Engine: Case statement with when/in in single ERB tag not backward compatible with ActionView

[joelhawksley]

The following is invalid in Herb:

[error] A case statement with when/in in a single ERB tag cannot be formatted. Use separate tags for case and its conditions. (ERB_CASE_WITH_CONDITIONS_ERROR) (parser-no-errors)

***.html.erb:59:32

  →  59 │                                 <% case @foo
        │                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     60 │                                    when :bar %>

 Rule offenses:
  parser-no-errors (1 offense in 1 file)

But it is valid in Erubi.

(In this case, we compile and run this template with just ERB.new, which requires the case/when to be in a single ERB tag. But the fact stands that this is valid Erubi and thus makes Herb not backwards-compatible with this pattern.)

[marcoroth]

Thanks for bringing this up @joelhawksley!

This is a really tricky one. We intentionally introduced the new error in #1112 as it was not being parsed/compiled/rendered correctly, and the error at least prevented it from actually compiling it with the wrong behavior. To fix this we would have had to split up the single ERB tag into two, one for the case and one for the when statement. At the time it seemed easier to just introduce a new error to stop the wrong behavior and to be able to surface it.

But, after thinking about this for a bit more now, I came up with the approach in #1263. We create a synthetic ERBWhenNode or ERBCaseInNode so we are able to still express the semantic in the AST as if it would have been two nodes.

I put this new behavior behind the strict mode for now, since this is not ideal for parsing, compiling, formatting, and in terms of how we have to handle it. That way, at least in the non-strict mode, we can provide full backwards compatibility with Erubi, which I think is reasonable.