[clang-format] Add experimental option to remove LLVM braces

See the style examples at:
https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

Differential Revision: https://reviews.llvm.org/D116316

clang/docs/ClangFormatStyleOptions.rst

@@ -3402,6 +3402,62 @@ the configuration (without a prefix: ``Auto``).
      /* second veryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryLongComment with plenty of
       * information */
 
+**RemoveBracesLLVM** (``Boolean``) :versionbadge:`clang-format 14`
+  Remove optional braces of control statements (``if``, ``else``, ``for``,
+  and ``while``) in C++ according to the LLVM coding style.
+
+  .. warning:: 
+
+   This option will be renamed and expanded to support other styles.
+
+  .. warning:: 
+
+   Setting this option to `true` could lead to incorrect code formatting due
+   to clang-format's lack of complete semantic information. As such, extra
+   care should be taken to review code changes made by this option.
+
+  .. code-block:: c++
+
+    false:                                     true:
+
+    if (isa<FunctionDecl>(D)) {        vs.     if (isa<FunctionDecl>(D))
+      handleFunctionDecl(D);                     handleFunctionDecl(D);
+    } else if (isa<VarDecl>(D)) {              else if (isa<VarDecl>(D))
+      handleVarDecl(D);                          handleVarDecl(D);
+    }
+
+    if (isa<VarDecl>(D)) {             vs.     if (isa<VarDecl>(D)) {
+      for (auto *A : D.attrs()) {                for (auto *A : D.attrs())
+        if (shouldProcessAttr(A)) {                if (shouldProcessAttr(A))
+          handleAttr(A);                             handleAttr(A);
+        }                                      }
+      }
+    }
+
+    if (isa<FunctionDecl>(D)) {        vs.     if (isa<FunctionDecl>(D))
+      for (auto *A : D.attrs()) {                for (auto *A : D.attrs())
+        handleAttr(A);                             handleAttr(A);
+      }
+    }
+
+    if (auto *D = (T)(D)) {            vs.     if (auto *D = (T)(D)) {
+      if (shouldProcess(D)) {                    if (shouldProcess(D))
+        handleVarDecl(D);                          handleVarDecl(D);
+      } else {                                   else
+        markAsIgnored(D);                          markAsIgnored(D);
+      }                                        }
+    }
+
+    if (a) {                           vs.     if (a)
+      b();                                       b();
+    } else {                                   else if (c)
+      if (c) {                                   d();
+        d();                                   else
+      } else {                                   e();
+        e();
+      }
+    }
+
 **SeparateDefinitionBlocks** (``SeparateDefinitionStyle``) :versionbadge:`clang-format 14`
   Specifies the use of empty lines to separate definition blocks, including
   classes, structs, enums, and functions.

clang/docs/ReleaseNotes.rst

@@ -310,6 +310,9 @@ clang-format
   `const` `volatile` `static` `inline` `constexpr` `restrict`
   to be controlled relative to the `type`.
 
+- Option ``RemoveBracesLLVM`` has been added to remove optional braces of
+  control statements for the LLVM style.
+
 - Option ``SeparateDefinitionBlocks`` has been added to insert or remove empty
   lines between definition blocks including functions, classes, structs, enums,
   and namespaces.

clang/include/clang/Format/Format.h

@@ -3049,6 +3049,60 @@ struct FormatStyle {
   bool ReflowComments;
   // clang-format on
 
+  /// Remove optional braces of control statements (``if``, ``else``, ``for``,
+  /// and ``while``) in C++ according to the LLVM coding style.
+  /// \warning
+  ///  This option will be renamed and expanded to support other styles.
+  /// \endwarning
+  /// \warning
+  ///  Setting this option to `true` could lead to incorrect code formatting due
+  ///  to clang-format's lack of complete semantic information. As such, extra
+  ///  care should be taken to review code changes made by this option.
+  /// \endwarning
+  /// \code
+  ///   false:                                     true:
+  ///
+  ///   if (isa<FunctionDecl>(D)) {        vs.     if (isa<FunctionDecl>(D))
+  ///     handleFunctionDecl(D);                     handleFunctionDecl(D);
+  ///   } else if (isa<VarDecl>(D)) {              else if (isa<VarDecl>(D))
+  ///     handleVarDecl(D);                          handleVarDecl(D);
+  ///   }
+  ///
+  ///   if (isa<VarDecl>(D)) {             vs.     if (isa<VarDecl>(D)) {
+  ///     for (auto *A : D.attrs()) {                for (auto *A : D.attrs())
+  ///       if (shouldProcessAttr(A)) {                if (shouldProcessAttr(A))
+  ///         handleAttr(A);                             handleAttr(A);
+  ///       }                                      }
+  ///     }
+  ///   }
+  ///
+  ///   if (isa<FunctionDecl>(D)) {        vs.     if (isa<FunctionDecl>(D))
+  ///     for (auto *A : D.attrs()) {                for (auto *A : D.attrs())
+  ///       handleAttr(A);                             handleAttr(A);
+  ///     }
+  ///   }
+  ///
+  ///   if (auto *D = (T)(D)) {            vs.     if (auto *D = (T)(D)) {
+  ///     if (shouldProcess(D)) {                    if (shouldProcess(D))
+  ///       handleVarDecl(D);                          handleVarDecl(D);
+  ///     } else {                                   else
+  ///       markAsIgnored(D);                          markAsIgnored(D);
+  ///     }                                        }
+  ///   }
+  ///
+  ///   if (a) {                           vs.     if (a)
+  ///     b();                                       b();
+  ///   } else {                                   else if (c)
+  ///     if (c) {                                   d();
+  ///       d();                                   else
+  ///     } else {                                   e();
+  ///       e();
+  ///     }
+  ///   }
+  /// \endcode
+  /// \version 14
+  bool RemoveBracesLLVM;
+
   /// \brief The style if definition blocks should be separated.
   enum SeparateDefinitionStyle {
     /// Leave definition blocks as they are.
@@ -3858,6 +3912,7 @@ struct FormatStyle {
            QualifierOrder == R.QualifierOrder &&
            RawStringFormats == R.RawStringFormats &&
            ReferenceAlignment == R.ReferenceAlignment &&
+           RemoveBracesLLVM == R.RemoveBracesLLVM &&
            SeparateDefinitionBlocks == R.SeparateDefinitionBlocks &&
            ShortNamespaceLines == R.ShortNamespaceLines &&
            SortIncludes == R.SortIncludes &&

