fixes #5215.

Add note about squashing in contributor guide.

We tell people how to do this all the time in the Issues tracker,
so now we can just point them here.

See
https://github.com/rails/rails/commit/cb7d19b66badfbd1846c195913419c331701074e#commitcomment-2127148.

not raise so many exceptions:

  https://github.com/rails/rails/blob/master/actionpack/lib/action_view/template.rb#L126

irb(main):001:0> class Foo; def method_missing(*args); super; end end
=> nil
irb(main):002:0> $DEBUG = true
=> true
irb(main):003:0> Array(Foo.new)
Exception `NoMethodError' at (irb):1 - undefined method `to_ary' for #<Foo:0x007f854390e488>
Exception `NoMethodError' at (irb):1 - undefined method `to_a' for #<Foo:0x007f854390e488>
=> [#<Foo:0x007f854390e488>]
irb(main):004:0>