llvm-project/clang/lib/Tooling/Syntax/BuildTree.cpp

//===- BuildTree.cpp ------------------------------------------*- C++ -*-=====//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "clang/Tooling/Syntax/BuildTree.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclBase.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/AST/Stmt.h"
#include "clang/Basic/LLVM.h"
#include "clang/Basic/SourceLocation.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/TokenKinds.h"
#include "clang/Lex/Lexer.h"
#include "clang/Tooling/Syntax/Nodes.h"
#include "clang/Tooling/Syntax/Tokens.h"
#include "clang/Tooling/Syntax/Tree.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/FormatVariadic.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/raw_ostream.h"
#include <map>

using namespace clang;

LLVM_ATTRIBUTE_UNUSED
static bool isImplicitExpr(clang::Expr *E) { return E->IgnoreImplicit() != E; }

/// A helper class for constructing the syntax tree while traversing a clang
/// AST.
///
/// At each point of the traversal we maintain a list of pending nodes.
/// Initially all tokens are added as pending nodes. When processing a clang AST
/// node, the clients need to:
///   - create a corresponding syntax node,
///   - assign roles to all pending child nodes with 'markChild' and
///     'markChildToken',
///   - replace the child nodes with the new syntax node in the pending list
///     with 'foldNode'.
///
/// Note that all children are expected to be processed when building a node.
///
/// Call finalize() to finish building the tree and consume the root node.
class syntax::TreeBuilder {
public:
  TreeBuilder(syntax::Arena &Arena) : Arena(Arena), Pending(Arena) {
    for (const auto &T : Arena.tokenBuffer().expandedTokens())
      LocationToToken.insert({T.location().getRawEncoding(), &T});
  }

  llvm::BumpPtrAllocator &allocator() { return Arena.allocator(); }

  /// Populate children for \p New node, assuming it covers tokens from \p
  /// Range.
  void foldNode(llvm::ArrayRef<syntax::Token> Range, syntax::Tree *New);

  /// Must be called with the range of each `DeclaratorDecl`. Ensures the
  /// corresponding declarator nodes are covered by `SimpleDeclaration`.
  void noticeDeclaratorRange(llvm::ArrayRef<syntax::Token> Range);

  /// Notifies that we should not consume trailing semicolon when computing
  /// token range of \p D.
  void noticeDeclaratorWithoutSemicolon(Decl *D);

  /// Mark the \p Child node with a corresponding \p Role. All marked children
  /// should be consumed by foldNode.
  /// (!) when called on expressions (clang::Expr is derived from clang::Stmt),
  ///     wraps expressions into expression statement.
  void markStmtChild(Stmt *Child, NodeRole Role);
  /// Should be called for expressions in non-statement position to avoid
  /// wrapping into expression statement.
  void markExprChild(Expr *Child, NodeRole Role);

  /// Set role for a token starting at \p Loc.
  void markChildToken(SourceLocation Loc, NodeRole R);

  /// Finish building the tree and consume the root node.
  syntax::TranslationUnit *finalize() && {
    auto Tokens = Arena.tokenBuffer().expandedTokens();
    assert(!Tokens.empty());
    assert(Tokens.back().kind() == tok::eof);

    // Build the root of the tree, consuming all the children.
    Pending.foldChildren(Arena, Tokens.drop_back(),
                         new (Arena.allocator()) syntax::TranslationUnit);

