[clang-scan-deps] Implement P2223R2 for DependencyDirectiveScanner.cpp (#143950)

P2223R2 allows the line-continuation slash `\` to be followed by
additional whitespace. The Clang lexer already follows this behavior,
also for versions prior to C++23. The dependency directive scanner
however only implements it for `#define` directives (15d5f5d).

This fully implements P2223R2 for the dependency directive scanner (for
any C++ standard) and aligns the dependency directive scanner's splicing
behavior with that of the Clang lexer.

For example, the following code was previously not scanned correctly by
`clang-scan-deps` but now works as expected:

```cpp
import \<whitespace here>
A;
```

2 files changed, 113 insertions, 10 deletions

diff --git a/clang/lib/Lex/DependencyDirectivesScanner.cpp b/clang/lib/Lex/DependencyDirectivesScanner.cpp
index 4606b85..1b6b16c 100644
--- a/clang/lib/Lex/DependencyDirectivesScanner.cpp
+++ b/clang/lib/Lex/DependencyDirectivesScanner.cpp
@@ -323,10 +323,6 @@ static unsigned skipNewline(const char *&First, const char *End) {
   return Len;
 }
 
-static bool wasLineContinuation(const char *First, unsigned EOLLen) {
-  return *(First - (int)EOLLen - 1) == '\\';
-}
-
 static void skipToNewlineRaw(const char *&First, const char *const End) {
   for (;;) {
     if (First == End)
@@ -336,13 +332,16 @@ static void skipToNewlineRaw(const char *&First, const char *const End) {
     if (Len)
       return;
 
+    char LastNonWhitespace = ' ';
     do {
+      if (!isHorizontalWhitespace(*First))
+        LastNonWhitespace = *First;
       if (++First == End)
         return;
       Len = isEOL(First, End);
     } while (!Len);
 
-    if (First[-1] != '\\')
+    if (LastNonWhitespace != '\\')
       return;
 
     First += Len;
@@ -394,6 +393,7 @@ static bool isQuoteCppDigitSeparator(const char *const Start,
 }
 
 void Scanner::skipLine(const char *&First, const char *const End) {
+  char LastNonWhitespace = ' ';
   for (;;) {
     assert(First <= End);
     if (First == End)
@@ -419,6 +419,8 @@ void Scanner::skipLine(const char *&First, const char *const End) {
       // Iterate over comments correctly.
       if (*First != '/' || End - First < 2) {
         LastTokenPtr = First;
+        if (!isWhitespace(*First))
+          LastNonWhitespace = *First;
         ++First;
         continue;
       }
@@ -431,6 +433,8 @@ void Scanner::skipLine(const char *&First, const char *const End) {
 
       if (First[1] != '*') {
         LastTokenPtr = First;
+        if (!isWhitespace(*First))
+          LastNonWhitespace = *First;
         ++First;
         continue;
       }
@@ -442,8 +446,9 @@ void Scanner::skipLine(const char *&First, const char *const End) {
       return;
 
     // Skip over the newline.
-    unsigned Len = skipNewline(First, End);
-    if (!wasLineContinuation(First, Len)) // Continue past line-continuations.
+    skipNewline(First, End);
+
+    if (LastNonWhitespace != '\\')
       break;
   }
 }
@@ -468,9 +473,16 @@ static void skipWhitespace(const char *&First, const char *const End) {
     if (End - First < 2)
       return;
 
-    if (First[0] == '\\' && isVerticalWhitespace(First[1])) {
-      skipNewline(++First, End);
-      continue;
+    if (*First == '\\') {
+      const char *Ptr = First + 1;
+      while (Ptr < End && isHorizontalWhitespace(*Ptr))
+        ++Ptr;
+      if (Ptr != End && isVerticalWhitespace(*Ptr)) {
+        skipNewline(Ptr, End);
+        First = Ptr;
+        continue;
+      }
+      return;
     }
 
     // Check for a non-comment character.
diff --git a/clang/unittests/Lex/DependencyDirectivesScannerTest.cpp b/clang/unittests/Lex/DependencyDirectivesScannerTest.cpp
index 377c066..61f7492 100644
--- a/clang/unittests/Lex/DependencyDirectivesScannerTest.cpp
+++ b/clang/unittests/Lex/DependencyDirectivesScannerTest.cpp
@@ -789,6 +789,97 @@ TEST(MinimizeSourceToDependencyDirectivesTest,
                Out.data());
 }
 
