[clang-format] Break non-trailing block comments

[krasimir]

Summary

This patch is an alternative to https://reviews.llvm.org/D36614, by resolving a
non-idempotency issue by breaking non-trailing comments:

Consider formatting the following code with column limit at V:

                    V
const /* comment comment */ A = B;

The comment is not a trailing comment, breaking before it doesn't bring it under
the column limit. The formatter breaks after it, resulting in:

                    V
const /* comment comment */
    A = B;

For a next reformat, the formatter considers the comment as a trailing comment,
so it is free to break it further, resulting in:

                    V
const /* comment
         comment */
    A = B;

This patch improves the situation by directly producing the third case.

[djasper]

If no tests break with this, lets just go for it. We can follow up and fix individual cases if we find undesired behavior.

lib/Format/ContinuationIndenter.cpp

if ((Text.endswith(Postfix = "\"") &&
         (Text.startswith(Prefix = "_T(\"") && Text.endswith(Postfix = "\")"))) {
       Token.reset(new BreakableStringLiteral(Current, StartColumn, Prefix,
                                              Postfix, State.Line->InPPDirective,
                                              Encoding, Style));
     } else {
       return 0;
     }
   } else if (Current.is(TT_BlockComment)) {
-    if (!Current.isTrailingComment() || !Style.ReflowComments ||
+    if (!Style.ReflowComments ||
         // If a comment token switches formatting, like
         // /* clang-format on */, we don't want to break it further,
         // but we may still want to adjust its indentation.
         switchesFormatting(Current))
       return addMultilineToken(Current, State);
     Token.reset(new BreakableBlockComment(
         Current, StartColumn, Current.OriginalColumn, !Current.Previous,
         State.Line->InPPDirective, Encoding, Style));

unittests/Format/FormatTestComments.cpp

EXPECT_EQ("/*\n"
             " * long */",
             format("/*\n"
                    " * long long long long\n"
                    " * long\n"
                    "  *\n"
                    "* long */",
                    getLLVMStyleWithColumns(20)));
 }
+
+TEST_F(FormatTestComments, NonTrailingBlockComments) {
+  verifyFormat("const /** comment comment */ A = B;",
+               getLLVMStyleWithColumns(40));
+
+  verifyFormat("const /** comment comment comment */ A =\n"
+               "    B;",
+               getLLVMStyleWithColumns(40));
+
+  EXPECT_EQ("const /** comment comment comment\n"
+            "         comment */\n"
+            "    A = B;",
+            format("const /** comment comment comment comment */\n"
+                   "    A = B;",
+                   getLLVMStyleWithColumns(40)));
+}
 } // end namespace
 } // end namespace format
 } // end namespace clang