Merge pull request swiftlang#36386 from ahoppen/pr/cancellable-backtracking-scope

[Parser] Distinguish between backtracking scopes that can be cancelled or not

Author: ahoppen

include/swift/Parse/Parser.h

@@ -463,7 +463,11 @@ class Parser {
   /// RAII object that, when it is destructed, restores the parser and lexer to
   /// their positions at the time the object was constructed.  Will not jump
   /// forward in the token stream.
-  class BacktrackingScope {
+  /// Actual uses of the backtracking scope should choose either \c
+  /// BacktrackingScope, which will always backtrack or \c
+  /// CancellableBacktrackingScope which can be cancelled.
+  class BacktrackingScopeImpl {
+  protected:
     Parser &P;
     ParserPosition PP;
     DiagnosticTransaction DT;
@@ -501,16 +505,32 @@ class Parser {
       }
     } TempReceiver;
 
-  public:
-    BacktrackingScope(Parser &P)
+    BacktrackingScopeImpl(Parser &P)
         : P(P), PP(P.getParserPosition()), DT(P.Diags),
           TempReceiver(P.TokReceiver) {
       SynContext.emplace(P.SyntaxContext);
-      SynContext->setBackTracking();
     }
 
-    ~BacktrackingScope();
+  public:
+    ~BacktrackingScopeImpl();
     bool willBacktrack() const { return Backtrack; }
+  };
+
+  /// A backtracking scope that will always backtrack when destructed.
+  class BacktrackingScope final : public BacktrackingScopeImpl {
+  public:
+    BacktrackingScope(Parser &P) : BacktrackingScopeImpl(P) {
+      SynContext->disable();
+    }
+  };
+
+  /// A backtracking scope whose backtracking can be disabled by calling
+  /// \c cancelBacktrack.
+  class CancellableBacktrackingScope final : public BacktrackingScopeImpl {
+  public:
+    CancellableBacktrackingScope(Parser &P) : BacktrackingScopeImpl(P) {
+      SynContext->setBackTracking();
+    }
 
     void cancelBacktrack();
   };

include/swift/SyntaxParse/SyntaxTreeCreator.h

@@ -48,8 +48,6 @@ class SyntaxTreeCreator final : public SyntaxParseActions {
   /// tree.
   SyntaxParsingCache *SyntaxCache;
 
-  llvm::BumpPtrAllocator ScratchAlloc;
-
 public:
   SyntaxTreeCreator(SourceManager &SM, unsigned bufferID,
                     SyntaxParsingCache *syntaxCache,

lib/Parse/ParseDecl.cpp

@@ -1120,7 +1120,7 @@ static Optional<AccessorKind> isAccessorLabel(const Token &token) {
 /// or to `nullptr` if not. A missing base type is not considered an error.
 static bool parseBaseTypeForQualifiedDeclName(Parser &P, TypeRepr *&baseType) {
   baseType = nullptr;
-  Parser::BacktrackingScope backtrack(P);
+  Parser::CancellableBacktrackingScope backtrack(P);
 
   // If base type cannot be parsed, return false (no error).
   if (!P.canParseBaseTypeForQualifiedDeclName())
@@ -3028,7 +3028,7 @@ bool Parser::canParseTypeAttribute() {
 /// \returns true on error, false on success
 static bool parseDifferentiableTypeAttributeArgument(
     Parser &P, TypeAttributes &Attributes, bool emitDiagnostics) {
-  Parser::BacktrackingScope backtrack(P);
+  Parser::CancellableBacktrackingScope backtrack(P);
 
   // Match '( <identifier> )', and store the identifier token to `argument`.
   if (!P.consumeIf(tok::l_paren))
@@ -3226,7 +3226,7 @@ bool Parser::parseTypeAttribute(TypeAttributes &Attributes, SourceLoc AtLoc,
     SyntaxParsingContext TokListContext(SyntaxContext, SyntaxKind::TokenList);
 
     if (Tok.is(tok::l_paren) && getEndOfPreviousLoc() == Tok.getLoc()) {
-      BacktrackingScope backtrack(*this);
+      CancellableBacktrackingScope backtrack(*this);
       skipSingle();
       // If we found '->', or 'throws' after paren, it's likely a parameter
       // of function type.
@@ -5046,7 +5046,7 @@ bool Parser::canDelayMemberDeclParsing(bool &HasOperatorDeclarations,
 