    return cast<syntax::TranslationUnit>(std::move(Pending).finalize());
  }

  /// getRange() finds the syntax tokens corresponding to the passed source
  /// locations.
  /// \p First is the start position of the first token and \p Last is the start
  /// position of the last token.
  llvm::ArrayRef<syntax::Token> getRange(SourceLocation First,
                                         SourceLocation Last) const {
    assert(First.isValid());
    assert(Last.isValid());
    assert(First == Last ||
           Arena.sourceManager().isBeforeInTranslationUnit(First, Last));
    return llvm::makeArrayRef(findToken(First), std::next(findToken(Last)));
  }
  llvm::ArrayRef<syntax::Token> getRange(const Decl *D) const {
    auto Tokens = getRange(D->getBeginLoc(), D->getEndLoc());
    if (llvm::isa<NamespaceDecl>(D))
      return Tokens;
    if (DeclsWithoutSemicolons.count(D))
      return Tokens;
    // FIXME: do not consume trailing semicolon on function definitions.
    // Most declarations own a semicolon in syntax trees, but not in clang AST.
    return withTrailingSemicolon(Tokens);
  }
  llvm::ArrayRef<syntax::Token> getExprRange(const Expr *E) const {
    return getRange(E->getBeginLoc(), E->getEndLoc());
  }
  /// Find the adjusted range for the statement, consuming the trailing
  /// semicolon when needed.
  llvm::ArrayRef<syntax::Token> getStmtRange(const Stmt *S) const {
    auto Tokens = getRange(S->getBeginLoc(), S->getEndLoc());
    if (isa<CompoundStmt>(S))
      return Tokens;

    // Some statements miss a trailing semicolon, e.g. 'return', 'continue' and
    // all statements that end with those. Consume this semicolon here.
    if (Tokens.back().kind() == tok::semi)
      return Tokens;
    return withTrailingSemicolon(Tokens);
  }

private:
  llvm::ArrayRef<syntax::Token>
  withTrailingSemicolon(llvm::ArrayRef<syntax::Token> Tokens) const {
    assert(!Tokens.empty());
    assert(Tokens.back().kind() != tok::eof);
    // (!) we never consume 'eof', so looking at the next token is ok.
    if (Tokens.back().kind() != tok::semi && Tokens.end()->kind() == tok::semi)
      return llvm::makeArrayRef(Tokens.begin(), Tokens.end() + 1);
    return Tokens;
  }

  /// Finds a token starting at \p L. The token must exist.
  const syntax::Token *findToken(SourceLocation L) const;

  /// A collection of trees covering the input tokens.
  /// When created, each tree corresponds to a single token in the file.
  /// Clients call 'foldChildren' to attach one or more subtrees to a parent
  /// node and update the list of trees accordingly.
  ///
  /// Ensures that added nodes properly nest and cover the whole token stream.
  struct Forest {
    Forest(syntax::Arena &A) {
      assert(!A.tokenBuffer().expandedTokens().empty());
      assert(A.tokenBuffer().expandedTokens().back().kind() == tok::eof);
      // Create all leaf nodes.
      // Note that we do not have 'eof' in the tree.
      for (auto &T : A.tokenBuffer().expandedTokens().drop_back()) {
        auto *L = new (A.allocator()) syntax::Leaf(&T);
        L->Original = true;
        L->CanModify = A.tokenBuffer().spelledForExpanded(T).hasValue();
        Trees.insert(Trees.end(), {&T, NodeAndRole{L}});
      }
    }

    ~Forest() { assert(DelayedFolds.empty()); }

    void assignRole(llvm::ArrayRef<syntax::Token> Range,
                    syntax::NodeRole Role) {
      assert(!Range.empty());
      auto It = Trees.lower_bound(Range.begin());
      assert(It != Trees.end() && "no node found");
      assert(It->first == Range.begin() && "no child with the specified range");
      assert((std::next(It) == Trees.end() ||
              std::next(It)->first == Range.end()) &&
             "no child with the specified range");
      It->second.Role = Role;
    }

    /// Add \p Node to the forest and attach child nodes based on \p Tokens.
    void foldChildren(const syntax::Arena &A,
                      llvm::ArrayRef<syntax::Token> Tokens,
                      syntax::Tree *Node) {
      // Execute delayed folds inside `Tokens`.
      auto BeginExecuted = DelayedFolds.lower_bound(Tokens.begin());
      auto It = BeginExecuted;
      for (; It != DelayedFolds.end() && It->second.End <= Tokens.end(); ++It)
        foldChildrenEager(A, llvm::makeArrayRef(It->first, It->second.End),
                          It->second.Node);
      DelayedFolds.erase(BeginExecuted, It);

