clang-format: chromium style: Disable across-block include reordering.

[thakis]

How's the work going to undo r357695 btw?

[sammccall]

In D66650#1642747, @thakis wrote:

How's the work going to undo r357695 btw?

I'm not sure anyone's chasing it at the moment, the focus has been on trying to improve the heuristic for cases that apply to all languages.
sThe heuristic was good enough to stick for C++, but uncovered lots of ambiguity in our style guide and working out what the stylists want has been slow. @gribozavr can give more details if you want them.

cfe/trunk/lib/Format/Format.cpp

GoogleStyle.IncludeStyle.IncludeBlocks =
         tooling::IncludeStyle::IBS_Preserve;
   }
 
   return GoogleStyle;
 }
 
 FormatStyle getChromiumStyle(FormatStyle::LanguageKind Language) {
   FormatStyle ChromiumStyle = getGoogleStyle(Language);
+
+  // Disable include reordering across blocks in Chromium code.
+  // - clang-format tries to detect that foo.h is the "main" header for
+  //   foo.cc and foo_unittest.cc via IncludeIsMainRegex. However, Chromium
+  //   uses many other suffices (_win.cc, _mac.mm, _posix.cc, _browsertest.cc,
+  //   _private.cc, _impl.cc etc) in different permutations
+  //   (_win_browsertest.cc) so disable this until IncludeIsMainRegex has a
+  //   better default for Chromium code.
+  // - The default for .cc and .mm files is different (r357695) for Google style
+  //   for the same reason. The plan is to unify this again once the main
+  //   header detection works for Google's ObjC code, but this hasn't happened
+  //   yet. Since Chromium has some ObjC code, switching Chromium is blocked
+  //   on that.
+  // - Finally, "If include reordering is harmful, put things in different
+  //   blocks to prevent it" has been a recommendation for a long time that
+  //   people are used to. We'll need a dev education push to change this to
+  //   "If include reordering is harmful, put things in a different block and
+  //   _prepend that with a comment_ to prevent it" before changing behavior.
+  ChromiumStyle.IncludeStyle.IncludeBlocks =
+      tooling::IncludeStyle::IBS_Preserve;
+
   if (Language == FormatStyle::LK_Java) {
     ChromiumStyle.AllowShortIfStatementsOnASingleLine =
         FormatStyle::SIS_WithoutElse;
     ChromiumStyle.BreakAfterJavaFieldAnnotations = true;
     ChromiumStyle.ContinuationIndentWidth = 8;
     ChromiumStyle.IndentWidth = 4;
     // See styleguide for import groups:
     // https://chromium.googlesource.com/chromium/src/+/master/styleguide/java/java.md#Import-Order