[clang-format] Don't reflow comment lines starting with '@'.

[krasimir]

Summary

This patch stops reflowing comment lines starting with '@', since they commonly
have a special meaning.

[djasper]

Looks good.

lib/Format/BreakableToken.cpp

const FormatToken &BreakableComment::tokenAt(unsigned LineIndex) const {
   return Tokens[LineIndex] ? *Tokens[LineIndex] : Tok;
 }
 
 static bool mayReflowContent(StringRef Content) {
   Content = Content.trim(Blanks);
   // Simple heuristic for what to reflow: content should contain at least two
   // characters and either the first or second character must be
   // non-punctuation.
-  return Content.size() >= 2 && !Content.endswith("\\") &&
+  return Content.size() >= 2 &&
+         // Lines starting with '@' commonly have special meaning.
+         !Content.startswith("@") && !Content.endswith("\\") &&
          // Note that this is UTF-8 safe, since if isPunctuation(Content[0]) is
          // true, then the first code point must be 1 byte long.
          (!isPunctuation(Content[0]) || !isPunctuation(Content[1]));
 }
 
 bool BreakableComment::mayReflow(unsigned LineIndex) const {
   return LineIndex > 0 && mayReflowContent(Content[LineIndex]) &&
          !Tok.Finalized && !switchesFormatting(tokenAt(LineIndex)) &&

unittests/Format/FormatTest.cpp

TEST_F(FormatTest, ReflowsComments) {
   // Don't reflow lines starting with two punctuation characters.
   EXPECT_EQ("// long long long\n"
             "// long\n"
             "// ... --- ...",
             format(
                 "// long long long long\n"
                 "// ... --- ...",
                 getLLVMStyleWithColumns(20)));
+
+  // Don't reflow lines starting with '@'.
+  EXPECT_EQ("// long long long\n"
+            "// long\n"
+            "// @param arg",
+            format("// long long long long\n"
+                   "// @param arg",
+                   getLLVMStyleWithColumns(20)));
+
   // Reflow lines that have a non-punctuation character among their first 2
   // characters.
   EXPECT_EQ("// long long long\n"
             "// long 'long'",
             format(
                 "// long long long long\n"
                 "// 'long'",
                 getLLVMStyleWithColumns(20)));