Allow entirely opting out of deprecation warnings

[ghiculescu]

Previously if you did app.config.active_support.deprecation = :silence, some work would still be done on each call to ActiveSupport::Deprecation.warn. Specifically checking the backtrace, generating the deprecation warning, and checking if the warning is disallowed.

In very hot paths, this could cause performance issues. This PR lets you turn off deprecation reporting entirely for a specific environment.

config.active_support.report_deprecations = false

^ has the same outcome as:

config.active_support.deprecation = :silence
config.active_support.disallowed_deprecation = :silence

But it will short circuit here.

[ghiculescu]

Here is a benchmark:

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
  gem "rails", github: "rails/rails", branch: "main"
  gem "benchmark-ips"
end

require "active_support"

class MainBranch < ActiveSupport::Deprecation
end

class WithThisPR < ActiveSupport::Deprecation
  def initialize(*)
    super
    self.silenced = true
  end
end

WithThisPR.behavior = :silence
MainBranch.behavior = :silence

Benchmark.ips do |x|
  x.report("entirely silenced with this PR") { WithThisPR.warn("foo") }
  x.report("behavior = :silence on main branch") { MainBranch.warn("foo") }
  x.compare!
end

Warming up --------------------------------------
entirely silenced with this PR
                        18.693k i/100ms
behavior = :silence on main branch
                        19.373k i/100ms
Calculating -------------------------------------
entirely silenced with this PR
                        421.698k (±39.6%) i/s -      1.626M in   5.055943s
behavior = :silence on main branch
                        174.997k (±10.0%) i/s -    871.785k in   5.028267s

Comparison:
entirely silenced with this PR:   421697.8 i/s
behavior = :silence on main branch:   174996.5 i/s - 2.41x  (± 0.00) slower

[rafaelfranca]

Instead of introducing a new option, can we should change the behavior of the value :silence to set all the right configs?

[rafaelfranca]

Ah, but there is also the disallowed_deprecation. Yeah, I think a new option is better.