Handling of post-condition loops

[whitequark]

It just came to my attention that parser currently does not handle loops with post-condition (https://github.com/whitequark/parser/blob/master/doc/AST_FORMAT.md#with-postcondition):

begin
  foo
end while true

This must be handled before 2.0 release.

[bbatsov]

Ah, yes - they're not quite the same as regular while/until nodes - http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/6745.

[whitequark]

Unfortunately, this will require some (begin) nodes to become (kwbegin)... they're not semantically equivalent. For example, if you make a rewriter which adds while foo after a (begin) node, i.e. ( ), the outcome will be different than with a (kwbegin), i.e. begin end.