Do not emit unexpected keyword rest by rbs prototype rb

[pocke]

Problem

rbs prototype rb command generates kwrest argument unexpectedly if the method as keyword argument(s).

# a.rb
class C
  def foo(x:) end
end

$ exe/rbs prototype rb a.rb
# a.rb
class C
  def foo: (x: untyped x, **untyped) -> untyped
end

Cause

Because a kwrest node is not falsy if the method has keyword arguments.

pp RubyVM::AbstractSyntaxTree.parse <<~RUBY
  def with_keyword_arg(x:) end
  def without_keyowrd_arg(x) end
RUBY

(SCOPE@1:0-2:30
 tbl: []
 args: nil
 body:
   (BLOCK@1:0-2:30
      (DEFN@1:0-1:28
       mid: :with_keyword_arg
       body:
         (SCOPE@1:0-1:28
          tbl: [:x, nil]
          args:
            (ARGS@1:21-1:23
             pre_num: 0
             pre_init: nil
             opt: nil
             first_post: nil
             post_num: 0
             post_init: nil
             rest: nil
             kw:
               (KW_ARG@1:21-1:23
                  (LASGN@1:21-1:23 :x :NODE_SPECIAL_REQUIRED_KEYWORD) nil)
             kwrest: (DVAR@1:21-1:23 nil)
             block: nil)
          body: nil))
      (DEFN@2:0-2:30
       mid: :without_keyowrd_arg
       body:
         (SCOPE@2:0-2:30
          tbl: [:x]
          args:
            (ARGS@2:24-2:25
             pre_num: 1
             pre_init: nil
             opt: nil
             first_post: nil
             post_num: 0
             post_init: nil
             rest: nil
             kw: nil
             kwrest: nil
             block: nil)
          body: nil))))

---

This pull request will fix the problem.

[soutaro]

Good catch!