[clang-format] New flag - BraceWrapping.AfterExternBlock

Summary:
Bug: https://bugs.llvm.org/show_bug.cgi?id=34016 - **"extern C part"**

**Problem:**

Due to the lack of "brace wrapping extern" flag, clang format does parse the block after **extern** keyword moving the opening bracket to the header line always!

**Patch description:**

A new style added, new configuration flag - **BraceWrapping.AfterExternBlock** that allows us to decide whether we want a break before brace or not.

Reviewers: djasper, krasimir

Reviewed By: krasimir

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D37845

Contributed by @PrimeE!

llvm-svn: 313354

clang/docs/ClangFormatStyleOptions.rst

@@ -661,6 +661,21 @@ the configuration (without a prefix: ``Auto``).
         int x;
       }
 
+  * ``bool AfterExternBlock`` Wrap extern blocks.
+
+    .. code-block:: c++
+
+      true:
+      extern "C"
+      {
+        int foo();
+      }
+
+      false:
+      extern "C" {
+      int foo();
+      }
+
   * ``bool BeforeCatch`` Wrap before ``catch``.
 
     .. code-block:: c++

clang/include/clang/Format/Format.h

@@ -681,6 +681,20 @@ struct FormatStyle {
     ///   }
     /// \endcode
     bool AfterUnion;
+    /// \brief Wrap extern blocks.
+    /// \code
+    ///   true:
+    ///   extern "C"
+    ///   {
+    ///     int foo();
+    ///   }
+    ///
+    ///   false:
+    ///   extern "C" {
+    ///   int foo();
+    ///   }
+    /// \endcode
+    bool AfterExternBlock;
     /// \brief Wrap before ``catch``.
     /// \code
     ///   true:

clang/lib/Format/Format.cpp

@@ -420,6 +420,7 @@ template <> struct MappingTraits<FormatStyle::BraceWrappingFlags> {
     IO.mapOptional("AfterObjCDeclaration", Wrapping.AfterObjCDeclaration);
     IO.mapOptional("AfterStruct", Wrapping.AfterStruct);
     IO.mapOptional("AfterUnion", Wrapping.AfterUnion);
+    IO.mapOptional("AfterExternBlock", Wrapping.AfterExternBlock);
     IO.mapOptional("BeforeCatch", Wrapping.BeforeCatch);
     IO.mapOptional("BeforeElse", Wrapping.BeforeElse);
     IO.mapOptional("IndentBraces", Wrapping.IndentBraces);
@@ -500,9 +501,9 @@ static FormatStyle expandPresets(const FormatStyle &Style) {
   if (Style.BreakBeforeBraces == FormatStyle::BS_Custom)
     return Style;
   FormatStyle Expanded = Style;
-  Expanded.BraceWrapping = {false, false, false, false, false, false,
-                            false, false, false, false, false, true,
-                            true,  true};
+  Expanded.BraceWrapping = {false, false, false, false, false, false, 
+                            false, false, false, false, false, false, 
+                            true,  true,  true};
   switch (Style.BreakBeforeBraces) {
   case FormatStyle::BS_Linux:
     Expanded.BraceWrapping.AfterClass = true;
@@ -515,6 +516,7 @@ static FormatStyle expandPresets(const FormatStyle &Style) {
     Expanded.BraceWrapping.AfterFunction = true;
     Expanded.BraceWrapping.AfterStruct = true;
     Expanded.BraceWrapping.AfterUnion = true;
+    Expanded.BraceWrapping.AfterExternBlock = true;
     Expanded.BraceWrapping.SplitEmptyFunction = true;
     Expanded.BraceWrapping.SplitEmptyRecord = false;
     break;
@@ -531,13 +533,14 @@ static FormatStyle expandPresets(const FormatStyle &Style) {
     Expanded.BraceWrapping.AfterNamespace = true;
     Expanded.BraceWrapping.AfterObjCDeclaration = true;
     Expanded.BraceWrapping.AfterStruct = true;
+    Expanded.BraceWrapping.AfterExternBlock = true;
     Expanded.BraceWrapping.BeforeCatch = true;
     Expanded.BraceWrapping.BeforeElse = true;
     break;
   case FormatStyle::BS_GNU:
-    Expanded.BraceWrapping = {true, true, true, true, true, true,
-                              true, true, true, true, true, true,
-                              true, true};
+    Expanded.BraceWrapping = {true, true, true, true, true, true, 
+                              true, true, true, true, true, true, 
+                              true, true, true};
     break;
   case FormatStyle::BS_WebKit:
     Expanded.BraceWrapping.AfterFunction = true;
@@ -573,9 +576,9 @@ FormatStyle getLLVMStyle() {
   LLVMStyle.BreakBeforeBinaryOperators = FormatStyle::BOS_None;
   LLVMStyle.BreakBeforeTernaryOperators = true;
   LLVMStyle.BreakBeforeBraces = FormatStyle::BS_Attach;
-  LLVMStyle.BraceWrapping = {false, false, false, false, false, false,
-                             false, false, false, false, false, true,
-                             true,  true};
+  LLVMStyle.BraceWrapping = {false, false, false, false, false, false, 
+                             false, false, false, false, false, false, 
+                             true,  true,  true};
   LLVMStyle.BreakAfterJavaFieldAnnotations = false;
   LLVMStyle.BreakConstructorInitializers = FormatStyle::BCIS_BeforeColon;
   LLVMStyle.BreakBeforeInheritanceComma = false;

