[libc++] Add missing __format__ attributes

[ldionne]

Summary

-Wformat-nonliteral was turned on in https://reviews.llvm.org/D112927,
however we forgot to apply some format attributes in Linux specific
code paths, which led to warnings when building on Linux. This patch
addresses that oversight.

[ldionne]

Thanks for looking into this! I missed those because I only built the library on Darwin, and CI passed since those are only warnings 🤦🏼‍♂️.

libcxx/include/__bsd_locale_fallbacks.h

112

I believe we should instead move those to _LIBCPP_FORMAT_PRINTF. The usage is documented here: https://clang.llvm.org/docs/AttributeReference.html#format.

[uabelho]

libcxx/include/__bsd_locale_fallbacks.h

112

Would you feel like doing it if it should be fixed in some other way that just disabling the warning?

I really don't know this stuff, I just saw one of our downstream bots signalling there were a bunch of new warnings and saw you disabled warnings this way in your original patch so did the same thing here.

[ldionne]

libcxx/include/__bsd_locale_fallbacks.h

112

Yup, no worries. Thanks for bringing that to our attention!

[ldionne]

Update with format attributes instead of pragmas to turn off the warning.

[Mordante]

LGTM modulo one remark.

libcxx/include/__config

1355

I like the comment, but let's expand it a bit.

This change looks very familiar. I'm quite sure something similar has been up for review before, but seems it never landed.

[uabelho]

libcxx/include/__bsd_locale_fallbacks.h

112

Great! Thank you!

[ldionne]

Address review comments and avoid adding _LIBCPP_HIDE_FROM_ABI, which breaks the GCC build.
We can address that separately.

[ldionne]

Will ship once CI passes.

libcxx/include/__config

1355

Yeah, now that you mention it this does seem to ring a bell for me too. But in any case, it hasn't landed yet so I guess we'll eventually stumble upon the change while clearing the review queue.

libcxx/include/__bsd_locale_fallbacks.h

 size_t __libcpp_mbsrtowcs_l(wchar_t *__dest, const char **__src, size_t __len,
                      mbstate_t *__ps, locale_t __l)
 {
     __libcpp_locale_guard __current(__l);
     return mbsrtowcs(__dest, __src, __len, __ps);
 }
 #endif
 
-inline
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 4, 5)
 int __libcpp_snprintf_l(char *__s, size_t __n, locale_t __l, const char *__format, ...) {
     va_list __va;
     va_start(__va, __format);
     __libcpp_locale_guard __current(__l);
     int __res = vsnprintf(__s, __n, __format, __va);
     va_end(__va);
     return __res;
 }
 
-inline
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 4)
 int __libcpp_asprintf_l(char **__s, locale_t __l, const char *__format, ...) {
     va_list __va;
     va_start(__va, __format);
     __libcpp_locale_guard __current(__l);
     int __res = vasprintf(__s, __format, __va);
     va_end(__va);
     return __res;
 }
 
-inline
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_ATTRIBUTE_FORMAT(__scanf__, 3, 4)
 int __libcpp_sscanf_l(const char *__s, locale_t __l, const char *__format, ...) {
     va_list __va;
     va_start(__va, __format);
     __libcpp_locale_guard __current(__l);
     int __res = vsscanf(__s, __format, __va);
     va_end(__va);
     return __res;
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_BSD_LOCALE_FALLBACKS_DEFAULTS_H

libcxx/include/__config

 #   else
 #     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(100)))
 #   endif
 #else
 # define _LIBCPP_INIT_PRIORITY_MAX
 #endif
 
 #if defined(__GNUC__) || defined(__clang__)
-#define _LIBCPP_FORMAT_PRINTF(a, b)                                            \
-  __attribute__((__format__(__printf__, a, b)))
+  // The attribute uses 1-based indices.
+# define _LIBCPP_ATTRIBUTE_FORMAT(archetype, format_string_index, first_format_arg_index) \
+    __attribute__((__format__(archetype, format_string_index, first_format_arg_index)))
 #else
-#define _LIBCPP_FORMAT_PRINTF(a, b)
+# define _LIBCPP_ATTRIBUTE_FORMAT(archetype, format_string_index, first_format_arg_index) /* nothing */
 #endif
 