      // Attach children to `Node`.
      foldChildrenEager(A, Tokens, Node);
    }

    /// Schedule a call to `foldChildren` that will only be executed when
    /// containing node is folded. The range of delayed nodes can be extended by
    /// calling `extendDelayedFold`. Only one delayed node for each starting
    /// token is allowed.
    void foldChildrenDelayed(llvm::ArrayRef<syntax::Token> Tokens,
                             syntax::Tree *Node) {
      assert(!Tokens.empty());
      bool Inserted =
          DelayedFolds.insert({Tokens.begin(), DelayedFold{Tokens.end(), Node}})
              .second;
      (void)Inserted;
      assert(Inserted && "Multiple delayed folds start at the same token");
    }

    /// If there a delayed fold, starting at `ExtendedRange.begin()`, extends
    /// its endpoint to `ExtendedRange.end()` and returns true.
    /// Otherwise, returns false.
    bool extendDelayedFold(llvm::ArrayRef<syntax::Token> ExtendedRange) {
      assert(!ExtendedRange.empty());
      auto It = DelayedFolds.find(ExtendedRange.data());
      if (It == DelayedFolds.end())
        return false;
      assert(It->second.End <= ExtendedRange.end());
      It->second.End = ExtendedRange.end();
      return true;
    }

    // EXPECTS: all tokens were consumed and are owned by a single root node.
    syntax::Node *finalize() && {
      assert(Trees.size() == 1);
      auto *Root = Trees.begin()->second.Node;
      Trees = {};
      return Root;
    }

    std::string str(const syntax::Arena &A) const {
      std::string R;
      for (auto It = Trees.begin(); It != Trees.end(); ++It) {
        unsigned CoveredTokens =
            It != Trees.end()
                ? (std::next(It)->first - It->first)
                : A.tokenBuffer().expandedTokens().end() - It->first;

        R += llvm::formatv("- '{0}' covers '{1}'+{2} tokens\n",
                           It->second.Node->kind(),
                           It->first->text(A.sourceManager()), CoveredTokens);
        R += It->second.Node->dump(A);
      }
      return R;
    }

  private:
    /// Implementation detail of `foldChildren`, does acutal folding ignoring
    /// delayed folds.
    void foldChildrenEager(const syntax::Arena &A,
                           llvm::ArrayRef<syntax::Token> Tokens,
                           syntax::Tree *Node) {
      assert(Node->firstChild() == nullptr && "node already has children");

      auto *FirstToken = Tokens.begin();
      auto BeginChildren = Trees.lower_bound(FirstToken);
      assert((BeginChildren == Trees.end() ||
              BeginChildren->first == FirstToken) &&
             "fold crosses boundaries of existing subtrees");
      auto EndChildren = Trees.lower_bound(Tokens.end());
      assert(
          (EndChildren == Trees.end() || EndChildren->first == Tokens.end()) &&
          "fold crosses boundaries of existing subtrees");

      // (!) we need to go in reverse order, because we can only prepend.
      for (auto It = EndChildren; It != BeginChildren; --It)
        Node->prependChildLowLevel(std::prev(It)->second.Node,
                                   std::prev(It)->second.Role);

      // Mark that this node came from the AST and is backed by the source code.
      Node->Original = true;
      Node->CanModify = A.tokenBuffer().spelledForExpanded(Tokens).hasValue();

      Trees.erase(BeginChildren, EndChildren);
      Trees.insert({FirstToken, NodeAndRole(Node)});
    }
    /// A with a role that should be assigned to it when adding to a parent.
    struct NodeAndRole {
      explicit NodeAndRole(syntax::Node *Node)
          : Node(Node), Role(NodeRole::Unknown) {}

      syntax::Node *Node;
      NodeRole Role;
    };

    /// Maps from the start token to a subtree starting at that token.
    /// Keys in the map are pointers into the array of expanded tokens, so
    /// pointer order corresponds to the order of preprocessor tokens.
    /// FIXME: storing the end tokens is redundant.
    /// FIXME: the key of a map is redundant, it is also stored in NodeForRange.
    std::map<const syntax::Token *, NodeAndRole> Trees;

