Use C++20 consteval to verify translation strings #30379

It is possible to write _(str_ptr), which is wrong, see #30267 (review).

I am not familiar with the translation build system integration, but it would be good to effectively change the signature of _ from auto _(const char*) to consteval auto _(const char*).

My understanding is that this would catch this error at compile time, saving review effort and follow-up fixup changes.

Assumed compiler error message:

error: the value of 'reason' is not usable in a constant expression
      |   _(reason);
      |     ^~~~~~

diff --git a/src/util/translation.h b/src/util/translation.h
index d33fd2d0a0..7d82264a43 100644
--- a/src/util/translation.h
+++ b/src/util/translation.h
@@ -71,7 +71,7 @@ const extern std::function<std::string(const char*)> G_TRANSLATION_FUN;
  * Translation function.
  * If no translation function is set, simply return the input.
  */
-inline bilingual_str _(const char* psz)
+inline consteval bilingual_str _(const char* psz)
 {
     return bilingual_str{psz, G_TRANSLATION_FUN ? (G_TRANSLATION_FUN)(psz) : psz};
 }

does not work:

./util/translation.h:74:32: error: consteval function never produces a constant expression [-Winvalid-constexpr]
   74 | inline consteval bilingual_str _(const char* psz)
      |                                ^
./util/translation.h:76:31: note: non-constexpr function 'operator bool' cannot be used in a constant expression
   76 |     return bilingual_str{psz, G_TRANSLATION_FUN ? (G_TRANSLATION_FUN)(psz) : psz};
      |                               ^

So I guess this needs a clang-tidy plugin.

nvm, I'll open a pull request soon.

So I guess this needs a clang-tidy plugin.

Clang-tidy plugin is not actually required because #30383 can catch the errors, IIUC.