Cop/StaticTranslationDefinition: Check multiline strings

What does this MR do and why?

To support multiline strings we need to allow dynamic strings as well. Both are dstr.

With this, we check for:

  _('long string ' \
    'ends here')

In Ruby AST it's a dstr but later it's just a plain string 'long string ends here' and thus normally used by gettext.

Now, we also check for:

  _("something with #{interpolation}")

This is because it's also dstr from AST point of view. Interpolated strings are not recognized by gettext. This fact we have to catch in a separate cop.

Found while investigating for #374091 (closed).

How to set up and validate locally

1. Apply the following patch to simulate an offense with multiline strings:

diff --git a/app/controllers/concerns/verifies_with_email.rb b/app/controllers/concerns/verifies_with_email.rb
index d00952015e1..fee79a60d47 100644
--- a/app/controllers/concerns/verifies_with_email.rb
+++ b/app/controllers/concerns/verifies_with_email.rb
@@ -102,6 +102,11 @@ def verify_token(user, token)
     end
   end
 
+  def self.m
+    @m ||= s_('IdentityVerification|Maximum login attempts exceeded. '\
+      'Wait %{interval} and try again.') % { interval: user_sign_in_interval }
+  end
+
   def render_sign_in_rate_limited
     message = s_('IdentityVerification|Maximum login attempts exceeded. '\
       'Wait %{interval} and try again.') % { interval: user_sign_in_interval }

On master 💥

No offense found although expected 💥

bundle exec rubocop -C false --only Cop/StaticTranslationDefinition app/controllers/concerns/verifies_with_email.rb

Inspecting 1 file
.

1 file inspected, no offenses detected

On MR ✅

Offense found ✅

$ bundle exec rubocop -C false --only Cop/StaticTranslationDefinition app/controllers/concerns/verifies_with_email.rb
Inspecting 1 file
C

Offenses:

app/controllers/concerns/verifies_with_email.rb:106:12: C: Cop/StaticTranslationDefinition: Translation is defined in static scope. Keep translations dynamic. See https://docs.gitlab.com/ee/development/i18n/externalization.html#keep-translations-dynamic
    @m ||= s_('IdentityVerification|Maximum login attempts exceeded. '\ ...
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected

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