Alternate implementation for https://thomasleecopeland.com/2018/02/12/increment-decrement-matcher.html

increment_and_decrement.rb

module IncrementAndDecrement
  def increment(&block)
    matcher = change(&block).by(1)
    RSpec::Matchers::AliasedMatcher.new(matcher, lambda do |desc|
      desc.gsub("changed", "incremented").gsub("change", "increment")
    end)
  end

  def decrement(&block)
    matcher = change(&block).by(-1)
    RSpec::Matchers::AliasedMatcher.new(matcher, lambda do |desc|
      desc.gsub("changed", "decremented").gsub("change", "decrement").gsub("-", "")
    end)
  end
end

RSpec.configure do |c|
  c.include IncrementAndDecrement
end

Technically, RSpec::Matchers::AliasedMatchers isn't part of RSpec's public API, so if you want to stick only with public APIs, you can do:

RSpec::Matchers.alias_matcher :increment, :change do |desc|
  desc.gsub("changed", "incremented").gsub("change", "increment")
end

RSpec::Matchers.alias_matcher :decrement, :change do |desc|
  desc.gsub("changed", "decremented").gsub("change", "decrement").gsub("-", "")
end

module IncrementAndDecrement
  def increment(&block)
    super(&block).by(1)
  end

  def decrement(&block)
    super(&block).by(-1)
  end
end

RSpec.configure do |c|
  c.include IncrementAndDecrement
end

RSpec::Matchers.alias_matcher is part of the public API, so you don't have to worry about private API breakages with this, but it's also more convoluted, so YMMV.

Brilliant! That's a big improvement, thank you Myron! Post updated.