clang/lib/Format/Format.cpp

@@ -781,6 +781,7 @@ template <> struct MappingTraits<FormatStyle> {
     IO.mapOptional("RawStringFormats", Style.RawStringFormats);
     IO.mapOptional("ReferenceAlignment", Style.ReferenceAlignment);
     IO.mapOptional("ReflowComments", Style.ReflowComments);
+    IO.mapOptional("RemoveBracesLLVM", Style.RemoveBracesLLVM);
     IO.mapOptional("SeparateDefinitionBlocks", Style.SeparateDefinitionBlocks);
     IO.mapOptional("ShortNamespaceLines", Style.ShortNamespaceLines);
     IO.mapOptional("SortIncludes", Style.SortIncludes);
@@ -1214,6 +1215,7 @@ FormatStyle getLLVMStyle(FormatStyle::LanguageKind Language) {
   LLVMStyle.UseCRLF = false;
   LLVMStyle.UseTab = FormatStyle::UT_Never;
   LLVMStyle.ReflowComments = true;
+  LLVMStyle.RemoveBracesLLVM = false;
   LLVMStyle.SpacesInParentheses = false;
   LLVMStyle.SpacesInSquareBrackets = false;
   LLVMStyle.SpaceInEmptyBlock = false;
@@ -1748,6 +1750,45 @@ FormatStyle::GetLanguageStyle(FormatStyle::LanguageKind Language) const {
 
 namespace {
 
+class BracesRemover : public TokenAnalyzer {
+public:
+  BracesRemover(const Environment &Env, const FormatStyle &Style)
+      : TokenAnalyzer(Env, Style) {}
+
+  std::pair<tooling::Replacements, unsigned>
+  analyze(TokenAnnotator &Annotator,
+          SmallVectorImpl<AnnotatedLine *> &AnnotatedLines,
+          FormatTokenLexer &Tokens) override {
+    AffectedRangeMgr.computeAffectedLines(AnnotatedLines);
+    tooling::Replacements Result;
+    removeBraces(AnnotatedLines, Result);
+    return {Result, 0};
+  }
+
+private:
+  // Remove optional braces.
+  void removeBraces(SmallVectorImpl<AnnotatedLine *> &Lines,
+                    tooling::Replacements &Result) {
+    const auto &SourceMgr = Env.getSourceManager();
+    for (AnnotatedLine *Line : Lines) {
+      removeBraces(Line->Children, Result);
+      if (!Line->Affected)
+        continue;
+      for (FormatToken *Token = Line->First; Token; Token = Token->Next) {
+        if (!Token->Optional)
+          continue;
+        assert(Token->isOneOf(tok::l_brace, tok::r_brace));
+        const auto Start = Token == Line->Last
+                               ? Token->WhitespaceRange.getBegin()
+                               : Token->Tok.getLocation();
+        const auto Range =
+            CharSourceRange::getCharRange(Start, Token->Tok.getEndLoc());
+        cantFail(Result.add(tooling::Replacement(SourceMgr, Range, "")));
+      }
+    }
+  }
+};
+
 class JavaScriptRequoter : public TokenAnalyzer {
 public:
   JavaScriptRequoter(const Environment &Env, const FormatStyle &Style)
@@ -3049,6 +3090,11 @@ reformat(const FormatStyle &Style, StringRef Code,
     });
   }
 
+  if (Style.isCpp() && Style.RemoveBracesLLVM)
+    Passes.emplace_back([&](const Environment &Env) {
+      return BracesRemover(Env, Expanded).process();
+    });
+
   if (Style.Language == FormatStyle::LK_Cpp) {
     if (Style.FixNamespaceComments)
       Passes.emplace_back([&](const Environment &Env) {

clang/lib/Format/FormatToken.h

@@ -442,6 +442,9 @@ public:
   /// This starts an array initializer.
   bool IsArrayInitializer = false;
 
