Deprecate custom BigDecimal serialization

[davidcelis]

Rails currently provides an extension to BigDecimal that redefines how
it is serialized to YAML. However, as noted in #12467, this does not
work as expected. When ActiveSupport is required, BigDecimal YAML
serialization does not maintain the object type. It instead ends up
serializing the number represented by the BigDecimal itself which, when
loaded by YAML later, becomes a Float:

require 'yaml'
require 'bigdecimal'

yaml = BigDecimal('13.37').to_yaml
# => "--- !ruby/object:BigDecimal 18:0.1337E2\n...\n"
YAML.load(yaml).class
# => BigDecimal

require 'active_support/all'

yaml = BigDecimal('13.37').to_yaml
# => "--- 13.37\n...\n"
YAML.load(yaml).class
# => Float

@tenderlove posits that we should deprecate the custom BigDecimal
serialization and let Ruby handle it. For the time being, users who
require this serialization for backwards compatibility can manually
require 'active_support/core_ext/big_decimal/conversions'. I would
agree. So, remove active_support/core_ext/big_decimal so that this
extension is no longer required along with active_support/all.

This will close #12467 and deprecate the custom BigDecimal#to_yaml.

Signed-off-by: David Celis me@davidcel.is

[davidcelis]

I haven't included tests in this PR because I'm not sure how you guys typically approach tests for a mere deprecation. Just let me know if there's something on that front I should do.

[zzak]

require 'active_support/core_ext/big_decimal/conversions' seems like a lot of work, I will see if we can get this merged in upstream BigDecimal. It should be possible given that yaml is supported by stdlib.

[rafaelfranca]

This broke the whole test suite. Can you take a look?

Also I think we should only deprecate the yaml conversion. Right now we are removing more than we need to.

[rafaelfranca]

BTW, thanks @davidcelis for working on this

[davidcelis]

I had removed the whole file due to the discussion in the linked Issue, but I can remove only the YAML serialization if that's what people decide is a better idea.

[rafaelfranca]

We still need the custom to_s implementation so we can't remove it:

lib/active_record/connection_adapters/abstract/quoting.rb
32:        when BigDecimal then value.to_s('F')
70:        when BigDecimal then value.to_s('F')

So I think we should split that file in 2, deprecate the yaml serialization and leave the custom to_s implementation.

[davidcelis]

Gotcha. I'm assuming I should leave the to_d method there as well?

[rafaelfranca]

We can remove the to_d since it was only to Ruby 1.8 compatibility

[davidcelis]

Great. I'll add a separate commit for that; I can squash it later if you want.

[rafaelfranca]

We can remove this require.

[davidcelis]

Woops, thought I had; nice catch.

[davidcelis]

Thanks!