Internalize hack for dynamic translations change so that it also works for Rust apps

api/cpp/include/slint.h

@@ -1276,10 +1276,7 @@ inline SharedString translate(const SharedString &original, const SharedString &
 /// Example
 /// ```cpp
 ///     my_ui->global<LanguageSettings>().on_french_selected([] {
-///        // trick from https://www.gnu.org/software/gettext/manual/html_node/gettext-grok.html
 ///        setenv("LANGUAGE", langs[l], true);
-///        extern int _nl_msg_cat_cntr;
-///        ++_nl_msg_cat_cntr;
 ///        slint::update_all_translations();
 ///    });
 /// ```

examples/printerdemo/cpp/main.cpp

@@ -80,9 +80,6 @@ int main()
     printer_demo->global<PrinterSettings>().on_change_language([](int l) {
         static const char *langs[] = { "en", "fr" };
         setenv("LANGUAGE", langs[l], true);
-        // trick from https://www.gnu.org/software/gettext/manual/html_node/gettext-grok.html
-        extern int _nl_msg_cat_cntr;
-        ++_nl_msg_cat_cntr;
         slint::update_all_translations();
     });
 #endif

internal/core/translations.rs

@@ -225,6 +225,20 @@ fn translate_gettext(string: &str, ctx: &str, domain: &str, n: i32, plural: &str
 }
 
 pub fn mark_all_translations_dirty() {
+    #[cfg(all(feature = "gettext-rs", target_family = "unix"))]
+    {
+        // SAFETY: This trick from https://www.gnu.org/software/gettext/manual/html_node/gettext-grok.html
+        // is merely incrementing a generational counter that will invalidate gettext's internal cache for translations.
+        // If in the worst case it won't invalidate, then old translations are shown.
+        #[allow(unsafe_code)]
+        unsafe {
+            extern "C" {
+                static mut _nl_msg_cat_cntr: std::ffi::c_int;
+            }
+            _nl_msg_cat_cntr += 1;
+        }
+    }
+
     crate::context::GLOBAL_CONTEXT.with(|ctx| {
         let Some(ctx) = ctx.get() else { return };
         ctx.0.translations_dirty.mark_dirty();