   // Skip until the matching right curly bracket; if we find a pound directive,
   // we can't lazily parse.
-  BacktrackingScope BackTrack(*this);
+  CancellableBacktrackingScope BackTrack(*this);
   bool HasPoundDirective;
   bool HasNestedTypeDeclarations;
   skipUntilMatchingRBrace(*this,
@@ -5975,7 +5975,7 @@ ParserStatus Parser::parseGetSet(ParseDeclOptions Flags,
   };
 
   // Prepare backtracking for implicit getter.
-  Optional<BacktrackingScope> backtrack;
+  Optional<CancellableBacktrackingScope> backtrack;
   backtrack.emplace(*this);
 
   bool Invalid = false;
@@ -7114,7 +7114,7 @@ Parser::parseDeclEnumCase(ParseDeclOptions Flags,
       // For recovery, see if the user typed something resembling a switch
       // "case" label.
       {
-        BacktrackingScope backtrack(*this);
+        CancellableBacktrackingScope backtrack(*this);
         llvm::SaveAndRestore<decltype(InVarOrLetPattern)>
         T(InVarOrLetPattern, Parser::IVOLP_InMatchingPattern);
         parseMatchingPattern(/*isExprBasic*/false);

lib/Parse/ParseExpr.cpp

@@ -2083,7 +2083,7 @@ static bool tryParseArgLabelList(Parser &P, Parser::DeclNameOptions flags,
 
   // Try to parse a compound name.
   SyntaxParsingContext ArgsCtxt(P.SyntaxContext, SyntaxKind::DeclNameArguments);
-  Parser::BacktrackingScope backtrack(P);
+  Parser::CancellableBacktrackingScope backtrack(P);
 
   lparenLoc = P.consumeToken(tok::l_paren);
   while (P.Tok.isNot(tok::r_paren)) {

lib/Parse/ParsePattern.cpp

@@ -968,8 +968,8 @@ ParserResult<Pattern> Parser::parseTypedPattern() {
       // Disable this tentative parse when in code-completion mode, otherwise
       // code-completion may enter the delayed-decl state twice.
       if (Tok.isFollowingLParen() && !L->isCodeCompletion()) {
-        BacktrackingScope backtrack(*this);
-        
+        CancellableBacktrackingScope backtrack(*this);
+
         // Create a local context if needed so we can parse trailing closures.
         LocalContext dummyContext;
         Optional<ContextChange> contextChange;

lib/Parse/ParseType.cpp

@@ -999,7 +999,7 @@ ParserResult<TypeRepr> Parser::parseTypeTupleBody() {
     // 'inout' here can be a obsoleted use of the marker in an argument list,
     // consume it in backtracking context so we can determine it's really a
     // deprecated use of it.
-    llvm::Optional<BacktrackingScope> Backtracking;
+    llvm::Optional<CancellableBacktrackingScope> Backtracking;
     SourceLoc ObsoletedInOutLoc;
     if (Tok.is(tok::kw_inout)) {
       Backtracking.emplace(*this);

lib/Parse/Parser.cpp

@@ -214,14 +214,14 @@ void Parser::performCodeCompletionSecondPassImpl(
   State->restoreCodeCompletionDelayedDeclState(info);
 }
 
-swift::Parser::BacktrackingScope::~BacktrackingScope() {
+swift::Parser::BacktrackingScopeImpl::~BacktrackingScopeImpl() {
   if (Backtrack) {
     P.backtrackToPosition(PP);
     DT.abort();
   }
 }
 
-void swift::Parser::BacktrackingScope::cancelBacktrack() {
+void swift::Parser::CancellableBacktrackingScope::cancelBacktrack() {
   if (!Backtrack)
     return;
 
@@ -783,8 +783,8 @@ void Parser::skipListUntilDeclRBrace(SourceLoc startLoc, tok T1, tok T2) {
         if (possibleDeclStartsLine && !hasDelimiter) {
           break;
         }
-        
-        Parser::BacktrackingScope backtrack(*this);
+
+        Parser::CancellableBacktrackingScope backtrack(*this);
         // Consume the let or var
         consumeToken();