[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[Corentin Jabot via cfe-commits]

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@@ -1119,43 +1115,158 @@ bool Preprocessor::LexHeaderName(Token &FilenameTok, bool AllowMacroExpansion) {
   return false;
 }
 
+// We represent the primary and partition names as 'Paths' which are sections
+// of the hierarchical access path for a clang module.  However for C++20
+// the periods in a name are just another character, and we will need to
+// flatten them into a string.
+std::string ModuleLoader::getFlatNameFromPath(ModuleIdPath Path) {
+  std::string Name;
+  if (Path.empty())
+    return Name;
+
+  for (auto &Piece : Path) {
+    assert(Piece.getIdentifierInfo() && Piece.getLoc().isValid());
+    if (!Name.empty())
+      Name += ".";
+    Name += Piece.getIdentifierInfo()->getName();
+  }
+  return Name;
+}
+
+bool Preprocessor::LexModuleNameContinue(Token &Tok, SourceLocation UseLoc,
+                                         SmallVectorImpl<Token> &Suffix,
+                                         SmallVectorImpl<IdentifierLoc> &Path,
+                                         bool AllowMacroExpansion) {
+  auto ConsumeToken = [&]() {
+    if (AllowMacroExpansion)
+      Lex(Tok);
+    else
+      LexUnexpandedToken(Tok);
+    Suffix.push_back(Tok);
+  };
+
+  Suffix.push_back(Tok);
+  while (true) {
+    if (Tok.isNot(tok::identifier))
+      return true;
+
+    // Record this part of the module path.
+    Path.emplace_back(Tok.getLocation(), Tok.getIdentifierInfo());
+    ConsumeToken();
+
+    if (Tok.isNot(tok::period))
+      return false;
+
+    ConsumeToken();
+  }
+}
+
+/// P1857R3: Modules Dependency Discovery
+///
+/// At the start of phase 4 an import or module token is treated as starting a
+/// directive and are converted to their respective keywords iff:
+///   - After skipping horizontal whitespace are
+///     - at the start of a logical line, or
+///     - preceded by an 'export' at the start of the logical line.
+///   - Are followed by an identifier pp token (before macro expansion), or
+///     - <, ", or : (but not ::) pp tokens for 'import', or
+///     - ; for 'module'
+/// Otherwise the token is treated as an identifier.
+bool Preprocessor::HandleModuleContextualKeyword(
+    Token &Result, bool TokAtPhysicalStartOfLine) {
+  if (!getLangOpts().CPlusPlusModules || !Result.isModuleContextualKeyword())
+    return false;
+
+  if (Result.is(tok::kw_export)) {
+    LastTokenWasExportKeyword = {Result, TokAtPhysicalStartOfLine};
+    return false;
+  }
+
+  if (LastTokenWasExportKeyword.isValid()) {
+    // The export keyword was not at the start of line, it's not a
+    // directive-introducing token.
+    if (!LastTokenWasExportKeyword.isAtPhysicalStartOfLine())
+      return false;
+    // [cpp.pre]/1.4
+    // export                  // not a preprocessing directive
+    // import foo;             // preprocessing directive (ill-formed at phase
+    // 7)
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cor3ntin wrote:

nit: can you fix the formatting here?

https://github.com/llvm/llvm-project/pull/107168