refactor: add function xstrnsave

xstrnsave is a clone of vim_strnsave that uses char* instead of char_u*.
Its purpose short-term is to help reduce the number of casts and for
long-term to replace vim_strnsave as the need to use char_u is
eliminated.

src/nvim/ex_eval.c

@@ -388,12 +388,12 @@ char *get_exception_string(void *value, except_type_T type, char_u *cmdname, int
     mesg = ((struct msglist *)value)->throw_msg;
     if (cmdname != NULL && *cmdname != NUL) {
       size_t cmdlen = STRLEN(cmdname);
-      ret = (char *)vim_strnsave((char_u *)"Vim(", 4 + cmdlen + 2 + STRLEN(mesg));
+      ret = xstrnsave("Vim(", 4 + cmdlen + 2 + STRLEN(mesg));
       STRCPY(&ret[4], cmdname);
       STRCPY(&ret[4 + cmdlen], "):");
       val = ret + 4 + cmdlen + 2;
     } else {
-      ret = (char *)vim_strnsave((char_u *)"Vim:", 4 + STRLEN(mesg));
+      ret = xstrnsave("Vim:", 4 + STRLEN(mesg));
       val = ret + 4;
     }
 

src/nvim/if_cscope.c

@@ -433,7 +433,7 @@ static int cs_add_common(char *arg1, char *arg2, char *flags)
   if (fname == NULL) {
     goto add_err;
   }
-  fname = (char *)vim_strnsave((char_u *)fname, len);
+  fname = xstrnsave(fname, len);
   xfree(fbuf);
   FileInfo file_info;
   bool file_info_ok  = os_fileinfo(fname, &file_info);

src/nvim/menu.c

@@ -1275,7 +1275,7 @@ static char *popup_mode_name(char *name, int idx)
   size_t len = STRLEN(name);
   assert(len >= 4);
 
-  char *p = (char *)vim_strnsave((char_u *)name, len + 1);
+  char *p = xstrnsave(name, len + 1);
   memmove(p + 6, p + 5, len - 4);
   p[5] = menu_mode_chars[idx];
 
@@ -1308,7 +1308,7 @@ static char *menu_text(const char *str, int *mnemonic, char **actext)
       *actext = xstrdup(p + 1);
     }
     assert(p >= str);
-    text = (char *)vim_strnsave((char_u *)str, (size_t)(p - str));
+    text = xstrnsave(str, (size_t)(p - str));
   } else {
     text = xstrdup(str);
   }
@@ -1560,7 +1560,7 @@ void ex_menutranslate(exarg_T *eap)
       from = xstrdup(from);
       from_noamp = menu_text(from, NULL, NULL);
       assert(arg >= to);
-      to = (char *)vim_strnsave((char_u *)to, (size_t)(arg - to));
+      to = xstrnsave(to, (size_t)(arg - to));
       menu_translate_tab_and_shift(from);
       menu_translate_tab_and_shift(to);
       menu_unescape_name(from);

src/nvim/strings.c

@@ -67,6 +67,13 @@ char_u *vim_strnsave(const char_u *string, size_t len)
   return (char_u *)strncpy(xmallocz(len), (char *)string, len);
 }
 
+/// A clone of vim_strnsave() that uses char* instead of char_u*
+char *xstrnsave(const char *string, size_t len)
+  FUNC_ATTR_NONNULL_RET FUNC_ATTR_MALLOC FUNC_ATTR_NONNULL_ALL
+{
+  return strncpy(xmallocz(len), string, len);  // NOLINT(runtime/printf)
+}
+
 /*
  * Same as vim_strsave(), but any characters found in esc_chars are preceded
  * by a backslash.