+  /// Is optional and can be removed.
+  bool Optional = false;
+
   /// If this token starts a block, this contains all the unwrapped lines
   /// in it.
   SmallVector<AnnotatedLine *, 1> Children;

clang/lib/Format/TokenAnnotator.cpp

@@ -780,6 +780,7 @@ private:
       unsigned CommaCount = 0;
       while (CurrentToken) {
         if (CurrentToken->is(tok::r_brace)) {
+          assert(Left->Optional == CurrentToken->Optional);
           Left->MatchingParen = CurrentToken;
           CurrentToken->MatchingParen = Left;
           if (Style.AlignArrayOfStructures != FormatStyle::AIAS_None) {

clang/lib/Format/UnwrappedLineParser.cpp

@@ -14,6 +14,7 @@
 
 #include "UnwrappedLineParser.h"
 #include "FormatToken.h"
+#include "TokenAnnotator.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -315,6 +316,7 @@ void UnwrappedLineParser::reset() {
   PreprocessorDirectives.clear();
   CurrentLines = &Lines;
   DeclarationScopeStack.clear();
+  NestedTooDeep.clear();
   PPStack.clear();
   Line->FirstStartColumn = FirstStartColumn;
 }
@@ -428,7 +430,47 @@ void UnwrappedLineParser::parseCSharpAttribute() {
   } while (!eof());
 }
 
-void UnwrappedLineParser::parseLevel(bool HasOpeningBrace) {
+bool UnwrappedLineParser::precededByCommentOrPPDirective() const {
+  if (!Lines.empty() && Lines.back().InPPDirective)
+    return true;
+
+  const FormatToken *Previous = Tokens->getPreviousToken();
+  return Previous && Previous->is(tok::comment) &&
+         (Previous->IsMultiline || Previous->NewlinesBefore > 0);
+}
+
+bool UnwrappedLineParser::mightFitOnOneLine() const {
+  const auto ColumnLimit = Style.ColumnLimit;
+  if (ColumnLimit == 0)
+    return true;
+
+  if (Lines.empty())
+    return true;
+
+  const auto &PreviousLine = Lines.back();
+  const auto &Tokens = PreviousLine.Tokens;
+  assert(!Tokens.empty());
+  const auto *LastToken = Tokens.back().Tok;
+  assert(LastToken);
+  if (!LastToken->isOneOf(tok::semi, tok::comment))
+    return true;
+
+  AnnotatedLine Line(PreviousLine);
+  assert(Line.Last == LastToken);
+
+  TokenAnnotator Annotator(Style, Keywords);
+  Annotator.annotate(Line);
+  Annotator.calculateFormattingInformation(Line);
+
+  return Line.Level * Style.IndentWidth + LastToken->TotalLength <= ColumnLimit;
+}
+
+// Returns true if a simple block, or false otherwise. (A simple block has a
+// single statement that fits on a single line.)
+bool UnwrappedLineParser::parseLevel(bool HasOpeningBrace, IfStmtKind *IfKind) {
+  const bool IsPrecededByCommentOrPPDirective =
+      !Style.RemoveBracesLLVM || precededByCommentOrPPDirective();
+  unsigned StatementCount = 0;
   bool SwitchLabelEncountered = false;
   do {
     tok::TokenKind kind = FormatTok->Tok.getKind();
@@ -449,11 +491,24 @@ void UnwrappedLineParser::parseLevel(bool HasOpeningBrace) {
       if (!FormatTok->is(TT_MacroBlockBegin) && tryToParseBracedList())
         continue;
       parseBlock();
+      ++StatementCount;
+      assert(StatementCount > 0 && "StatementCount overflow!");
       addUnwrappedLine();
       break;
     case tok::r_brace:
-      if (HasOpeningBrace)
-        return;
+      if (HasOpeningBrace) {
+        if (!Style.RemoveBracesLLVM)
+          return false;
+        if (FormatTok->isNot(tok::r_brace) || StatementCount != 1 ||
+            IsPrecededByCommentOrPPDirective ||
+            precededByCommentOrPPDirective()) {
+          return false;
+        }
+        const FormatToken *Next = Tokens->peekNextToken();
+        if (Next->is(tok::comment) && Next->NewlinesBefore == 0)
+          return false;
+        return mightFitOnOneLine();
+      }
       nextToken();
       addUnwrappedLine();
       break;
@@ -493,10 +548,13 @@ void UnwrappedLineParser::parseLevel(bool HasOpeningBrace) {
       }
       LLVM_FALLTHROUGH;
     default:
-      parseStructuralElement(!HasOpeningBrace);
+      parseStructuralElement(IfKind, !HasOpeningBrace);
+      ++StatementCount;
+      assert(StatementCount > 0 && "StatementCount overflow!");
       break;
     }
   } while (!eof());
+  return false;
 }
 
 void UnwrappedLineParser::calculateBraceTypes(bool ExpectClassBody) {
@@ -652,11 +710,13 @@ size_t UnwrappedLineParser::computePPHash() const {
   return h;
 }
 
