RSpec: `let`s leak across examples when invoked from before(:all)

foo_spec.rb

puts "rspec-core #{RSpec::Core::Version::STRING}"

class InstantiationCounter
  def initialize(k)
    $count ||= { }
    $count[k] ||= 0
    $count[k] += 1
    @k = k
  end

  def count
    $count[@k]
  end
end

describe 'Foo' do
  if ENV['BEFORE_ALL']
    before(:all) do
      puts "Before all bar: #{bar.count}"
    end
  end

  let(:bar) { InstantiationCounter.new('bar') }
  let(:baz) { InstantiationCounter.new('baz') }

  it 'is in the first one' do
    puts "Bar: #{bar.count}"
    puts "Baz: #{baz.count}"
  end

  it 'is in the second one' do
    puts "Bar: #{bar.count}"
    puts "Baz: #{baz.count}"
  end

  it 'is in the third one' do
    puts "Bar: #{bar.count}"
    puts "Baz: #{baz.count}"
  end
end

with_before_all.txt

# Output from
#     $ BEFORE_ALL=y rspec foo_spec.rb -fn

rspec-core 2.8.0.rc1

Foo
Before all bar: 1
Bar: 1
Baz: 1
  is in the first one
Bar: 1
Baz: 1
  is in the second one
Bar: 1
Baz: 1
  is in the third one

Finished in 0.00096 seconds
3 examples, 0 failures

without_before_all.txt

# Output from
#     $ rspec foo_spec.rb -fn

rspec-core 2.8.0.rc1

Foo
Bar: 1
Baz: 1
  is in the first one
Bar: 2
Baz: 2
  is in the second one
Bar: 3
Baz: 3
  is in the third one

Finished in 0.00082 seconds
3 examples, 0 failures

Example output is from 2.8.0.rc1, but the same behavior is present with all 2.x versions (checked 2.0.1, 2.1.0, 2.3.2, 2.4.0, 2.5.2, 2.6.4, and 2.7.1).