clang-format: Avoid line breaks before the first <<.

This puts a slight penalty on the linebreak before the first "<<", so
that clang-format generally tries to keep things on the first line.

User feedback has shown that this is generally desirable.

Before:
  llvm::outs()
      << "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa =" << aaaaaaaaaaaaaaaaaaaaaaaaaaa;

After:
  llvm::outs() << "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa ="
               << aaaaaaaaaaaaaaaaaaaaaaaaaaa;

llvm-svn: 186115

clang/include/clang/Format/Format.h

@@ -45,6 +45,9 @@ struct FormatStyle {
   /// \brief The penalty for each character outside of the column limit.
   unsigned PenaltyExcessCharacter;
 
+  /// \brief The penalty for breaking before the first "<<".
+  unsigned PenaltyBreakFirstLessLess;
+
   /// \brief Set whether & and * bind to the type as opposed to the variable.
   bool PointerBindsToType;
 
@@ -173,8 +176,9 @@ struct FormatStyle {
            IndentWidth == R.IndentWidth &&
            MaxEmptyLinesToKeep == R.MaxEmptyLinesToKeep &&
            ObjCSpaceBeforeProtocolList == R.ObjCSpaceBeforeProtocolList &&
-           PenaltyBreakString == R.PenaltyBreakString &&
            PenaltyBreakComment == R.PenaltyBreakComment &&
+           PenaltyBreakFirstLessLess == R.PenaltyBreakFirstLessLess &&
+           PenaltyBreakString == R.PenaltyBreakString &&
            PenaltyExcessCharacter == R.PenaltyExcessCharacter &&
            PenaltyReturnTypeOnItsOwnLine == R.PenaltyReturnTypeOnItsOwnLine &&
            PointerBindsToType == R.PointerBindsToType &&

clang/lib/Format/Format.cpp

@@ -102,6 +102,8 @@ template <> struct MappingTraits<clang::format::FormatStyle> {
                    Style.ObjCSpaceBeforeProtocolList);
     IO.mapOptional("PenaltyBreakComment", Style.PenaltyBreakComment);
     IO.mapOptional("PenaltyBreakString", Style.PenaltyBreakString);
+    IO.mapOptional("PenaltyBreakFirstLessLess",
+                   Style.PenaltyBreakFirstLessLess);
     IO.mapOptional("PenaltyExcessCharacter", Style.PenaltyExcessCharacter);
     IO.mapOptional("PenaltyReturnTypeOnItsOwnLine",
                    Style.PenaltyReturnTypeOnItsOwnLine);
@@ -123,6 +125,13 @@ template <> struct MappingTraits<clang::format::FormatStyle> {
 namespace clang {
 namespace format {
 
+void setDefaultPenalties(FormatStyle &Style) {
+  Style.PenaltyBreakComment = 45;
+  Style.PenaltyBreakFirstLessLess = 100;
+  Style.PenaltyBreakString = 1000;
+  Style.PenaltyExcessCharacter = 1000000;
+}
+
 FormatStyle getLLVMStyle() {
   FormatStyle LLVMStyle;
   LLVMStyle.AccessModifierOffset = -2;
@@ -140,10 +149,6 @@ FormatStyle getLLVMStyle() {
   LLVMStyle.IndentCaseLabels = false;
   LLVMStyle.MaxEmptyLinesToKeep = 1;
   LLVMStyle.ObjCSpaceBeforeProtocolList = true;
-  LLVMStyle.PenaltyBreakComment = 45;
-  LLVMStyle.PenaltyBreakString = 1000;
-  LLVMStyle.PenaltyExcessCharacter = 1000000;
-  LLVMStyle.PenaltyReturnTypeOnItsOwnLine = 60;
   LLVMStyle.PointerBindsToType = false;
   LLVMStyle.SpacesBeforeTrailingComments = 1;
   LLVMStyle.SpacesInBracedLists = true;
@@ -152,6 +157,10 @@ FormatStyle getLLVMStyle() {
   LLVMStyle.UseTab = false;
   LLVMStyle.BreakBeforeBraces = FormatStyle::BS_Attach;
   LLVMStyle.IndentFunctionDeclarationAfterType = false;
+
+  setDefaultPenalties(LLVMStyle);
+  LLVMStyle.PenaltyReturnTypeOnItsOwnLine = 60;
+
   return LLVMStyle;
 }
 
