Assert scope is not match/guard when using escaped regexes

[sabiwara]

Following on #14720, I've been thinking if we could/should improve the error message when trying to use regex module attributes in guards/match scope.
This PR explores the idea, but feel free to close if this is overthinking on my part.

Before (confusing: :re.compile mentioned but not in the code)

After

[josevalim]

I think in this case some code duplication is fine. Let's just reimplement the error message directly in the Regex module, since that's what we would expect library authors to do too.

[josevalim]

Instead of generating additional code, we should call Regex.__import__(unquote(...)), where __import__ itself is a macro which also checks the environment.

[sabiwara]

This sounds much better indeed 👍

[sabiwara]

Arg, this might actually not work, since unlike Kernel we need to require Regex.

       quote do
            require Regex
            Regex.__import_pattern__(unquote(Macro.escape(exported)))
          end

Which then makes it impossible to compile elixir:

==> elixir (compile)
error: you are trying to use/import/require the module Regex which is currently being defined.

(although it seems weird to warn here, since it's happening within quote 🤔)

[sabiwara]

Pushed the broken commit just in case: c1dee89

[josevalim]

I believe the issue happens because we escape the struct defined by defstruct inside Regex. An easy fix is to add this clause to the top of the cond in __escape__:

        is_nil(regex.re_pattern) ->
          nil

And then it compiles fine!

[sabiwara]

Thanks @josevalim !
Are we happy with the fact we need to emit require though?
We end up with as much code as __assert_assert_no_match_or_guard_scope__, but the benefit of the latter is that users could reuse it in their own code.
Will fix the CI for the require approach though if we want to go this way!

[sabiwara]

Arf, it doesn't work either invalid pattern in match, require is not allowed in matches.