[clang-format] Add option to omit wrapping for empty records

Currently, clang-format does not allow empty records to be formatted
on a single line if the corresponding `BraceWrapping.After*` option
is set to true.

This results in unnecessarily wrapped code:

  struct foo
  {
      int i;
  };

  struct bar
  {
  };

This patch adds the `BraceWrapping.WrapEmptyRecord` option, which
allows `class`, `struct`, and `union` declarations with empty bodies
to be formatted as one-liners, even when `AfterRecord: true`.

As such, the following becomes possible:

  struct foo
  {
      int i;
  };

  struct bar {};

Author: itzexpoexpo

clang/include/clang/Format/Format.h

@@ -1355,6 +1355,32 @@ struct FormatStyle {
     BWACS_Always
   };
 
+  enum BraceWrapEmptyRecordStyle : int8_t {
+    /// Use default wrapping rules for records
+    /// (AfterClass,AfterStruct,AfterUnion)
+    /// \code
+    /// class foo
+    /// {
+    ///   int foo;
+    /// };
+    ///
+    /// class foo
+    /// {
+    /// };
+    /// \endcode
+    BWER_Default,
+    /// Override wrapping for empty records
+    /// \code
+    /// class foo
+    /// {
+    ///   int foo;
+    /// };
+    ///
+    /// class foo {};
+    /// \endcode
+    BWER_Never
+  };
+
   /// Precise control over the wrapping of braces.
   /// \code
   ///   # Should be declared this way:
@@ -1585,6 +1611,8 @@ struct FormatStyle {
     /// \endcode
     ///
     bool SplitEmptyNamespace;
+    /// Wrap empty record (``class``/``struct``/``union``).
+    BraceWrapEmptyRecordStyle WrapEmptyRecord;
   };
 
   /// Control of individual brace wrapping cases.

clang/lib/Format/Format.cpp

@@ -200,6 +200,7 @@ template <> struct MappingTraits<FormatStyle::BraceWrappingFlags> {
     IO.mapOptional("SplitEmptyFunction", Wrapping.SplitEmptyFunction);
     IO.mapOptional("SplitEmptyRecord", Wrapping.SplitEmptyRecord);
     IO.mapOptional("SplitEmptyNamespace", Wrapping.SplitEmptyNamespace);
+    IO.mapOptional("WrapEmptyRecord", Wrapping.WrapEmptyRecord);
   }
 };
 
@@ -232,6 +233,15 @@ struct ScalarEnumerationTraits<
   }
 };
 
+template <>
+struct ScalarEnumerationTraits<FormatStyle::BraceWrapEmptyRecordStyle> {
+  static void enumeration(IO &IO,
+                          FormatStyle::BraceWrapEmptyRecordStyle &Value) {
+    IO.enumCase(Value, "Default", FormatStyle::BWER_Default);
+    IO.enumCase(Value, "Never", FormatStyle::BWER_Never);
+  }
+};
+
 template <>
 struct ScalarEnumerationTraits<
     FormatStyle::BreakBeforeConceptDeclarationsStyle> {
@@ -1392,7 +1402,8 @@ static void expandPresetsBraceWrapping(FormatStyle &Expanded) {
                             /*IndentBraces=*/false,
                             /*SplitEmptyFunction=*/true,
                             /*SplitEmptyRecord=*/true,
-                            /*SplitEmptyNamespace=*/true};
+                            /*SplitEmptyNamespace=*/true,
+                            /*WrapEmptyRecord=*/FormatStyle::BWER_Default};
   switch (Expanded.BreakBeforeBraces) {
   case FormatStyle::BS_Linux:
     Expanded.BraceWrapping.AfterClass = true;

clang/lib/Format/TokenAnnotator.cpp

@@ -5935,10 +5935,11 @@ bool TokenAnnotator::mustBreakBefore(const AnnotatedLine &Line,
 
     // Don't attempt to interpret struct return types as structs.
     if (Right.isNot(TT_FunctionLBrace)) {
-      return (Line.startsWith(tok::kw_class) &&
-              Style.BraceWrapping.AfterClass) ||
-             (Line.startsWith(tok::kw_struct) &&
-              Style.BraceWrapping.AfterStruct);
+      return ((Line.startsWith(tok::kw_class) &&
+               Style.BraceWrapping.AfterClass) ||
+              (Line.startsWith(tok::kw_struct) &&
+               Style.BraceWrapping.AfterStruct)) &&
+             Style.BraceWrapping.WrapEmptyRecord == FormatStyle::BWER_Default;
     }
   }
 

clang/lib/Format/UnwrappedLineParser.cpp

@@ -952,20 +952,26 @@ static bool isIIFE(const UnwrappedLine &Line,
 }
 
 static bool ShouldBreakBeforeBrace(const FormatStyle &Style,
-                                   const FormatToken &InitialToken) {
+                                   const FormatToken &InitialToken,
+                                   const FormatToken &NextToken) {
   tok::TokenKind Kind = InitialToken.Tok.getKind();
   if (InitialToken.is(TT_NamespaceMacro))
     Kind = tok::kw_namespace;
 