@@ -172,10 +181,6 @@ FormatStyle getGoogleStyle() {
   GoogleStyle.IndentCaseLabels = true;
   GoogleStyle.MaxEmptyLinesToKeep = 1;
   GoogleStyle.ObjCSpaceBeforeProtocolList = false;
-  GoogleStyle.PenaltyBreakComment = 45;
-  GoogleStyle.PenaltyBreakString = 1000;
-  GoogleStyle.PenaltyExcessCharacter = 1000000;
-  GoogleStyle.PenaltyReturnTypeOnItsOwnLine = 200;
   GoogleStyle.PointerBindsToType = true;
   GoogleStyle.SpacesBeforeTrailingComments = 2;
   GoogleStyle.SpacesInBracedLists = false;
@@ -184,6 +189,10 @@ FormatStyle getGoogleStyle() {
   GoogleStyle.UseTab = false;
   GoogleStyle.BreakBeforeBraces = FormatStyle::BS_Attach;
   GoogleStyle.IndentFunctionDeclarationAfterType = true;
+
+  setDefaultPenalties(GoogleStyle);
+  GoogleStyle.PenaltyReturnTypeOnItsOwnLine = 200;
+
   return GoogleStyle;
 }
 
@@ -503,6 +512,10 @@ private:
     const FormatToken &Current = *State.NextToken;
     const FormatToken &Previous = *State.NextToken->Previous;
 
+    // Extra penalty that needs to be added because of the way certain line
+    // breaks are chosen.
+    unsigned ExtraPenalty = 0;
+
     if (State.Stack.size() == 0 || Current.Type == TT_ImplicitStringLiteral) {
       // FIXME: Is this correct?
       int WhitespaceLength = SourceMgr.getSpellingColumnNumber(
@@ -621,6 +634,11 @@ private:
              Line.MustBeDeclaration))
           State.Stack.back().BreakBeforeParameter = true;
       }
+
+      // Breaking before the first "<<" is generally not desirable.
+      if (Current.is(tok::lessless) && State.Stack.back().FirstLessLess == 0)
+        ExtraPenalty += Style.PenaltyBreakFirstLessLess;
+
     } else {
       if (Current.is(tok::equal) &&
           (RootToken->is(tok::kw_for) || State.ParenLevel == 0) &&
@@ -699,7 +717,7 @@ private:
       }
     }
 
-    return moveStateToNextToken(State, DryRun);
+    return moveStateToNextToken(State, DryRun) + ExtraPenalty;
   }
 
   /// \brief Mark the next token as consumed in \p State and modify its stacks

clang/unittests/Format/FormatTest.cpp

@@ -3002,6 +3002,22 @@ TEST_F(FormatTest, AlignsPipes) {
   verifyFormat("llvm::outs() << \"aaaaaaaaaaaaaaaaaaaaaaaa: \"\n"
                "             << aaaaaaaaaaaaa(aaaaaaaaaaaaaaaaaaaaaaaaaaaa);");
 
+  // Breaking before the first "<<" is generally not desirable.
+  verifyFormat(
+      "llvm::errs()\n"
+      "    << \"aaaaaaaaaaaaaaaaaaa: \" << aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
+      "    << \"aaaaaaaaaaaaaaaaaaa: \" << aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
+      "    << \"aaaaaaaaaaaaaaaaaaa: \" << aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
+      "    << \"aaaaaaaaaaaaaaaaaaa: \" << aaaaaaaaaaaaaaaaaaaaaaaaaaaa;",
+      getLLVMStyleWithColumns(70));
+  verifyFormat("llvm::errs() << \"aaaaaaaaaaaaaaaaaaa: \"\n"
+               "             << aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
+               "             << \"aaaaaaaaaaaaaaaaaaa: \"\n"
+               "             << aaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
+               "             << \"aaaaaaaaaaaaaaaaaaa: \"\n"
+               "             << aaaaaaaaaaaaaaaaaaaaaaaaaaaa;",
+               getLLVMStyleWithColumns(70));
+
   verifyFormat(
       "llvm::errs() << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
       "                    .aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa();");