-void UnwrappedLineParser::parseBlock(bool MustBeDeclaration, unsigned AddLevels,
-                                     bool MunchSemi,
-                                     bool UnindentWhitesmithsBraces) {
+UnwrappedLineParser::IfStmtKind
+UnwrappedLineParser::parseBlock(bool MustBeDeclaration, unsigned AddLevels,
+                                bool MunchSemi,
+                                bool UnindentWhitesmithsBraces) {
   assert(FormatTok->isOneOf(tok::l_brace, TT_MacroBlockBegin) &&
          "'{' or macro block token expected");
+  FormatToken *Tok = FormatTok;
   const bool MacroBlock = FormatTok->is(TT_MacroBlockBegin);
   FormatTok->setBlockKind(BK_Block);
 
@@ -691,16 +751,28 @@ void UnwrappedLineParser::parseBlock(bool MustBeDeclaration, unsigned AddLevels,
                                           MustBeDeclaration);
   if (AddLevels > 0u && Style.BreakBeforeBraces != FormatStyle::BS_Whitesmiths)
     Line->Level += AddLevels;
-  parseLevel(/*HasOpeningBrace=*/true);
+
+  IfStmtKind IfKind = IfStmtKind::NotIf;
+  const bool SimpleBlock = parseLevel(/*HasOpeningBrace=*/true, &IfKind);
 
   if (eof())
-    return;
+    return IfKind;
 
   if (MacroBlock ? !FormatTok->is(TT_MacroBlockEnd)
                  : !FormatTok->is(tok::r_brace)) {
     Line->Level = InitialLevel;
     FormatTok->setBlockKind(BK_Block);
-    return;
+    return IfKind;
+  }
+
+  if (SimpleBlock && Tok->is(tok::l_brace)) {
+    assert(FormatTok->is(tok::r_brace));
+    const FormatToken *Previous = Tokens->getPreviousToken();
+    assert(Previous);
+    if (Previous->isNot(tok::r_brace) || Previous->Optional) {
+      Tok->MatchingParen = FormatTok;
+      FormatTok->MatchingParen = Tok;
+    }
   }
 
   size_t PPEndHash = computePPHash();
@@ -731,6 +803,8 @@ void UnwrappedLineParser::parseBlock(bool MustBeDeclaration, unsigned AddLevels,
           CurrentLines->size() - 1;
     }
   }
+
+  return IfKind;
 }
 
 static bool isGoogScope(const UnwrappedLine &Line) {
@@ -1185,7 +1259,8 @@ void UnwrappedLineParser::readTokenWithJavaScriptASI() {
     return addUnwrappedLine();
 }
 
-void UnwrappedLineParser::parseStructuralElement(bool IsTopLevel) {
+void UnwrappedLineParser::parseStructuralElement(IfStmtKind *IfKind,
+                                                 bool IsTopLevel) {
   if (Style.Language == FormatStyle::LK_TableGen &&
       FormatTok->is(tok::pp_include)) {
     nextToken();
@@ -1228,7 +1303,7 @@ void UnwrappedLineParser::parseStructuralElement(bool IsTopLevel) {
     if (Style.isJavaScript() && Line->MustBeDeclaration)
       // field/method declaration.
       break;
-    parseIfThenElse();
+    parseIfThenElse(IfKind);
     return;
   case tok::kw_for:
   case tok::kw_while:
@@ -2138,7 +2213,39 @@ void UnwrappedLineParser::parseSquare(bool LambdaIntroducer) {
   } while (!eof());
 }
 
