[Syntax] Add a helper to find expansion by its first spelled token

[ilya-biryukov]

Summary

Used in clangd for a code tweak that expands a macro.

[sammccall]

clang/include/clang/Tooling/Syntax/Tokens.h

204

macro directives -> preprocessor directives?

207

nit: "pragma"

207

these would be clearer without repeating the text, e.g. "Expands to an empty range"

209

also mention #include?

214

maybe mention # as it's the other common case

216

I think expansionStartingAt would be a clearer name. Given the current name, I would expect to be able to pass any of the tokens within the spelled range

[sammccall]

LG but needs tests

[ilya-biryukov]

• Address comments.
• s/findExpansion/expansionStartingAt.
• Add tests.

clang/include/clang/Tooling/Syntax/Tokens.h

209

Added an example and a FIXME mentioning this does not work yet.

[sammccall]

clang/lib/Tooling/Syntax/Tokens.cpp

208

This assert could use a message

[ilya-biryukov]

• Added a message to the assertion

clang/include/clang/Tooling/Syntax/Tokens.h

public:
   ///         a f1 => can't map
   ///        s1 s2 => can't map
   ///
   /// If \p Expanded is empty, the returned value is llvm::None.
   /// Complexity is logarithmic.
   llvm::Optional<llvm::ArrayRef<syntax::Token>>
   spelledForExpanded(llvm::ArrayRef<syntax::Token> Expanded) const;
 
+  /// An expansion produced by the preprocessor, includes macro expansions and
+  /// macro directives. Preprocessor always maps a non-empty range of spelled
+  /// tokens to a (possibly empty) range of expanded tokens.
+  /// Here is a few examples of expansions:
+  ///    #pragme once      // Expansion from "#pragma once" to an empty range.
+  ///    #define FOO 1 2 3 // Expansion from "#define FOO 1" to an empty range.
+  ///    FOO               // Expansion from "FOO" to "1 2 3".
+  struct Expansion {
+    llvm::ArrayRef<syntax::Token> Spelled;
+    llvm::ArrayRef<syntax::Token> Expanded;
+  };
+  /// If \p Spelled starts a mapping (e.g. if it's a macro name) return the
+  /// subrange of expanded tokens that the macro expands to.
+  llvm::Optional<Expansion> findExpansion(const syntax::Token *Spelled) const;
+
   /// Lexed tokens of a file before preprocessing. E.g. for the following input
   ///     #define DECL(name) int name = 10
   ///     DECL(a);
   /// spelledTokens() returns {"#", "define", "DECL", "(", "name", ")", "eof"}.
   /// FIXME: we do not yet store tokens of directives, like #include, #define,
   ///        #pragma, etc.
   llvm::ArrayRef<syntax::Token> spelledTokens(FileID FID) const;
 
public:
   /// preprocessing starts, i.e. as a part of BeginSourceFile() or
   /// CreateASTConsumer().
   TokenCollector(Preprocessor &P);
 
   /// Finalizes token collection. Should be called after preprocessing is
   /// finished, i.e. after running Execute().
   LLVM_NODISCARD TokenBuffer consume() &&;
 
+
 private:
   /// Maps from a start location to an end location of each mapping.
   ///   1. range from '#' to the last token in the line for PP directives,
   ///   2. macro name and arguments for macro expansions.
   using SpelledMappings =
       llvm::DenseMap</*SourceLocation*/ int, SourceLocation>;
   class Builder;
   class Callbacks;

clang/lib/Tooling/Syntax/Tokens.cpp

TokenBuffer::spelledForExpanded(llvm::ArrayRef<syntax::Token> Expanded) const {
   // All is good, return the result.
   return llvm::makeArrayRef(
       BeginMapping ? File.SpelledTokens.data() + BeginMapping->BeginSpelled
                    : BeginSpelled,
       LastMapping ? File.SpelledTokens.data() + LastMapping->EndSpelled
                   : LastSpelled + 1);
 }
 
+llvm::Optional<TokenBuffer::Expansion>
+TokenBuffer::findExpansion(const syntax::Token *Spelled) const {
+  assert(Spelled);
+  assert(Spelled->location().isFileID() && "not a spelled token");
+  auto FileIt = Files.find(SourceMgr->getFileID(Spelled->location()));
+  assert(FileIt != Files.end());
+
+  auto &File = FileIt->second;
+  assert(File.SpelledTokens.data() <= Spelled &&
+         Spelled < (File.SpelledTokens.data() + File.SpelledTokens.size()));
+
+  unsigned SpelledIndex = Spelled - File.SpelledTokens.data();
+  auto M = llvm::bsearch(File.Mappings, [&](const Mapping &M) {
+    return SpelledIndex <= M.BeginSpelled;
+  });
+  if (M == File.Mappings.end() || M->BeginSpelled != SpelledIndex)
+    return llvm::None;
+
+  Expansion E;
+  E.Spelled = llvm::makeArrayRef(File.SpelledTokens.data() + M->BeginSpelled,
+                                 File.SpelledTokens.data() + M->EndSpelled);
+  E.Expanded = llvm::makeArrayRef(ExpandedTokens.data() + M->BeginExpanded,
+                                  ExpandedTokens.data() + M->EndExpanded);
+  return E;
+}
+
 std::vector<syntax::Token> syntax::tokenize(FileID FID, const SourceManager &SM,
                                             const LangOptions &LO) {
   std::vector<syntax::Token> Tokens;
   IdentifierTable Identifiers(LO);
   auto AddToken = [&](clang::Token T) {
     // Fill the proper token kind for keywords, etc.
     if (T.getKind() == tok::raw_identifier && !T.needsCleaning() &&
         !T.hasUCN()) { // FIXME: support needsCleaning and hasUCN cases.