+  bool IsEmptyBlock = NextToken.is(tok::r_brace);
+  bool WrapRecordAllowed =
+      !(IsEmptyBlock &&
+        Style.BraceWrapping.WrapEmptyRecord == FormatStyle::BWER_Never);
+
   switch (Kind) {
   case tok::kw_namespace:
     return Style.BraceWrapping.AfterNamespace;
   case tok::kw_class:
-    return Style.BraceWrapping.AfterClass;
+    return Style.BraceWrapping.AfterClass && WrapRecordAllowed;
   case tok::kw_union:
-    return Style.BraceWrapping.AfterUnion;
+    return Style.BraceWrapping.AfterUnion && WrapRecordAllowed;
   case tok::kw_struct:
-    return Style.BraceWrapping.AfterStruct;
+    return Style.BraceWrapping.AfterStruct && WrapRecordAllowed;
   case tok::kw_enum:
     return Style.BraceWrapping.AfterEnum;
   default:
@@ -3191,7 +3197,7 @@ void UnwrappedLineParser::parseNamespace() {
   if (FormatTok->is(tok::l_brace)) {
     FormatTok->setFinalizedType(TT_NamespaceLBrace);
 
-    if (ShouldBreakBeforeBrace(Style, InitialToken))
+    if (ShouldBreakBeforeBrace(Style, InitialToken, *Tokens->peekNextToken()))
       addUnwrappedLine();
 
     unsigned AddLevels =
@@ -3856,7 +3862,7 @@ bool UnwrappedLineParser::parseEnum() {
   }
 
   if (!Style.AllowShortEnumsOnASingleLine &&
-      ShouldBreakBeforeBrace(Style, InitialToken)) {
+      ShouldBreakBeforeBrace(Style, InitialToken, *Tokens->peekNextToken())) {
     addUnwrappedLine();
   }
   // Parse enum body.
@@ -4151,7 +4157,7 @@ void UnwrappedLineParser::parseRecord(bool ParseAsExpr, bool IsJavaRecord) {
     if (ParseAsExpr) {
       parseChildBlock();
     } else {
-      if (ShouldBreakBeforeBrace(Style, InitialToken))
+      if (ShouldBreakBeforeBrace(Style, InitialToken, *Tokens->peekNextToken()))
         addUnwrappedLine();
 
       unsigned AddLevels = Style.IndentAccessModifiers ? 2u : 1u;

clang/unittests/Format/FormatTest.cpp

@@ -15615,6 +15615,36 @@ TEST_F(FormatTest, NeverMergeShortRecords) {
                Style);
 }
 
+TEST_F(FormatTest, WrapEmptyRecords) {
+  FormatStyle Style = getLLVMStyle();
+
+  Style.BreakBeforeBraces = FormatStyle::BS_Custom;
+  Style.BraceWrapping.AfterStruct = true;
+  Style.BraceWrapping.AfterClass = true;
+  Style.BraceWrapping.AfterUnion = true;
+  Style.BraceWrapping.SplitEmptyRecord = false;
+
+  verifyFormat("class foo\n{\n  void bar();\n};", Style);
+  verifyFormat("class foo\n{};", Style);
+
+  verifyFormat("struct foo\n{\n  int bar;\n};", Style);
+  verifyFormat("struct foo\n{};", Style);
+
+  verifyFormat("union foo\n{\n  int bar;\n};", Style);
+  verifyFormat("union foo\n{};", Style);
+
+  Style.BraceWrapping.WrapEmptyRecord = FormatStyle::BWER_Never;
+
+  verifyFormat("class foo\n{\n  void bar();\n};", Style);
+  verifyFormat("class foo {};", Style);
+
+  verifyFormat("struct foo\n{\n  int bar;\n};", Style);
+  verifyFormat("struct foo {};", Style);
+
+  verifyFormat("union foo\n{\n  int bar;\n};", Style);
+  verifyFormat("union foo {};", Style);
+}
+
 TEST_F(FormatTest, UnderstandContextOfRecordTypeKeywords) {
   // Elaborate type variable declarations.
   verifyFormat("struct foo a = {bar};\nint n;");