+TEST(MinimizeSourceToDependencyDirectivesTest,
+     WhitespaceAfterLineContinuationSlashLineComment) {
+  SmallVector<char, 128> Out;
+
+  ASSERT_FALSE(minimizeSourceToDependencyDirectives("// some comment \\  \n"
+                                                    "module A;\n",
+                                                    Out));
+  EXPECT_STREQ("", Out.data());
+}
+
+TEST(MinimizeSourceToDependencyDirectivesTest,
+     WhitespaceAfterLineContinuationSlashAllDirectives) {
+  SmallVector<char, 512> Out;
+  SmallVector<dependency_directives_scan::Token, 16> Tokens;
+  SmallVector<Directive, 16> Directives;
+
+  StringRef Input = "#define \\   \n"
+                    "A\n"
+                    "#undef\t\\   \n"
+                    "A\n"
+                    "#endif \\\t\t\n"
+                    "\n"
+                    "#if \\     \t\n"
+                    "A\n"
+                    "#ifdef\t\\   \n"
+                    "A\n"
+                    "#ifndef \\ \t\n"
+                    "A\n"
+                    "#elifdef \\  \n"
+                    "A\n"
+                    "#elifndef \\ \n"
+                    "A\n"
+                    "#elif \\\t\t \n"
+                    "A\n"
+                    "#else \\\t \t\n"
+                    "\n"
+                    "#include \\  \n"
+                    "<A>\n"
+                    "#include_next \\    \n"
+                    "<A>\n"
+                    "#__include_macros\\ \n"
+                    "<A>\n"
+                    "#import \\ \t\n"
+                    "<A>\n"
+                    "@import \\\t \n"
+                    "A;\n"
+                    "#pragma clang \\   \n"
+                    "module \\    \n"
+                    "import A\n"
+                    "#pragma \\   \n"
+                    "push_macro(A)\n"
+                    "#pragma \\\t \n"
+                    "pop_macro(A)\n"
+                    "#pragma \\   \n"
+                    "include_alias(<A>,\\ \n"
+                    "<B>)\n"
+                    "export \\    \n"
+                    "module m;\n"
+                    "import\t\\\t \n"
+                    "m;\n"
+                    "#pragma\t\\  \n"
+                    "clang\t\\  \t\n"
+                    "system_header\n";
+  ASSERT_FALSE(
+      minimizeSourceToDependencyDirectives(Input, Out, Tokens, Directives));
+
+  EXPECT_EQ(pp_define, Directives[0].Kind);
+  EXPECT_EQ(pp_undef, Directives[1].Kind);
+  EXPECT_EQ(pp_endif, Directives[2].Kind);
+  EXPECT_EQ(pp_if, Directives[3].Kind);
+  EXPECT_EQ(pp_ifdef, Directives[4].Kind);
+  EXPECT_EQ(pp_ifndef, Directives[5].Kind);
+  EXPECT_EQ(pp_elifdef, Directives[6].Kind);
+  EXPECT_EQ(pp_elifndef, Directives[7].Kind);
+  EXPECT_EQ(pp_elif, Directives[8].Kind);
+  EXPECT_EQ(pp_else, Directives[9].Kind);
+  EXPECT_EQ(pp_include, Directives[10].Kind);
+  EXPECT_EQ(pp_include_next, Directives[11].Kind);
+  EXPECT_EQ(pp___include_macros, Directives[12].Kind);
+  EXPECT_EQ(pp_import, Directives[13].Kind);
+  EXPECT_EQ(decl_at_import, Directives[14].Kind);
+  EXPECT_EQ(pp_pragma_import, Directives[15].Kind);
+  EXPECT_EQ(pp_pragma_push_macro, Directives[16].Kind);
+  EXPECT_EQ(pp_pragma_pop_macro, Directives[17].Kind);
+  EXPECT_EQ(pp_pragma_include_alias, Directives[18].Kind);
+  EXPECT_EQ(cxx_export_module_decl, Directives[19].Kind);
+  EXPECT_EQ(cxx_import_decl, Directives[20].Kind);
+  EXPECT_EQ(pp_pragma_system_header, Directives[21].Kind);
+  EXPECT_EQ(pp_eof, Directives[22].Kind);
+}
+
 TEST(MinimizeSourceToDependencyDirectivesTest, PoundWarningAndError) {
   SmallVector<char, 128> Out;