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

libcxx/src/filesystem/filesystem_common.h

 
 #if defined(_LIBCPP_WIN32API)
 // Non anonymous, to allow access from two translation units.
 errc __win_err_to_errc(int err);
 #endif
 
 namespace {
 
-static _LIBCPP_FORMAT_PRINTF(1, 0) string
+static _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 1, 0) string
 format_string_impl(const char* msg, va_list ap) {
   array<char, 256> buf;
 
   va_list apcopy;
   va_copy(apcopy, ap);
   int ret = ::vsnprintf(buf.data(), buf.size(), msg, apcopy);
   va_end(apcopy);
 
   string result;
   if (static_cast<size_t>(ret) < buf.size()) {
     result.assign(buf.data(), static_cast<size_t>(ret));
   } else {
     // we did not provide a long enough buffer on our first attempt. The
     // return value is the number of bytes (excluding the null byte) that are
     // needed for formatting.
     size_t size_with_null = static_cast<size_t>(ret) + 1;
     result.__resize_default_init(size_with_null - 1);
     ret = ::vsnprintf(&result[0], size_with_null, msg, ap);
     _LIBCPP_ASSERT(static_cast<size_t>(ret) == (size_with_null - 1), "TODO");
   }
   return result;
 }
 
-static _LIBCPP_FORMAT_PRINTF(1, 2) string
+static _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 1, 2) string
 format_string(const char* msg, ...) {
   string ret;
   va_list ap;
   va_start(ap, msg);
 #ifndef _LIBCPP_NO_EXCEPTIONS
   try {
 #endif // _LIBCPP_NO_EXCEPTIONS
     ret = format_string_impl(msg, ap);
T report(const error_code& ec) const {
     case 1:
       __throw_filesystem_error(what, *p1_, ec);
     case 2:
       __throw_filesystem_error(what, *p1_, *p2_, ec);
     }
     _LIBCPP_UNREACHABLE();
   }
 
-  _LIBCPP_FORMAT_PRINTF(3, 0)
+  _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 0)
   void report_impl(const error_code& ec, const char* msg, va_list ap) const {
     if (ec_) {
       *ec_ = ec;
       return;
     }
     string what =
         string("in ") + func_name_ + ": " + format_string_impl(msg, ap);
     switch (bool(p1_) + bool(p2_)) {
     case 0:
       __throw_filesystem_error(what, ec);
     case 1:
       __throw_filesystem_error(what, *p1_, ec);
     case 2:
       __throw_filesystem_error(what, *p1_, *p2_, ec);
     }
     _LIBCPP_UNREACHABLE();
   }
 
-  _LIBCPP_FORMAT_PRINTF(3, 4)
+  _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 4)
   T report(const error_code& ec, const char* msg, ...) const {
     va_list ap;
     va_start(ap, msg);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try {
 #endif // _LIBCPP_NO_EXCEPTIONS
       report_impl(ec, msg, ap);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     } catch (...) {
       va_end(ap);
       throw;
     }
 #endif // _LIBCPP_NO_EXCEPTIONS
     va_end(ap);
     return error_value<T>();
   }
 
   T report(errc const& err) const {
     return report(make_error_code(err));
   }
 
-  _LIBCPP_FORMAT_PRINTF(3, 4)
+  _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 4)
   T report(errc const& err, const char* msg, ...) const {
     va_list ap;
     va_start(ap, msg);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try {
 #endif // _LIBCPP_NO_EXCEPTIONS
       report_impl(make_error_code(err), msg, ap);
 #ifndef _LIBCPP_NO_EXCEPTIONS