class Prism::Translation::Parser::Builder

def block(method_call, begin_t, args, body, end_t)

https://github.com/whitequark/parser/blob/v3.3.7.1/lib/parser/builders/default.rb#L1122-L1155

args = :it
block_type = :itblock
if args.type == :itarg

The following three lines have been added to support the `it` block parameter syntax in the source code below.
def block(method_call, begin_t, args, body, end_t)
  _receiver, _selector, *call_args = *method_call
  if method_call.type == :yield
    diagnostic :error, :block_given_to_yield, nil, method_call.loc.keyword, [loc(begin_t)]
  end
  last_arg = call_args.last
  if last_arg && (last_arg.type == :block_pass || last_arg.type == :forwarded_args)
    diagnostic :error, :block_and_blockarg, nil, last_arg.loc.expression, [loc(begin_t)]
  end
  if args.type == :itarg
    block_type = :itblock
    args = :it
  elsif args.type == :numargs
    block_type = :numblock
    args = args.children[0]
  else
    block_type = :block
  end
  if [:send, :csend, :index, :super, :zsuper, :lambda].include?(method_call.type)
    n(block_type, [ method_call, args, body ],
      block_map(method_call.loc.expression, begin_t, end_t))
  else
    # Code like "return foo 1 do end" is reduced in a weird sequence.
    # Here, method_call is actually (return).
    actual_send, = *method_call
    block =
      n(block_type, [ actual_send, args, body ],
        block_map(actual_send.loc.expression, begin_t, end_t))
    n(method_call.type, [ block ],
      method_call.loc.with_expression(join_exprs(method_call, block)))
  end
end