[SyntaxTree][Synthesis] Fix allocation in `createTree` for more general use

[eduucaldas]

Summary

Prior to this change createTree could not create arbitrary syntax
trees. Now it dispatches to the constructor of the concrete syntax tree
according to the NodeKind passed as argument. This allows reuse inside
the Synthesis API.

[eduucaldas]

Fix comment

clang/include/clang/Tooling/Syntax/BuildTree.h

 syntax::Leaf *createLeaf(syntax::Arena &A, tok::TokenKind K,
                          StringRef Spelling);
 
 /// Infer the token spelling from its `TokenKind`, then create `Leaf` from
 /// this token
 syntax::Leaf *createLeaf(syntax::Arena &A, tok::TokenKind K);
 
 // Synthesis of Trees
+/// Creates the concrete syntax node according to the specified `NodeKind` `K`.
+/// Returns it as a pointer to the base class `Tree`.
 syntax::Tree *
-createTree(Arena &A,
+createTree(syntax::Arena &A,
            std::vector<std::pair<syntax::Node *, syntax::NodeRole>> Children,
            syntax::NodeKind K);
 
 // Synthesis of Syntax Nodes
-clang::syntax::EmptyStatement *createEmptyStatement(clang::syntax::Arena &A);
+syntax::EmptyStatement *createEmptyStatement(syntax::Arena &A);
 
 } // namespace syntax
 } // namespace clang
 #endif

clang/lib/Tooling/Syntax/Synthesis.cpp

syntax::Leaf *clang::syntax::createLeaf(syntax::Arena &A, tok::TokenKind K) {
   const auto *Spelling = tok::getPunctuatorSpelling(K);
   if (!Spelling)
     Spelling = tok::getKeywordSpelling(K);
   assert(Spelling &&
          "Cannot infer the spelling of the token from its token kind.");
   return createLeaf(A, K, Spelling);
 }
 