clang/lib/Format/UnwrappedLineFormatter.cpp

@@ -233,9 +233,10 @@ class LineJoiner {
       if (Tok && Tok->is(tok::kw_typedef))
         Tok = Tok->getNextNonComment();
       if (Tok && Tok->isOneOf(tok::kw_class, tok::kw_struct, tok::kw_union,
-                              Keywords.kw_interface))
+                              tok::kw_extern, Keywords.kw_interface))
         return !Style.BraceWrapping.SplitEmptyRecord && EmptyBlock
-            ? tryMergeSimpleBlock(I, E, Limit) : 0;
+                   ? tryMergeSimpleBlock(I, E, Limit)
+                   : 0;
     }
 
     // FIXME: TheLine->Level != 0 might or might not be the right check to do.

clang/lib/Format/UnwrappedLineParser.cpp

@@ -1039,7 +1039,12 @@ void UnwrappedLineParser::parseStructuralElement() {
     if (FormatTok->Tok.is(tok::string_literal)) {
       nextToken();
       if (FormatTok->Tok.is(tok::l_brace)) {
-        parseBlock(/*MustBeDeclaration=*/true, /*AddLevel=*/false);
+        if (Style.BraceWrapping.AfterExternBlock) {
+          addUnwrappedLine();
+          parseBlock(/*MustBeDeclaration=*/true);
+        } else {
+          parseBlock(/*MustBeDeclaration=*/true, /*AddLevel=*/false);
+        }
         addUnwrappedLine();
         return;
       }

clang/unittests/Format/FormatTest.cpp

@@ -1704,7 +1704,42 @@ TEST_F(FormatTest, FormatsCompactNamespaces) {
                    Style));
 }
 
-TEST_F(FormatTest, FormatsExternC) { verifyFormat("extern \"C\" {\nint a;"); }
+TEST_F(FormatTest, FormatsExternC) {
+  verifyFormat("extern \"C\" {\nint a;");
+  verifyFormat("extern \"C\" {}");
+  verifyFormat("extern \"C\" {\n"
+               "int foo();\n"
+               "}");
+  verifyFormat("extern \"C\" int foo() {}");
+  verifyFormat("extern \"C\" int foo();");
+  verifyFormat("extern \"C\" int foo() {\n"
+               "  int i = 42;\n"
+               "  return i;\n"
+               "}");
+
+  FormatStyle Style = getLLVMStyle();
+  Style.BreakBeforeBraces = FormatStyle::BS_Custom;
+  Style.BraceWrapping.AfterFunction = true;
+  verifyFormat("extern \"C\" int foo() {}", Style);
+  verifyFormat("extern \"C\" int foo();", Style);
+  verifyFormat("extern \"C\" int foo()\n"
+               "{\n"
+               "  int i = 42;\n"
+               "  return i;\n"
+               "}",
+               Style);
+
+  Style.BraceWrapping.AfterExternBlock = true;
+  Style.BraceWrapping.SplitEmptyRecord = false;
+  verifyFormat("extern \"C\"\n"
+               "{}",
+               Style);
+  verifyFormat("extern \"C\"\n"
+               "{\n"
+               "  int foo();\n"
+               "}",
+               Style);
+}
 
 TEST_F(FormatTest, FormatsInlineASM) {
   verifyFormat("asm(\"xyz\" : \"=a\"(a), \"=d\"(b) : \"a\"(data));");
@@ -9979,6 +10014,7 @@ TEST_F(FormatTest, ParsesConfigurationBools) {
   CHECK_PARSE_NESTED_BOOL(BraceWrapping, AfterObjCDeclaration);
   CHECK_PARSE_NESTED_BOOL(BraceWrapping, AfterStruct);
   CHECK_PARSE_NESTED_BOOL(BraceWrapping, AfterUnion);
+  CHECK_PARSE_NESTED_BOOL(BraceWrapping, AfterExternBlock);
   CHECK_PARSE_NESTED_BOOL(BraceWrapping, BeforeCatch);
   CHECK_PARSE_NESTED_BOOL(BraceWrapping, BeforeElse);
   CHECK_PARSE_NESTED_BOOL(BraceWrapping, IndentBraces);