    /// See documentation of `foldChildrenDelayed` for details.
    struct DelayedFold {
      const syntax::Token *End = nullptr;
      syntax::Tree *Node = nullptr;
    };
    std::map<const syntax::Token *, DelayedFold> DelayedFolds;
  };

  /// For debugging purposes.
  std::string str() { return Pending.str(Arena); }

  syntax::Arena &Arena;
  /// To quickly find tokens by their start location.
  llvm::DenseMap</*SourceLocation*/ unsigned, const syntax::Token *>
      LocationToToken;
  Forest Pending;
  llvm::DenseSet<Decl *> DeclsWithoutSemicolons;
};

namespace {
class BuildTreeVisitor : public RecursiveASTVisitor<BuildTreeVisitor> {
public:
  explicit BuildTreeVisitor(ASTContext &Ctx, syntax::TreeBuilder &Builder)
      : Builder(Builder), LangOpts(Ctx.getLangOpts()) {}

  bool shouldTraversePostOrder() const { return true; }

  bool WalkUpFromDeclaratorDecl(DeclaratorDecl *D) {
    // Ensure declarators are covered by SimpleDeclaration.
    Builder.noticeDeclaratorRange(Builder.getRange(D));
    // FIXME: build nodes for the declarator too.
    return true;
  }
  bool WalkUpFromTypedefNameDecl(TypedefNameDecl *D) {
    // Also a declarator.
    Builder.noticeDeclaratorRange(Builder.getRange(D));
    // FIXME: build nodes for the declarator too.
    return true;
  }

  bool VisitDecl(Decl *D) {
    assert(!D->isImplicit());
    Builder.foldNode(Builder.getRange(D),
                     new (allocator()) syntax::UnknownDeclaration());
    return true;
  }

  bool WalkUpFromTagDecl(TagDecl *C) {
    // Avoid building UnknownDeclaration here, syntatically 'struct X {}' and
    // similar are part of declaration specifiers and do not introduce a new
    // top-level declaration.
    return true;
  }

  bool WalkUpFromTranslationUnitDecl(TranslationUnitDecl *TU) {
    // (!) we do not want to call VisitDecl(), the declaration for translation
    // unit is built by finalize().
    return true;
  }

  bool WalkUpFromCompoundStmt(CompoundStmt *S) {
    using NodeRole = syntax::NodeRole;

    Builder.markChildToken(S->getLBracLoc(), NodeRole::OpenParen);
    for (auto *Child : S->body())
      Builder.markStmtChild(Child, NodeRole::CompoundStatement_statement);
    Builder.markChildToken(S->getRBracLoc(), NodeRole::CloseParen);

    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::CompoundStatement);
    return true;
  }

  // Some statements are not yet handled by syntax trees.
  bool WalkUpFromStmt(Stmt *S) {
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::UnknownStatement);
    return true;
  }

  bool TraverseCXXForRangeStmt(CXXForRangeStmt *S) {
    // We override to traverse range initializer as VarDecl.
    // RAV traverses it as a statement, we produce invalid node kinds in that
    // case.
    // FIXME: should do this in RAV instead?
    if (S->getInit() && !TraverseStmt(S->getInit()))
      return false;
    if (S->getLoopVariable() && !TraverseDecl(S->getLoopVariable()))
      return false;
    if (S->getRangeInit() && !TraverseStmt(S->getRangeInit()))
      return false;
    if (S->getBody() && !TraverseStmt(S->getBody()))
      return false;
    return true;
  }

  bool TraverseStmt(Stmt *S) {
    if (auto *DS = llvm::dyn_cast_or_null<DeclStmt>(S)) {
      // We want to consume the semicolon, make sure SimpleDeclaration does not.
      for (auto *D : DS->decls())
        Builder.noticeDeclaratorWithoutSemicolon(D);
    } else if (auto *E = llvm::dyn_cast_or_null<Expr>(S)) {
      // (!) do not recurse into subexpressions.
      // we do not have syntax trees for expressions yet, so we only want to see
      // the first top-level expression.
      return WalkUpFromExpr(E->IgnoreImplicit());
    }
    return RecursiveASTVisitor::TraverseStmt(S);
  }