+namespace {
+// Allocates the concrete syntax `Tree` according to its `NodeKind`.
+syntax::Tree *allocateTree(syntax::Arena &A, syntax::NodeKind Kind) {
+  switch (Kind) {
+  case syntax::NodeKind::Leaf:
+    assert(false);
+  case syntax::NodeKind::TranslationUnit:
+    return new (A.getAllocator()) syntax::TranslationUnit;
+  case syntax::NodeKind::UnknownExpression:
+    return new (A.getAllocator()) syntax::UnknownExpression;
+  case syntax::NodeKind::ParenExpression:
+    return new (A.getAllocator()) syntax::ParenExpression;
+  case syntax::NodeKind::ThisExpression:
+    return new (A.getAllocator()) syntax::ThisExpression;
+  case syntax::NodeKind::IntegerLiteralExpression:
+    return new (A.getAllocator()) syntax::IntegerLiteralExpression;
+  case syntax::NodeKind::CharacterLiteralExpression:
+    return new (A.getAllocator()) syntax::CharacterLiteralExpression;
+  case syntax::NodeKind::FloatingLiteralExpression:
+    return new (A.getAllocator()) syntax::FloatingLiteralExpression;
+  case syntax::NodeKind::StringLiteralExpression:
+    return new (A.getAllocator()) syntax::StringLiteralExpression;
+  case syntax::NodeKind::BoolLiteralExpression:
+    return new (A.getAllocator()) syntax::BoolLiteralExpression;
+  case syntax::NodeKind::CxxNullPtrExpression:
+    return new (A.getAllocator()) syntax::CxxNullPtrExpression;
+  case syntax::NodeKind::IntegerUserDefinedLiteralExpression:
+    return new (A.getAllocator()) syntax::IntegerUserDefinedLiteralExpression;
+  case syntax::NodeKind::FloatUserDefinedLiteralExpression:
+    return new (A.getAllocator()) syntax::FloatUserDefinedLiteralExpression;
+  case syntax::NodeKind::CharUserDefinedLiteralExpression:
+    return new (A.getAllocator()) syntax::CharUserDefinedLiteralExpression;
+  case syntax::NodeKind::StringUserDefinedLiteralExpression:
+    return new (A.getAllocator()) syntax::StringUserDefinedLiteralExpression;
+  case syntax::NodeKind::PrefixUnaryOperatorExpression:
+    return new (A.getAllocator()) syntax::PrefixUnaryOperatorExpression;
+  case syntax::NodeKind::PostfixUnaryOperatorExpression:
+    return new (A.getAllocator()) syntax::PostfixUnaryOperatorExpression;
+  case syntax::NodeKind::BinaryOperatorExpression:
+    return new (A.getAllocator()) syntax::BinaryOperatorExpression;
+  case syntax::NodeKind::UnqualifiedId:
+    return new (A.getAllocator()) syntax::UnqualifiedId;
+  case syntax::NodeKind::IdExpression:
+    return new (A.getAllocator()) syntax::IdExpression;
+  case syntax::NodeKind::CallExpression:
+    return new (A.getAllocator()) syntax::CallExpression;
+  case syntax::NodeKind::UnknownStatement:
+    return new (A.getAllocator()) syntax::UnknownStatement;
+  case syntax::NodeKind::DeclarationStatement:
+    return new (A.getAllocator()) syntax::DeclarationStatement;
+  case syntax::NodeKind::EmptyStatement:
+    return new (A.getAllocator()) syntax::EmptyStatement;
+  case syntax::NodeKind::SwitchStatement:
+    return new (A.getAllocator()) syntax::SwitchStatement;
+  case syntax::NodeKind::CaseStatement:
+    return new (A.getAllocator()) syntax::CaseStatement;
+  case syntax::NodeKind::DefaultStatement:
+    return new (A.getAllocator()) syntax::DefaultStatement;
+  case syntax::NodeKind::IfStatement:
+    return new (A.getAllocator()) syntax::IfStatement;
+  case syntax::NodeKind::ForStatement:
+    return new (A.getAllocator()) syntax::ForStatement;
+  case syntax::NodeKind::WhileStatement:
+    return new (A.getAllocator()) syntax::WhileStatement;
+  case syntax::NodeKind::ContinueStatement:
+    return new (A.getAllocator()) syntax::ContinueStatement;
+  case syntax::NodeKind::BreakStatement:
+    return new (A.getAllocator()) syntax::BreakStatement;
+  case syntax::NodeKind::ReturnStatement:
+    return new (A.getAllocator()) syntax::ReturnStatement;
+  case syntax::NodeKind::RangeBasedForStatement:
+    return new (A.getAllocator()) syntax::RangeBasedForStatement;
+  case syntax::NodeKind::ExpressionStatement:
+    return new (A.getAllocator()) syntax::ExpressionStatement;
