add option -fallback-style=fail (fail if -style=file falls back)

[pseyfert]

Summary

I want to provide information to the clang-format user if the style
is/can be used, or if clang-format sees itself forced to fall back to
the fallback-style. I.e. in contrast to doing nothing, as with
-fallback-style=none, I want clang-format to fail. Kind of as a reminder
to the user "hey, maybe you forgot to add your .clang-format file to
your project".

I was thinking about doing this through an option
-fallback-style=fail. Do you think that is the right approach?

If yes, I gave it a try and wrote the attached one-off patch, which adds
the option -fallback-style=fail.

I *briefly* tested it on my system by building make clang-format[1] and
tried what happens when no .clang-format file exists and I run
clang-format -style=file -fallback-style=fail main.cc on a minimal
hello world c++ file.

NB: the patch does contain an update of the unit test, but I did not
manage to run it (problems with the system I was running on …).

Thanks in advance for feedback,

Paul

For reference:
This addresses what I asked on Stackoverflow
http://stackoverflow.com/questions/41758865/determine-if-clang-format-finds-clang-format-file-in-a-shell-or-cmake-script

and is a duplication of my pull request on github
https://github.com/llvm-mirror/clang/pull/21

[1]: when following steps 1-3 from
https://clang.llvm.org/get_started.html and then running `make
clang-format` in step 7 instead of make.

[JDevlieghere]

Can you make a full-context diff please?

[djasper]

lib/Format/Format.cpp

1984

Invert the order here (remove the "!"), so that the error case becomes and early exist.

1986

No "else" after "return" (http://llvm.org/docs/CodingStandards.html#don-t-use-else-after-a-return).

Also, we generally don't use braces around single-statement ifs.

[pseyfert]

Implementing comments by @djasper

[djasper]

Looks good.

docs/ClangFormat.rst

-cursor=<uint>            - The position of the cursor when invoking
                                 clang-format from an editor integration
     -dump-config              - Dump configuration options to stdout and exit.
                                 Can be used with -style option.
     -fallback-style=<string>  - The name of the predefined style used as a
                                 fallback in case clang-format is invoked with
                                 -style=file, but can not find the .clang-format
                                 file to use.
                                 Use -fallback-style=none to skip formatting.
+                                Use -fallback-style=fail to exit with an error.
     -i                        - Inplace edit <file>s, if specified.
     -length=<uint>            - Format a range of this length (in bytes).
                                 Multiple ranges can be formatted by specifying
                                 several -offset and -length pairs.
                                 When only a single -offset is specified without
                                 -length, clang-format will format up to the end
                                 of the file.
                                 Can only be used with one input file.

lib/Format/Format.cpp

bool getPredefinedStyle(StringRef Name, FormatStyle::LanguageKind Language,
   } else if (Name.equals_lower("mozilla")) {
     *Style = getMozillaStyle();
   } else if (Name.equals_lower("google")) {
     *Style = getGoogleStyle(Language);
   } else if (Name.equals_lower("webkit")) {
     *Style = getWebKitStyle();
   } else if (Name.equals_lower("gnu")) {
     *Style = getGNUStyle();
-  } else if (Name.equals_lower("none")) {
+  } else if (Name.equals_lower("none") || Name.equals_lower("fail")) {
     *Style = getNoStyle();
   } else {
     return false;
   }
 
   Style->Language = Language;
   return true;
 }
if (FoundConfigFile) {
       DEBUG(llvm::dbgs() << "Using configuration file " << ConfigFile << "\n");
       return Style;
     }
   }
   if (!UnsuitableConfigFiles.empty())
     return make_string_error("Configuration file(s) do(es) not support " +
                              getLanguageName(Style.Language) + ": " +
                              UnsuitableConfigFiles);
+
+  if (FallbackStyleName.equals_lower("fail"))
+    return make_string_error("Configuration file not found.");
   return FallbackStyle;
 }
 
 } // namespace format
 } // namespace clang

tools/clang-format/ClangFormat.cpp

Style("style",
           cl::desc(clang::format::StyleOptionHelpDescription),
           cl::init("file"), cl::cat(ClangFormatCategory));
 static cl::opt<std::string>
 FallbackStyle("fallback-style",
               cl::desc("The name of the predefined style used as a\n"
                        "fallback in case clang-format is invoked with\n"
                        "-style=file, but can not find the .clang-format\n"
                        "file to use.\n"
-                       "Use -fallback-style=none to skip formatting."),
+                       "Use -fallback-style=none to skip formatting.\n"
+                       "Use -fallback-style=fail to exit with an error."),
               cl::init("LLVM"), cl::cat(ClangFormatCategory));
 
 static cl::opt<std::string>
 AssumeFileName("assume-filename",
                cl::desc("When reading from stdin, clang-format assumes this\n"
                         "filename to look for a style config file (with\n"
                         "-style=file) and to determine the language."),
                cl::init("<stdin>"), cl::cat(ClangFormatCategory));

unittests/Format/FormatTest.cpp

TEST(FormatStyle, GetStyleOfFile) {
   ASSERT_TRUE((bool)Style2);
   ASSERT_EQ(*Style2, getMozillaStyle());
 
   // Test 2.2: no format on 'none' fallback style.
   Style2 = getStyle("file", "/b/test.cpp", "none", "", &FS);
   ASSERT_TRUE((bool)Style2);
   ASSERT_EQ(*Style2, getNoStyle());
 
-  // Test 2.3: format if config is found with no based style while fallback is
+  // Test 2.3: no format on 'fail' fallback style.
+  Style2 = getStyle("file", "/b/test.cpp", "fail", "", &FS);
+  ASSERT_FALSE((bool)Style2);
+  llvm::consumeError(Style2.takeError());
+
+  // Test 2.4: format if config is found with no based style while fallback is
   // 'none'.
   ASSERT_TRUE(FS.addFile("/b/.clang-format", 0,
                          llvm::MemoryBuffer::getMemBuffer("IndentWidth: 2")));
   Style2 = getStyle("file", "/b/test.cpp", "none", "", &FS);
   ASSERT_TRUE((bool)Style2);
   ASSERT_EQ(*Style2, getLLVMStyle());
 
-  // Test 2.4: format if yaml with no based style, while fallback is 'none'.
+  // Test 2.5: format if yaml with no based style, while fallback is 'none'.
   Style2 = getStyle("{}", "a.h", "none", "", &FS);
   ASSERT_TRUE((bool)Style2);
   ASSERT_EQ(*Style2, getLLVMStyle());
 
   // Test 3: format file in parent directory.
   ASSERT_TRUE(
       FS.addFile("/c/.clang-format", 0,
                  llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: Google")));