Implement filesystem_error::what() and improve reporting.

This patch implements the `what()` for filesystem errors. The message
includes the 'what_arg', any paths that were specified, and the
error code message.

Additionally this patch refactors how errors are created, making it easier
to report them correctly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337664 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/format_string.hpp

@@ -0,0 +1,68 @@
+#ifndef TEST_SUPPORT_FORMAT_STRING_HPP
+#define TEST_SUPPORT_FORMAT_STRING_HPP
+
+#include <cstdio>
+#include <string>
+#include <memory>
+#include <array>
+
+namespace format_string_detail {
+inline std::string format_string_imp(const char* msg, ...) {
+  // we might need a second shot at this, so pre-emptivly make a copy
+  struct GuardVAList {
+    va_list& target;
+    bool active = true;
+    void clear() {
+      if (active)
+        va_end(target);
+      active = false;
+    }
+    ~GuardVAList() {
+      if (active)
+        va_end(target);
+    }
+  };
+  va_list args;
+  va_start(args, msg);
+  GuardVAList args_guard = {args};
+
+  va_list args_cp;
+  va_copy(args_cp, args);
+  GuardVAList args_copy_guard = {args_cp};
+
+  std::array<char, 256> local_buff;
+  std::size_t size = local_buff.size();
+  auto ret = ::vsnprintf(local_buff.data(), size, msg, args_cp);
+
+  args_copy_guard.clear();
+
+  // handle empty expansion
+  if (ret == 0)
+    return std::string{};
+  if (static_cast<std::size_t>(ret) < size)
+    return std::string(local_buff.data());
+
+  // we did not provide a long enough buffer on our first attempt.
+  // add 1 to size to account for null-byte in size cast to prevent overflow
+  size = static_cast<std::size_t>(ret) + 1;
+  auto buff_ptr = std::unique_ptr<char[]>(new char[size]);
+  ret = ::vsnprintf(buff_ptr.get(), size, msg, args);
+  return std::string(buff_ptr.get());
+}
+
+const char* unwrap(std::string& s) { return s.c_str(); }
+template <class Arg>
+Arg const& unwrap(Arg& a) {
+  static_assert(!std::is_class<Arg>::value, "cannot pass class here");
+  return a;
+}
+
+} // namespace format_string_detail
+
+template <class... Args>
+std::string format_string(const char* fmt, Args const&... args) {
+  return format_string_detail::format_string_imp(
+      fmt, format_string_detail::unwrap(const_cast<Args&>(args))...);
+}
+
+#endif // TEST_SUPPORT_FORMAT_STRING_HPP