class RSpec::Matchers::BuiltIn::Compound::NestedEvaluator

This is necessary so that the ‘expect` block is only executed once.
}.to bar
expect { x }.to foo
expect {
`expect { x }.to foo.and bar` becomes:
For block expectations, we need to nest them instead, so that
expect(x).to bar
expect(x).to foo
`expect(x).to foo.and bar`, this becomes:
Normally, we evaluate the matching sequentially. For an expression like

def self.matcher_expects_call_stack_jump?(matcher) 

def self.matcher_expects_call_stack_jump?(matcher)
  matcher.expects_call_stack_jump?
rescue NoMethodError
  false
end

def initialize(actual, matcher_1, matcher_2) 

def initialize(actual, matcher_1, matcher_2)
  @actual        = actual
  @matcher_1     = matcher_1
  @matcher_2     = matcher_2
  @match_results = {}
  inner, outer = order_block_matchers
  @match_results[outer] = outer.matches?(Proc.new do |*args|
    @match_results[inner] = inner.matches?(inner_matcher_block(args))
  end)
end

def inner_matcher_block(outer_args)

the args on to the `expect` block.
When such a matcher is used as the outer matcher, we need to forward the
Some block matchers (such as `yield_xyz`) pass args to the `expect` block. 
def inner_matcher_block(outer_args)
  return @actual if outer_args.empty?
  Proc.new do |*inner_args|
    unless inner_args.empty?
      raise ArgumentError, "(#{@matcher_1.description}) and " \
        "(#{@matcher_2.description}) cannot be combined in a compound expectation " \
        "since both matchers pass arguments to the block."
    end
    @actual.call(*outer_args)
  end
end

def matcher_matches?(matcher) 

def matcher_matches?(matcher)
  @match_results.fetch(matcher) do
    raise ArgumentError, "Your #{matcher.description} has no match " \
     "results, this can occur when an unexpected call stack or " \
     "local jump occurs. Perhaps one of your matchers needs to " \
     "declare `expects_call_stack_jump?` as `true`?"
  end
end

def order_block_matchers

should be the outer matcher.
This method figures out which matcher should be the inner matcher and which

`raise_error` logic, so only the former case will work properly.
get executed because the `raise "boom"` line would jump to the `rescue` in the
In the latter case, the after-block logic in the `change` matcher would never

}.to raise_error("boom")
}.to change { x }.by(1)
raise "boom"
x += 1
expect {
expect {

...rather than:

}.to change { x }.by(1)
}.to raise_error("boom")
raise "boom"
x += 1
expect {
expect {

For example, we need it to be this:
up the call stack, we need to order things so that it is the inner matcher.
For a matcher like `raise_error` or `throw_symbol`, where the block will jump 
def order_block_matchers
  return @matcher_1, @matcher_2 unless self.class.matcher_expects_call_stack_jump?(@matcher_2)
  return @matcher_2, @matcher_1 unless self.class.matcher_expects_call_stack_jump?(@matcher_1)
  raise ArgumentError, "(#{@matcher_1.description}) and " \
    "(#{@matcher_2.description}) cannot be combined in a compound expectation " \
    "because they both expect a call stack jump."
end