Add toggle_locale() fcns

These are designed to temporarily switch the locale for a cateogry
around some operation that needs it to be different than the current
one.  They will be used in the next commit.

These will eventually replace the more unwieldy
_is_cur_LC_category_utf8() function, which toggles as a side effect

embed.fnc

@@ -3328,6 +3328,12 @@ Sr	|void	|setlocale_failure_panic_i|const unsigned int cat_index	\
 				|NN const char * failed			\
 				|const line_t caller_0_line		\
 				|const line_t caller_1_line
+So	|const char *|toggle_locale_i|const unsigned switch_cat_index	\
+				|NN const char * new_locale		\
+				|const line_t caller_line
+So	|void	|restore_toggled_locale_i|const unsigned cat_index	\
+                                |NULLOK const char * original_locale    \
+				|const line_t caller_line
 #    if defined(USE_POSIX_2008_LOCALE)
 S	|const char*|emulate_setlocale_i|const unsigned int index	\
 				    |NULLOK const char* new_locale	\

locale.c

@@ -202,6 +202,14 @@ STATIC_ASSERT_DECL(STRLENs(UTF8NESS_PREFIX) == 1);
                                       locale, result)
 #  endif
 
+#  define toggle_locale_i(index, locale)                                    \
+                 S_toggle_locale_i(aTHX_ index, locale, __LINE__)
+#  define toggle_locale_c(cat, locale)  toggle_locale_i(cat##_INDEX_, locale)
+#  define restore_toggled_locale_i(index, locale)                           \
+                 S_restore_toggled_locale_i(aTHX_ index, locale, __LINE__)
+#  define restore_toggled_locale_c(cat, locale)                             \
+                             restore_toggled_locale_i(cat##_INDEX_, locale)
+
 /* Two parallel arrays indexed by our mapping of category numbers into small
  * non-negative indexes; first the locale categories Perl uses on this system,
  * used to do the inverse mapping.  The second array is their names.  These
@@ -4952,6 +4960,95 @@ S_print_bytes_for_locale(pTHX_
 
 #  endif   /* #ifdef DEBUGGING */
 
+STATIC const char *
+S_toggle_locale_i(pTHX_ const unsigned cat_index,
+                        const char * new_locale,
+                        const line_t caller_line)
+{
+    /* Changes the locale for the category specified by 'index' to 'new_locale,
+     * if they aren't already the same.
+     *
+     * Returns a copy of the name of the original locale for 'cat_index'
+     * so can be switched back to with the companion function
+     * restore_toggled_locale_i(),  (NULL if no restoral is necessary.) */
+
+    const char * locale_to_restore_to = NULL;
+
+    PERL_ARGS_ASSERT_TOGGLE_LOCALE_I;
+    assert(cat_index <= NOMINAL_LC_ALL_INDEX);
+
+    /* Find the original locale of the category we may need to change, so that
+     * it can be restored to later */
+
+    locale_to_restore_to = querylocale_i(cat_index);
+
+    DEBUG_Lv(PerlIO_printf(Perl_debug_log,
+             "(%" LINE_Tf "): toggle_locale_i: index=%d(%s), wanted=%s,"
+             " actual=%s\n",
+             caller_line, cat_index, category_names[cat_index],
+             new_locale, locale_to_restore_to));
+
+    if (! locale_to_restore_to) {
+        locale_panic_(Perl_form(aTHX_ "Could not find current %s locale, errno=%d",
+                                category_names[cat_index], errno));
+    }
+
+    /* If the locales are the same, there's nothing to do */
+    if (strEQ(locale_to_restore_to, new_locale)) {
+        DEBUG_Lv(PerlIO_printf(Perl_debug_log,
+                               "(%d): %s locale unchanged as %s\n",
+                               caller_line, category_names[cat_index],
+                               new_locale));
+
+        return NULL;
+    }
+
+    /* Finally, change the locale to the new one */
+    void_setlocale_i(cat_index, new_locale);
+
+    DEBUG_Lv(PerlIO_printf(Perl_debug_log, "(%" LINE_Tf "): %s locale switched to %s\n",
+                           caller_line, category_names[cat_index], new_locale));
+
+    return locale_to_restore_to;
+
+#ifndef DEBUGGING
+    PERL_UNUSED_ARG(caller_line);
+#endif
+
+}
+
+STATIC void
+S_restore_toggled_locale_i(pTHX_ const unsigned int cat_index,
+                                 const char * restore_locale,
+                                 const line_t caller_line)
+{
+    /* Restores the locale for LC_category corresponding to cat_indes to
+     * 'restore_locale' (which is a copy that will be freed by this function),
+     * or do nothing if the latter parameter is NULL */
+
+    PERL_ARGS_ASSERT_RESTORE_TOGGLED_LOCALE_I;
+    assert(cat_index <= NOMINAL_LC_ALL_INDEX);
+
+    if (restore_locale == NULL) {
+        DEBUG_Lv(PerlIO_printf(Perl_debug_log,
+                               "(%" LINE_Tf "): No need to restore %s\n",
+                               caller_line, category_names[cat_index]));
+        return;
+    }
+
+    DEBUG_Lv(PerlIO_printf(Perl_debug_log,
+                           "(%" LINE_Tf "): %s restoring locale to %s\n",
+                           caller_line, category_names[cat_index],
+                           restore_locale));
+
+    void_setlocale_i(cat_index, restore_locale);
+
+#ifndef DEBUGGING
+    PERL_UNUSED_ARG(caller_line);
+#endif
+
+}
+
 STATIC const char *
 S_switch_category_locale_to_template(pTHX_ const int switch_category,
                                      const int template_category,

proto.h

@@ -5646,6 +5646,8 @@ STATIC void	S_new_numeric(pTHX_ const char* newnum);
 #define PERL_ARGS_ASSERT_NEW_NUMERIC
 STATIC void	S_restore_switched_locale(pTHX_ const int category, const char * const original_locale);
 #define PERL_ARGS_ASSERT_RESTORE_SWITCHED_LOCALE
+STATIC void	S_restore_toggled_locale_i(pTHX_ const unsigned cat_index, const char * original_locale, const line_t caller_line);
+#define PERL_ARGS_ASSERT_RESTORE_TOGGLED_LOCALE_I
 STATIC const char *	S_save_to_buffer(const char * string, const char **buf, Size_t *buf_size);
 #define PERL_ARGS_ASSERT_SAVE_TO_BUFFER
 STATIC void	S_set_numeric_radix(pTHX_ const bool use_locale);
@@ -5659,6 +5661,9 @@ STATIC const char*	S_stdize_locale(pTHX_ const int category, const char* input_l
 #define PERL_ARGS_ASSERT_STDIZE_LOCALE
 STATIC const char*	S_switch_category_locale_to_template(pTHX_ const int switch_category, const int template_category, const char * template_locale);
 #define PERL_ARGS_ASSERT_SWITCH_CATEGORY_LOCALE_TO_TEMPLATE
+STATIC const char *	S_toggle_locale_i(pTHX_ const unsigned switch_cat_index, const char * new_locale, const line_t caller_line);
+#define PERL_ARGS_ASSERT_TOGGLE_LOCALE_I	\
+	assert(new_locale)
 #    if defined(USE_POSIX_2008_LOCALE)
 STATIC const char*	S_emulate_setlocale_i(pTHX_ const unsigned int index, const char* new_locale, const int recalc_LC_ALL, const line_t line);
 #define PERL_ARGS_ASSERT_EMULATE_SETLOCALE_I