FileSystem: Check for DTTOIF alone, not _DIRENT_HAVE_D_TYPE

[jordan_rose]

Summary

While d_type is a non-standard extension to struct dirent, only glibc signals its presence with another macro _DIRENT_HAVE_D_TYPE. However, any platform with d_type also includes a way to convert to mode_t values using the macro DTTOIF, so we can check for that alone and still be confident that the d_type member exists.

(If this turns out to be wrong, I'll go back and set up an actual CMake check.)

I couldn't think of how to write a test for this, because I couldn't think of how to test that a stat call doesn't happen without controlling the filesystem or intercepting stat, and there's no good cross-platform way to do that that I know of.

Follow-up (almost a year later) to D51918.

rdar://problem/50592673

[sammccall]

Thanks for the detailed comment, and apologies for missing this case the first time around.

As noted previously, if this check isn't sufficient, than we'll fail to compile, so it shouldn't go unnoticed.

[jordan_rose]

Committed as rL366486.

llvm/lib/Support/Unix/Path.inc

std::error_code detail::directory_iterator_destruct(detail::DirIterState &it) {
   it.IterationHandle = 0;
   it.CurrentEntry = directory_entry();
   return std::error_code();
 }
 
 static file_type direntType(dirent* Entry) {
   // Most platforms provide the file type in the dirent: Linux/BSD/Mac.
   // The DTTOIF macro lets us reuse our status -> type conversion.
-#if defined(_DIRENT_HAVE_D_TYPE) && defined(DTTOIF)
+  // Note that while glibc provides a macro to see if this is supported,
+  // _DIRENT_HAVE_D_TYPE, it's not defined on BSD/Mac, so we test for the
+  // d_type-to-mode_t conversion macro instead.
+#if defined(DTTOIF)
   return typeForMode(DTTOIF(Entry->d_type));
 #else
   // Other platforms such as Solaris require a stat() to get the type.
   return file_type::type_unknown;
 #endif
 }
 
 std::error_code detail::directory_iterator_increment(detail::DirIterState &It) {