  // Some expressions are not yet handled by syntax trees.
  bool WalkUpFromExpr(Expr *E) {
    assert(!isImplicitExpr(E) && "should be handled by TraverseStmt");
    Builder.foldNode(Builder.getExprRange(E),
                     new (allocator()) syntax::UnknownExpression);
    return true;
  }

  bool WalkUpFromNamespaceDecl(NamespaceDecl *S) {
    auto Tokens = Builder.getRange(S);
    if (Tokens.front().kind() == tok::coloncolon) {
      // Handle nested namespace definitions. Those start at '::' token, e.g.
      // namespace a^::b {}
      // FIXME: build corresponding nodes for the name of this namespace.
      return true;
    }
    Builder.foldNode(Tokens, new (allocator()) syntax::NamespaceDefinition);
    return true;
  }

  // The code below is very regular, it could even be generated with some
  // preprocessor magic. We merely assign roles to the corresponding children
  // and fold resulting nodes.
  bool WalkUpFromDeclStmt(DeclStmt *S) {
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::DeclarationStatement);
    return true;
  }

  bool WalkUpFromNullStmt(NullStmt *S) {
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::EmptyStatement);
    return true;
  }

  bool WalkUpFromSwitchStmt(SwitchStmt *S) {
    Builder.markChildToken(S->getSwitchLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::SwitchStatement);
    return true;
  }

  bool WalkUpFromCaseStmt(CaseStmt *S) {
    Builder.markChildToken(S->getKeywordLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markExprChild(S->getLHS(), syntax::NodeRole::CaseStatement_value);
    Builder.markStmtChild(S->getSubStmt(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::CaseStatement);
    return true;
  }

  bool WalkUpFromDefaultStmt(DefaultStmt *S) {
    Builder.markChildToken(S->getKeywordLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getSubStmt(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::DefaultStatement);
    return true;
  }

  bool WalkUpFromIfStmt(IfStmt *S) {
    Builder.markChildToken(S->getIfLoc(), syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getThen(),
                          syntax::NodeRole::IfStatement_thenStatement);
    Builder.markChildToken(S->getElseLoc(),
                           syntax::NodeRole::IfStatement_elseKeyword);
    Builder.markStmtChild(S->getElse(),
                          syntax::NodeRole::IfStatement_elseStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::IfStatement);
    return true;
  }

  bool WalkUpFromForStmt(ForStmt *S) {
    Builder.markChildToken(S->getForLoc(), syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::ForStatement);
    return true;
  }

  bool WalkUpFromWhileStmt(WhileStmt *S) {
    Builder.markChildToken(S->getWhileLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::WhileStatement);
    return true;
  }

  bool WalkUpFromContinueStmt(ContinueStmt *S) {
    Builder.markChildToken(S->getContinueLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::ContinueStatement);
    return true;
  }

  bool WalkUpFromBreakStmt(BreakStmt *S) {
    Builder.markChildToken(S->getBreakLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::BreakStatement);
    return true;
  }

  bool WalkUpFromReturnStmt(ReturnStmt *S) {
    Builder.markChildToken(S->getReturnLoc(),
                           syntax::NodeRole::IntroducerKeyword);
    Builder.markExprChild(S->getRetValue(),
                          syntax::NodeRole::ReturnStatement_value);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::ReturnStatement);
    return true;
  }

  bool WalkUpFromCXXForRangeStmt(CXXForRangeStmt *S) {
    Builder.markChildToken(S->getForLoc(), syntax::NodeRole::IntroducerKeyword);
    Builder.markStmtChild(S->getBody(), syntax::NodeRole::BodyStatement);
    Builder.foldNode(Builder.getStmtRange(S),
                     new (allocator()) syntax::RangeBasedForStatement);
    return true;
  }

