class RSpec::CallerFilter

internal method that raised an error.
the code using the library, which is far more useful than the particular
non-rspec lines. This enables errors to be reported at the call site in
Consistent implementation for “cleaning” the caller method to strip out

def self.first_non_rspec_line 

def self.first_non_rspec_line
  # `caller` is an expensive method that scales linearly with the size of
  # the stack. The performance hit for fetching it in chunks is small,
  # and since the target line is probably near the top of the stack, the
  # overall improvement of a chunked search like this is significant.
  #
  # See benchmarks/caller.rb for measurements.
  # Initial value here is mostly arbitrary, but is chosen to give good
  # performance on the common case of creating a double.
  increment = 5
  i         = 1
  line      = nil
  while !line
    stack = caller(i, increment)
    raise "No non-lib lines in stack" unless stack
    line = stack.find { |l| l !~ LIB_REGEX }
    i         += increment
    increment *= 2 # The choice of two here is arbitrary.
  end
  line
end

def self.first_non_rspec_line

fallback is logically the same, but slower.
Earlier rubies do not support the two argument form of `caller`. This 
def self.first_non_rspec_line
  caller.find { |line| line !~ LIB_REGEX }
end