+  case syntax::NodeKind::CompoundStatement:
+    return new (A.getAllocator()) syntax::CompoundStatement;
+  case syntax::NodeKind::UnknownDeclaration:
+    return new (A.getAllocator()) syntax::UnknownDeclaration;
+  case syntax::NodeKind::EmptyDeclaration:
+    return new (A.getAllocator()) syntax::EmptyDeclaration;
+  case syntax::NodeKind::StaticAssertDeclaration:
+    return new (A.getAllocator()) syntax::StaticAssertDeclaration;
+  case syntax::NodeKind::LinkageSpecificationDeclaration:
+    return new (A.getAllocator()) syntax::LinkageSpecificationDeclaration;
+  case syntax::NodeKind::SimpleDeclaration:
+    return new (A.getAllocator()) syntax::SimpleDeclaration;
+  case syntax::NodeKind::TemplateDeclaration:
+    return new (A.getAllocator()) syntax::TemplateDeclaration;
+  case syntax::NodeKind::ExplicitTemplateInstantiation:
+    return new (A.getAllocator()) syntax::ExplicitTemplateInstantiation;
+  case syntax::NodeKind::NamespaceDefinition:
+    return new (A.getAllocator()) syntax::NamespaceDefinition;
+  case syntax::NodeKind::NamespaceAliasDefinition:
+    return new (A.getAllocator()) syntax::NamespaceAliasDefinition;
+  case syntax::NodeKind::UsingNamespaceDirective:
+    return new (A.getAllocator()) syntax::UsingNamespaceDirective;
+  case syntax::NodeKind::UsingDeclaration:
+    return new (A.getAllocator()) syntax::UsingDeclaration;
+  case syntax::NodeKind::TypeAliasDeclaration:
+    return new (A.getAllocator()) syntax::TypeAliasDeclaration;
+  case syntax::NodeKind::SimpleDeclarator:
+    return new (A.getAllocator()) syntax::SimpleDeclarator;
+  case syntax::NodeKind::ParenDeclarator:
+    return new (A.getAllocator()) syntax::ParenDeclarator;
+  case syntax::NodeKind::ArraySubscript:
+    return new (A.getAllocator()) syntax::ArraySubscript;
+  case syntax::NodeKind::TrailingReturnType:
+    return new (A.getAllocator()) syntax::TrailingReturnType;
+  case syntax::NodeKind::ParametersAndQualifiers:
+    return new (A.getAllocator()) syntax::ParametersAndQualifiers;
+  case syntax::NodeKind::MemberPointer:
+    return new (A.getAllocator()) syntax::MemberPointer;
+  case syntax::NodeKind::GlobalNameSpecifier:
+    return new (A.getAllocator()) syntax::GlobalNameSpecifier;
+  case syntax::NodeKind::DecltypeNameSpecifier:
+    return new (A.getAllocator()) syntax::DecltypeNameSpecifier;
+  case syntax::NodeKind::IdentifierNameSpecifier:
+    return new (A.getAllocator()) syntax::IdentifierNameSpecifier;
+  case syntax::NodeKind::SimpleTemplateNameSpecifier:
+    return new (A.getAllocator()) syntax::SimpleTemplateNameSpecifier;
+  case syntax::NodeKind::NestedNameSpecifier:
+    return new (A.getAllocator()) syntax::NestedNameSpecifier;
+  case syntax::NodeKind::MemberExpression:
+    return new (A.getAllocator()) syntax::MemberExpression;
+  case syntax::NodeKind::CallArguments:
+    return new (A.getAllocator()) syntax::CallArguments;
+  case syntax::NodeKind::ParameterDeclarationList:
+    return new (A.getAllocator()) syntax::ParameterDeclarationList;
+  }
+  llvm_unreachable("unknown node kind");
+}
+} // namespace
+
 syntax::Tree *clang::syntax::createTree(
     syntax::Arena &A,
     std::vector<std::pair<syntax::Node *, syntax::NodeRole>> Children,
     syntax::NodeKind K) {
-  auto *T = new (A.getAllocator()) syntax::Tree(K);
+  auto *T = allocateTree(A, K);
   FactoryImpl::setCanModify(T);
   for (auto ChildIt = Children.rbegin(); ChildIt != Children.rend();
        std::advance(ChildIt, 1))
     FactoryImpl::prependChildLowLevel(T, ChildIt->first, ChildIt->second);
 
   T->assertInvariants();
   return T;
 }
 
 syntax::EmptyStatement *clang::syntax::createEmptyStatement(syntax::Arena &A) {
-  auto *S = new (A.getAllocator()) syntax::EmptyStatement;
-  FactoryImpl::setCanModify(S);
-  FactoryImpl::prependChildLowLevel(S, createLeaf(A, tok::semi),
-                                    NodeRole::Unknown);
-  S->assertInvariants();
-  return S;
+  return cast<EmptyStatement>(
+      createTree(A, {{createLeaf(A, tok::semi), NodeRole::Unknown}},
+                 NodeKind::EmptyStatement));
 }