  bool WalkUpFromEmptyDecl(EmptyDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::EmptyDeclaration);
    return true;
  }

  bool WalkUpFromStaticAssertDecl(StaticAssertDecl *S) {
    Builder.markExprChild(S->getAssertExpr(),
                          syntax::NodeRole::StaticAssertDeclaration_condition);
    Builder.markExprChild(S->getMessage(),
                          syntax::NodeRole::StaticAssertDeclaration_message);
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::StaticAssertDeclaration);
    return true;
  }

  bool WalkUpFromLinkageSpecDecl(LinkageSpecDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::LinkageSpecificationDeclaration);
    return true;
  }

  bool WalkUpFromNamespaceAliasDecl(NamespaceAliasDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::NamespaceAliasDefinition);
    return true;
  }

  bool WalkUpFromUsingDirectiveDecl(UsingDirectiveDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::UsingNamespaceDirective);
    return true;
  }

  bool WalkUpFromUsingDecl(UsingDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::UsingDeclaration);
    return true;
  }

  bool WalkUpFromUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::UsingDeclaration);
    return true;
  }

  bool WalkUpFromUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::UsingDeclaration);
    return true;
  }

  bool WalkUpFromTypeAliasDecl(TypeAliasDecl *S) {
    Builder.foldNode(Builder.getRange(S),
                     new (allocator()) syntax::TypeAliasDeclaration);
    return true;
  }

private:
  /// A small helper to save some typing.
  llvm::BumpPtrAllocator &allocator() { return Builder.allocator(); }

  syntax::TreeBuilder &Builder;
  const LangOptions &LangOpts;
};
} // namespace

void syntax::TreeBuilder::foldNode(llvm::ArrayRef<syntax::Token> Range,
                                   syntax::Tree *New) {
  Pending.foldChildren(Arena, Range, New);
}

void syntax::TreeBuilder::noticeDeclaratorRange(
    llvm::ArrayRef<syntax::Token> Range) {
  if (Pending.extendDelayedFold(Range))
    return;
  Pending.foldChildrenDelayed(Range,
                              new (allocator()) syntax::SimpleDeclaration);
}

void syntax::TreeBuilder::noticeDeclaratorWithoutSemicolon(Decl *D) {
  DeclsWithoutSemicolons.insert(D);
}

void syntax::TreeBuilder::markChildToken(SourceLocation Loc, NodeRole Role) {
  if (Loc.isInvalid())
    return;
  Pending.assignRole(*findToken(Loc), Role);
}

void syntax::TreeBuilder::markStmtChild(Stmt *Child, NodeRole Role) {
  if (!Child)
    return;

  auto Range = getStmtRange(Child);
  // This is an expression in a statement position, consume the trailing
  // semicolon and form an 'ExpressionStatement' node.
  if (auto *E = dyn_cast<Expr>(Child)) {
    Pending.assignRole(getExprRange(E),
                       NodeRole::ExpressionStatement_expression);
    // (!) 'getRange(Stmt)' ensures this already covers a trailing semicolon.
    Pending.foldChildren(Arena, Range,
                         new (allocator()) syntax::ExpressionStatement);
  }
  Pending.assignRole(Range, Role);
}

void syntax::TreeBuilder::markExprChild(Expr *Child, NodeRole Role) {
  if (!Child)
    return;

  Pending.assignRole(getExprRange(Child), Role);
}

const syntax::Token *syntax::TreeBuilder::findToken(SourceLocation L) const {
  auto It = LocationToToken.find(L.getRawEncoding());
  assert(It != LocationToToken.end());
  return It->second;
}

syntax::TranslationUnit *
syntax::buildSyntaxTree(Arena &A, const TranslationUnitDecl &TU) {
  TreeBuilder Builder(A);
  BuildTreeVisitor(TU.getASTContext(), Builder).TraverseAST(TU.getASTContext());
  return std::move(Builder).finalize();
}