-void UnwrappedLineParser::parseIfThenElse() {
+void UnwrappedLineParser::keepAncestorBraces() {
+  if (!Style.RemoveBracesLLVM)
+    return;
+
+  const int MaxNestingLevels = 2;
+  const int Size = NestedTooDeep.size();
+  if (Size >= MaxNestingLevels)
+    NestedTooDeep[Size - MaxNestingLevels] = true;
+  NestedTooDeep.push_back(false);
+}
+
+static void markOptionalBraces(FormatToken *LeftBrace) {
+  if (!LeftBrace)
+    return;
+
+  assert(LeftBrace->is(tok::l_brace));
+
+  FormatToken *RightBrace = LeftBrace->MatchingParen;
+  if (!RightBrace) {
+    assert(!LeftBrace->Optional);
+    return;
+  }
+
+  assert(RightBrace->is(tok::r_brace));
+  assert(RightBrace->MatchingParen == LeftBrace);
+  assert(LeftBrace->Optional == RightBrace->Optional);
+
+  LeftBrace->Optional = true;
+  RightBrace->Optional = true;
+}
+
+FormatToken *UnwrappedLineParser::parseIfThenElse(IfStmtKind *IfKind,
+                                                  bool KeepBraces) {
   auto HandleAttributes = [this]() {
     // Handle AttributeMacro, e.g. `if (x) UNLIKELY`.
     if (FormatTok->is(TT_AttributeMacro))
@@ -2155,10 +2262,17 @@ void UnwrappedLineParser::parseIfThenElse() {
   if (FormatTok->Tok.is(tok::l_paren))
     parseParens();
   HandleAttributes();
+
   bool NeedsUnwrappedLine = false;
+  keepAncestorBraces();
+
+  FormatToken *IfLeftBrace = nullptr;
+  IfStmtKind IfBlockKind = IfStmtKind::NotIf;
+
   if (FormatTok->Tok.is(tok::l_brace)) {
+    IfLeftBrace = FormatTok;
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
-    parseBlock();
+    IfBlockKind = parseBlock();
     if (Style.BraceWrapping.BeforeElse)
       addUnwrappedLine();
     else
@@ -2169,22 +2283,48 @@ void UnwrappedLineParser::parseIfThenElse() {
     parseStructuralElement();
     --Line->Level;
   }
+
+  bool KeepIfBraces = false;
+  if (Style.RemoveBracesLLVM) {
+    assert(!NestedTooDeep.empty());
+    KeepIfBraces = (IfLeftBrace && !IfLeftBrace->MatchingParen) ||
+                   NestedTooDeep.back() || IfBlockKind == IfStmtKind::IfOnly ||
+                   IfBlockKind == IfStmtKind::IfElseIf;
+  }
+
+  FormatToken *ElseLeftBrace = nullptr;
+  IfStmtKind Kind = IfStmtKind::IfOnly;
+
   if (FormatTok->Tok.is(tok::kw_else)) {
+    if (Style.RemoveBracesLLVM) {
+      NestedTooDeep.back() = false;
+      Kind = IfStmtKind::IfElse;
+    }
     nextToken();
     HandleAttributes();
     if (FormatTok->Tok.is(tok::l_brace)) {
+      ElseLeftBrace = FormatTok;
       CompoundStatementIndenter Indenter(this, Style, Line->Level);
-      parseBlock();
+      if (parseBlock() == IfStmtKind::IfOnly)
+        Kind = IfStmtKind::IfElseIf;
       addUnwrappedLine();
     } else if (FormatTok->Tok.is(tok::kw_if)) {
       FormatToken *Previous = Tokens->getPreviousToken();
-      bool PrecededByComment = Previous && Previous->is(tok::comment);
-      if (PrecededByComment) {
+      const bool IsPrecededByComment = Previous && Previous->is(tok::comment);
+      if (IsPrecededByComment) {
         addUnwrappedLine();
         ++Line->Level;
       }
-      parseIfThenElse();
-      if (PrecededByComment)
+      bool TooDeep = true;
+      if (Style.RemoveBracesLLVM) {
+        Kind = IfStmtKind::IfElseIf;
+        TooDeep = NestedTooDeep.pop_back_val();
+      }
+      ElseLeftBrace =
+          parseIfThenElse(/*IfKind=*/nullptr, KeepBraces || KeepIfBraces);
+      if (Style.RemoveBracesLLVM)
+        NestedTooDeep.push_back(TooDeep);
+      if (IsPrecededByComment)
         --Line->Level;
     } else {
       addUnwrappedLine();
@@ -2194,9 +2334,40 @@ void UnwrappedLineParser::parseIfThenElse() {
         addUnwrappedLine();
       --Line->Level;
     }
-  } else if (NeedsUnwrappedLine) {
-    addUnwrappedLine();
+  } else {
+    if (Style.RemoveBracesLLVM)
+      KeepIfBraces = KeepIfBraces || IfBlockKind == IfStmtKind::IfElse;
+    if (NeedsUnwrappedLine)
+      addUnwrappedLine();
   }
+
+  if (!Style.RemoveBracesLLVM)
+    return nullptr;
+
+  assert(!NestedTooDeep.empty());
+  const bool KeepElseBraces =
+      (ElseLeftBrace && !ElseLeftBrace->MatchingParen) || NestedTooDeep.back();
+
+  NestedTooDeep.pop_back();
+
+  if (!KeepBraces && !KeepIfBraces && !KeepElseBraces) {
+    markOptionalBraces(IfLeftBrace);
+    markOptionalBraces(ElseLeftBrace);
+  } else if (IfLeftBrace) {
+    FormatToken *IfRightBrace = IfLeftBrace->MatchingParen;
+    if (IfRightBrace) {
+      assert(IfRightBrace->MatchingParen == IfLeftBrace);
+      assert(!IfLeftBrace->Optional);
+      assert(!IfRightBrace->Optional);
+      IfLeftBrace->MatchingParen = nullptr;
+      IfRightBrace->MatchingParen = nullptr;
+    }
+  }
+
+  if (IfKind)
+    *IfKind = Kind;
+
+  return IfLeftBrace;
 }
 
 void UnwrappedLineParser::parseTryCatch() {
@@ -2234,6 +2405,9 @@ void UnwrappedLineParser::parseTryCatch() {
   if (Style.Language == FormatStyle::LK_Java && FormatTok->is(tok::l_paren)) {
     parseParens();
   }
+
+  keepAncestorBraces();
+
   if (FormatTok->is(tok::l_brace)) {
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
     parseBlock();
@@ -2267,8 +2441,11 @@ void UnwrappedLineParser::parseTryCatch() {
         parseParens();
         continue;
       }
-      if (FormatTok->isOneOf(tok::semi, tok::r_brace, tok::eof))
+      if (FormatTok->isOneOf(tok::semi, tok::r_brace, tok::eof)) {
+        if (Style.RemoveBracesLLVM)
+          NestedTooDeep.pop_back();
         return;
+      }
       nextToken();
     }
     NeedsUnwrappedLine = false;
@@ -2279,6 +2456,10 @@ void UnwrappedLineParser::parseTryCatch() {
     else
       NeedsUnwrappedLine = true;
   }
+
+  if (Style.RemoveBracesLLVM)
+    NestedTooDeep.pop_back();
+
   if (NeedsUnwrappedLine)
     addUnwrappedLine();
 }
@@ -2385,9 +2566,18 @@ void UnwrappedLineParser::parseForOrWhileLoop() {
     nextToken();
   if (FormatTok->Tok.is(tok::l_paren))
     parseParens();
+
+  keepAncestorBraces();
+
   if (FormatTok->Tok.is(tok::l_brace)) {
+    FormatToken *LeftBrace = FormatTok;
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
     parseBlock();
+    if (Style.RemoveBracesLLVM) {
+      assert(!NestedTooDeep.empty());
+      if (!NestedTooDeep.back())
+        markOptionalBraces(LeftBrace);
+    }
     addUnwrappedLine();
   } else {
     addUnwrappedLine();
@@ -2395,11 +2585,17 @@ void UnwrappedLineParser::parseForOrWhileLoop() {
     parseStructuralElement();
     --Line->Level;
   }
+
+  if (Style.RemoveBracesLLVM)
+    NestedTooDeep.pop_back();
 }
 
