[clang-format] Fix AlignConsecutive on PP blocks

Summary:
Currently the 'AlignConsecutive*' options incorrectly align across
elif and else statements, even if they are very far away and across
unrelated preprocessor macros.

This failed since on preprocessor run 2+, there is not enough context
about the #ifdefs to actually differentiate one block from another,
causing them to align across different blocks or even large sections of
the file.

Eg, with AlignConsecutiveAssignments:

```
\#if FOO      // Run 1
\#else        // Run 1
int a   = 1;  // Run 2, wrong
\#endif       // Run 1

\#if FOO      // Run 1
\#else        // Run 1
int bar = 1;  // Run 2
\#endif       // Run 1
```

is read as

```
int a   = 1;  // Run 2, wrong
int bar = 1;  // Run 2
```

The approach taken to fix this was to add a new flag to Token that
forces breaking alignment across groups of lines (MustBreakAlignBefore)
in a similar manner to the existing flag that forces a line break
(MustBreakBefore). This flag is set for the first Token after a
preprocessor statement or diff conflict marker.

Fixes #25167,#31281

Patch By: JakeMerdichAMD

Reviewed By: MyDeveloperDay

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D79388

Author: mydeveloperday

clang/lib/Format/FormatToken.h

@@ -182,6 +182,12 @@ struct FormatToken {
   /// before the token.
   bool MustBreakBefore = false;
 
+  /// Whether to not align across this token
+  ///
+  /// This happens for example when a preprocessor directive ended directly
+  /// before the token, but very rarely otherwise.
+  bool MustBreakAlignBefore = false;
+
   /// The raw text of the token.
   ///
   /// Contains the raw token text without leading whitespace and without leading

clang/lib/Format/UnwrappedLineParser.cpp

@@ -2968,6 +2968,7 @@ void UnwrappedLineParser::readToken(int LevelDifference) {
       }
       FormatTok = Tokens->getNextToken();
       FormatTok->MustBreakBefore = true;
+      FormatTok->MustBreakAlignBefore = true;
     }
 
     if (!PPStack.empty() && (PPStack.back().Kind == PP_Unreachable) &&
@@ -2992,6 +2993,7 @@ void UnwrappedLineParser::pushToken(FormatToken *Tok) {
   Line->Tokens.push_back(UnwrappedLineNode(Tok));
   if (MustBreakBeforeNextToken) {
     Line->Tokens.back().Tok->MustBreakBefore = true;
+    Line->Tokens.back().Tok->MustBreakAlignBefore = true;
     MustBreakBeforeNextToken = false;
   }
 }

clang/lib/Format/WhitespaceManager.cpp

@@ -377,9 +377,11 @@ static unsigned AlignTokens(const FormatStyle &Style, F &&Matches,
     if (Changes[i].NewlinesBefore != 0) {
       CommasBeforeMatch = 0;
       EndOfSequence = i;
-      // If there is a blank line, or if the last line didn't contain any
-      // matching token, the sequence ends here.
-      if (Changes[i].NewlinesBefore > 1 || !FoundMatchOnLine)
+      // If there is a blank line, there is a forced-align-break (eg,
+      // preprocessor), or if the last line didn't contain any matching token,
+      // the sequence ends here.
+      if (Changes[i].NewlinesBefore > 1 ||
+          Changes[i].Tok->MustBreakAlignBefore || !FoundMatchOnLine)
         AlignCurrentSequence();
 
       FoundMatchOnLine = false;
@@ -618,6 +620,8 @@ void WhitespaceManager::alignTrailingComments() {
     if (Changes[i].StartOfBlockComment)
       continue;
     Newlines += Changes[i].NewlinesBefore;
+    if (Changes[i].Tok->MustBreakAlignBefore)
+      BreakBeforeNext = true;
     if (!Changes[i].IsTrailingComment)
       continue;
 

clang/unittests/Format/FormatTest.cpp

@@ -11457,6 +11457,29 @@ TEST_F(FormatTest, AlignConsecutiveAssignments) {
   verifyFormat("int oneTwoThree = 123; // comment\n"
                "int oneTwo      = 12;  // comment",
                Alignment);
+
+  // Bug 25167
+  verifyFormat("#if A\n"
+               "#else\n"
+               "int aaaaaaaa = 12;\n"
+               "#endif\n"
+               "#if B\n"
+               "#else\n"
+               "int a = 12;\n"
+               "#endif\n",
+               Alignment);
+  verifyFormat("enum foo {\n"
+               "#if A\n"
+               "#else\n"
+               "  aaaaaaaa = 12;\n"
+               "#endif\n"
+               "#if B\n"
+               "#else\n"
+               "  a = 12;\n"
+               "#endif\n"
+               "};\n",
+               Alignment);
+
   EXPECT_EQ("int a = 5;\n"
             "\n"
             "int oneTwoThree = 123;",

clang/unittests/Format/FormatTestComments.cpp

@@ -2780,6 +2780,27 @@ TEST_F(FormatTestComments, AlignTrailingComments) {
                    "       // line 2 about b\n"
                    "       long b;",
                    getLLVMStyleWithColumns(80)));
+
+  // Checks an edge case in preprocessor handling.
+  // These comments should *not* be aligned
+  EXPECT_EQ(
+      "#if FOO\n"
+      "#else\n"
+      "long a; // Line about a\n"
+      "#endif\n"
+      "#if BAR\n"
+      "#else\n"
+      "long b_long_name; // Line about b\n"
+      "#endif\n",
+      format("#if FOO\n"
+             "#else\n"
+             "long a;           // Line about a\n" // Previous (bad) behavior
+             "#endif\n"
+             "#if BAR\n"
+             "#else\n"
+             "long b_long_name; // Line about b\n"
+             "#endif\n",
+             getLLVMStyleWithColumns(80)));
 }
 
 TEST_F(FormatTestComments, AlignsBlockCommentDecorations) {