Don't use heredoc inside interpolation

[makenowjust]

A heredoc inside interpolation is buggy. For example, this code cannot be compiled even though it was valid Ruby code:

p <<-HERE1
  here1-1
  #{<<-HERE2}
    here2
  HERE2
  here1-2
HERE1

And this code is compiled even though it was invalid Ruby code:

p "#{<<-HERE}
"
hello
HERE
# => "hello\n"

I tried to fix these, however I did't complete it and I found it is a bit hard to fix.
A heredoc inside interpolation is very tricky, so I suggest forbidding this.

Thank you.

[makenowjust]

ping @asterite. Could you review this?

[asterite]

@makenowjust no, sorry, I won't have time to review PRs related to the compiler. I'll just discuss issues and approve trivial PRs

[bew]

Maybe rebase? to make sure it still works who knows...

[makenowjust]

@RX14 I think this failed is irrelevant. Could you re-run CircleCI job?

[makenowjust]

Thank you.

[RX14]

@makenowjust just to clarify, is this a breaking change or not? (i.e. did this work before or is this just improving the error message)

[makenowjust]

@RX14 Just a bug fix (or bug hiding). There is no source code broken by this.