 void UnwrappedLineParser::parseDoWhile() {
   assert(FormatTok->Tok.is(tok::kw_do) && "'do' expected");
   nextToken();
+
+  keepAncestorBraces();
+
   if (FormatTok->Tok.is(tok::l_brace)) {
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
     parseBlock();
@@ -2412,6 +2608,9 @@ void UnwrappedLineParser::parseDoWhile() {
     --Line->Level;
   }
 
+  if (Style.RemoveBracesLLVM)
+    NestedTooDeep.pop_back();
+
   // FIXME: Add error handling.
   if (!FormatTok->Tok.is(tok::kw_while)) {
     addUnwrappedLine();
@@ -2481,6 +2680,9 @@ void UnwrappedLineParser::parseSwitch() {
   nextToken();
   if (FormatTok->Tok.is(tok::l_paren))
     parseParens();
+
+  keepAncestorBraces();
+
   if (FormatTok->Tok.is(tok::l_brace)) {
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
     parseBlock();
@@ -2491,6 +2693,9 @@ void UnwrappedLineParser::parseSwitch() {
     parseStructuralElement();
     --Line->Level;
   }
+
+  if (Style.RemoveBracesLLVM)
+    NestedTooDeep.pop_back();
 }
 
 void UnwrappedLineParser::parseAccessSpecifier() {

clang/lib/Format/UnwrappedLineParser.h

@@ -81,12 +81,21 @@ public:
   void parse();
 
 private:
+  enum class IfStmtKind {
+    NotIf,   // Not an if statement.
+    IfOnly,  // An if statement without the else clause.
+    IfElse,  // An if statement followed by else but not else if.
+    IfElseIf // An if statement followed by else if.
+  };
+
   void reset();
   void parseFile();
-  void parseLevel(bool HasOpeningBrace);
-  void parseBlock(bool MustBeDeclaration = false, unsigned AddLevels = 1u,
-                  bool MunchSemi = true,
-                  bool UnindentWhitesmithsBraces = false);
+  bool precededByCommentOrPPDirective() const;
+  bool mightFitOnOneLine() const;
+  bool parseLevel(bool HasOpeningBrace, IfStmtKind *IfKind = nullptr);
+  IfStmtKind parseBlock(bool MustBeDeclaration = false, unsigned AddLevels = 1u,
+                        bool MunchSemi = true,
+                        bool UnindentWhitesmithsBraces = false);
   void parseChildBlock();
   void parsePPDirective();
   void parsePPDefine();
@@ -96,13 +105,15 @@ private:
   void parsePPEndIf();
   void parsePPUnknown();
   void readTokenWithJavaScriptASI();
-  void parseStructuralElement(bool IsTopLevel = false);
+  void parseStructuralElement(IfStmtKind *IfKind = nullptr,
+                              bool IsTopLevel = false);
   bool tryToParseBracedList();
   bool parseBracedList(bool ContinueOnSemicolons = false, bool IsEnum = false,
                        tok::TokenKind ClosingBraceKind = tok::r_brace);
   void parseParens();
   void parseSquare(bool LambdaIntroducer = false);
-  void parseIfThenElse();
+  void keepAncestorBraces();
+  FormatToken *parseIfThenElse(IfStmtKind *IfKind, bool KeepBraces = false);
   void parseTryCatch();
   void parseForOrWhileLoop();
   void parseDoWhile();
@@ -235,6 +246,10 @@ private:
   // owned outside of and handed into the UnwrappedLineParser.
   ArrayRef<FormatToken *> AllTokens;
 
+  // Keeps a stack of the states of nested control statements (true if the
+  // statement contains more than some predefined number of nested statements).
+  SmallVector<bool, 8> NestedTooDeep;
+
   // Represents preprocessor branch type, so we can find matching
   // #if/#else/#endif directives.
   enum PPBranchKind {

clang/unittests/Format/FormatTest.cpp

@@ -18889,6 +18889,7 @@ TEST_F(FormatTest, ParsesConfigurationBools) {
   CHECK_PARSE_BOOL(ObjCSpaceBeforeProtocolList);
   CHECK_PARSE_BOOL(Cpp11BracedListStyle);
   CHECK_PARSE_BOOL(ReflowComments);
+  CHECK_PARSE_BOOL(RemoveBracesLLVM);
   CHECK_PARSE_BOOL(SortUsingDeclarations);
   CHECK_PARSE_BOOL(SpacesInParentheses);
   CHECK_PARSE_BOOL(SpacesInSquareBrackets);
@@ -23334,6 +23335,367 @@ TEST_F(FormatTest, ShortTemplatedArgumentLists) {
   verifyFormat("struct Z : X<decltype([] { return 0; }){}> {};", Style);
 }
 
+TEST_F(FormatTest, RemoveBraces) {
+  FormatStyle Style = getLLVMStyle();
+  Style.RemoveBracesLLVM = true;
+
+  // The following eight test cases are fully-braced versions of the examples at
+  // "llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-
+  // statement-bodies-of-if-else-loop-statements".
+
+  // 1. Omit the braces, since the body is simple and clearly associated with
+  // the if.
+  verifyFormat("if (isa<FunctionDecl>(D))\n"
+               "  handleFunctionDecl(D);\n"
+               "else if (isa<VarDecl>(D))\n"
+               "  handleVarDecl(D);",
+               "if (isa<FunctionDecl>(D)) {\n"
+               "  handleFunctionDecl(D);\n"
+               "} else if (isa<VarDecl>(D)) {\n"
+               "  handleVarDecl(D);\n"
+               "}",
+               Style);
+
+  // 2. Here we document the condition itself and not the body.
+  verifyFormat("if (isa<VarDecl>(D)) {\n"
+               "  // It is necessary that we explain the situation with this\n"
+               "  // surprisingly long comment, so it would be unclear\n"
+               "  // without the braces whether the following statement is in\n"
+               "  // the scope of the `if`.\n"
+               "  // Because the condition is documented, we can't really\n"
+               "  // hoist this comment that applies to the body above the\n"
+               "  // if.\n"
+               "  handleOtherDecl(D);\n"
+               "}",
+               Style);
+
+  // 3. Use braces on the outer `if` to avoid a potential dangling else
+  // situation.
+  verifyFormat("if (isa<VarDecl>(D)) {\n"
+               "  for (auto *A : D.attrs())\n"
+               "    if (shouldProcessAttr(A))\n"
+               "      handleAttr(A);\n"
+               "}",
+               "if (isa<VarDecl>(D)) {\n"
+               "  for (auto *A : D.attrs()) {\n"
+               "    if (shouldProcessAttr(A)) {\n"
+               "      handleAttr(A);\n"
+               "    }\n"
+               "  }\n"
+               "}",
+               Style);
+
+  // 4. Use braces for the `if` block to keep it uniform with the else block.
+  verifyFormat("if (isa<FunctionDecl>(D)) {\n"
+               "  handleFunctionDecl(D);\n"
+               "} else {\n"
+               "  // In this else case, it is necessary that we explain the\n"
+               "  // situation with this surprisingly long comment, so it\n"
+               "  // would be unclear without the braces whether the\n"
+               "  // following statement is in the scope of the `if`.\n"
+               "  handleOtherDecl(D);\n"
+               "}",
+               Style);
+
+  // 5. This should also omit braces.  The `for` loop contains only a single
+  // statement, so it shouldn't have braces.  The `if` also only contains a
+  // single simple statement (the for loop), so it also should omit braces.
+  verifyFormat("if (isa<FunctionDecl>(D))\n"
+               "  for (auto *A : D.attrs())\n"
+               "    handleAttr(A);",
+               "if (isa<FunctionDecl>(D)) {\n"
+               "  for (auto *A : D.attrs()) {\n"
+               "    handleAttr(A);\n"
+               "  }\n"
+               "}",
+               Style);
+
+  // 6. Use braces for the outer `if` since the nested `for` is braced.
+  verifyFormat("if (isa<FunctionDecl>(D)) {\n"
+               "  for (auto *A : D.attrs()) {\n"
+               "    // In this for loop body, it is necessary that we explain\n"
+               "    // the situation with this surprisingly long comment,\n"
+               "    // forcing braces on the `for` block.\n"
+               "    handleAttr(A);\n"
+               "  }\n"
+               "}",
+               Style);
+
+  // 7. Use braces on the outer block because there are more than two levels of
+  // nesting.
+  verifyFormat("if (isa<FunctionDecl>(D)) {\n"
+               "  for (auto *A : D.attrs())\n"
+               "    for (ssize_t i : llvm::seq<ssize_t>(count))\n"
+               "      handleAttrOnDecl(D, A, i);\n"
+               "}",
+               "if (isa<FunctionDecl>(D)) {\n"
+               "  for (auto *A : D.attrs()) {\n"
+               "    for (ssize_t i : llvm::seq<ssize_t>(count)) {\n"
+               "      handleAttrOnDecl(D, A, i);\n"
+               "    }\n"
+               "  }\n"
+               "}",
+               Style);
+
+  // 8. Use braces on the outer block because of a nested `if`, otherwise the
+  // compiler would warn: `add explicit braces to avoid dangling else`
+  verifyFormat("if (auto *D = dyn_cast<FunctionDecl>(D)) {\n"
+               "  if (shouldProcess(D))\n"
+               "    handleVarDecl(D);\n"
+               "  else\n"
+               "    markAsIgnored(D);\n"
+               "}",
+               "if (auto *D = dyn_cast<FunctionDecl>(D)) {\n"
+               "  if (shouldProcess(D)) {\n"
+               "    handleVarDecl(D);\n"
+               "  } else {\n"
+               "    markAsIgnored(D);\n"
+               "  }\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a)\n"
+               "  b; // comment\n"
+               "else if (c)\n"
+               "  d; /* comment */\n"
+               "else\n"
+               "  e;",
+               "if (a) {\n"
+               "  b; // comment\n"
+               "} else if (c) {\n"
+               "  d; /* comment */\n"
+               "} else {\n"
+               "  e;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  b;\n"
+               "  c;\n"
+               "} else if (d) {\n"
+               "  e;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "#undef NDEBUG\n"
+               "  b;\n"
+               "} else {\n"
+               "  c;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  // comment\n"
+               "} else if (b) {\n"
+               "  c;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  b;\n"
+               "} else {\n"
+               "  { c; }\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  if (b) // comment\n"
+               "    c;\n"
+               "} else if (d) {\n"
+               "  e;\n"
+               "}",
+               "if (a) {\n"
+               "  if (b) { // comment\n"
+               "    c;\n"
+               "  }\n"
+               "} else if (d) {\n"
+               "  e;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  if (b) {\n"
+               "    c;\n"
+               "    // comment\n"
+               "  } else if (d) {\n"
+               "    e;\n"
+               "  }\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  if (b)\n"
+               "    c;\n"
+               "}",
+               "if (a) {\n"
+               "  if (b) {\n"
+               "    c;\n"
+               "  }\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a)\n"
+               "  if (b)\n"
+               "    c;\n"
+               "  else\n"
+               "    d;\n"
+               "else\n"
+               "  e;",
+               "if (a) {\n"
+               "  if (b) {\n"
+               "    c;\n"
+               "  } else {\n"
+               "    d;\n"
+               "  }\n"
+               "} else {\n"
+               "  e;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  // comment\n"
+               "  if (b)\n"
+               "    c;\n"
+               "  else if (d)\n"
+               "    e;\n"
+               "} else {\n"
+               "  g;\n"
+               "}",
+               "if (a) {\n"
+               "  // comment\n"
+               "  if (b) {\n"
+               "    c;\n"
+               "  } else if (d) {\n"
+               "    e;\n"
+               "  }\n"
+               "} else {\n"
+               "  g;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a)\n"
+               "  b;\n"
+               "else if (c)\n"
+               "  d;\n"
+               "else\n"
+               "  e;",
+               "if (a) {\n"
+               "  b;\n"
+               "} else {\n"
+               "  if (c) {\n"
+               "    d;\n"
+               "  } else {\n"
+               "    e;\n"
+               "  }\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  if (b)\n"
+               "    c;\n"
+               "  else if (d)\n"
+               "    e;\n"
+               "} else {\n"
+               "  g;\n"
+               "}",
+               "if (a) {\n"
+               "  if (b)\n"
+               "    c;\n"
+               "  else {\n"
+               "    if (d)\n"
+               "      e;\n"
+               "  }\n"
+               "} else {\n"
+               "  g;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a)\n"
+               "  b;\n"
+               "else if (c)\n"
+               "  while (d)\n"
+               "    e;\n"
+               "// comment",
+               "if (a)\n"
+               "{\n"
+               "  b;\n"
+               "} else if (c) {\n"
+               "  while (d) {\n"
+               "    e;\n"
+               "  }\n"
+               "}\n"
+               "// comment",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  b;\n"
+               "} else if (c) {\n"
+               "  d;\n"
+               "} else {\n"
+               "  e;\n"
+               "  g;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  b;\n"
+               "} else if (c) {\n"
+               "  d;\n"
+               "} else {\n"
+               "  e;\n"
+               "} // comment",
+               Style);
+
+  verifyFormat("int abs = [](int i) {\n"
+               "  if (i >= 0)\n"
+               "    return i;\n"
+               "  return -i;\n"
+               "};",
+               "int abs = [](int i) {\n"
+               "  if (i >= 0) {\n"
+               "    return i;\n"
+               "  }\n"
+               "  return -i;\n"
+               "};",
+               Style);
+
+  Style.ColumnLimit = 20;
+
+  verifyFormat("if (a) {\n"
+               "  b = c + // 1 -\n"
+               "      d;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a) {\n"
+               "  b = c >= 0 ? d\n"
+               "             : e;\n"
+               "}",
+               "if (a) {\n"
+               "  b = c >= 0 ? d : e;\n"
+               "}",
+               Style);
+
+  verifyFormat("if (a)\n"
+               "  b = c > 0 ? d : e;",
+               "if (a) {\n"
+               "  b = c > 0 ? d : e;\n"
+               "}",
+               Style);
+
+  Style.ColumnLimit = 0;
+
+  verifyFormat("if (a)\n"
+               "  b234567890223456789032345678904234567890 = "
+               "c234567890223456789032345678904234567890;",
+               "if (a) {\n"
+               "  b234567890223456789032345678904234567890 = "
+               "c234567890223456789032345678904234567890;\n"
+               "}",
+               Style);
+}
+
 } // namespace
 } // namespace format
 } // namespace clang