C++: Stop parsing a declaration after two tries

If we fail to parse a declaration, we rewind, eat the token and look for the
next token that might be a good candidate for a declaration start (e.g. an
identifier). This becomes cpu and memory expensive with super long and invalid
expressions like

    typedef b:Ⓜ️:if_< b:Ⓜ️:bool_<
	(sizeof(fun((Dummy *) 0, (ThisT *) 0, (b:Ⓜ️:int_<70> *) 0)) ==
	 sizeof(defined_)) >, b:Ⓜ️:if_< b:Ⓜ️:bool_<
	(sizeof(fun((Dummy *) 0, (ThisT *) 0, (b:Ⓜ️:int_<71> *) 0)) ==
	 sizeof(defined_)) >, b:Ⓜ️:if_< b:Ⓜ️:bool_<
	(sizeof(fun((Dummy *) 0, (ThisT *) 0, (b:Ⓜ️:int_<72> *) 0)) ==
	 sizeof(defined_)) >, b:Ⓜ️:if_< b:Ⓜ️:bool_<
	(sizeof(fun((Dummy *) 0, (ThisT *) 0, (b:Ⓜ️:int_<73> *) 0)) ==
	 sizeof(defined_)) >, b:Ⓜ️:if_< b:Ⓜ️:bool_<
	(sizeof(fun((Dummy *) 0, (ThisT *) 0, (b:Ⓜ️:int_<74> *) 0)) ==
	 sizeof(defined_)) >, b:Ⓜ️:if_< b:Ⓜ️:bool_<
	(sizeof(fun((Dummy *) 0, (ThisT *) 0, (b:Ⓜ️:int_<75> *) 0)) ==
	 sizeof(defined_)) >, b:Ⓜ️:if_< b:Ⓜ️:bool_<
	// ...some more crazy lines like this

Therefore, stop trying after two failures by looking for the next semicolon or
closing curly brace.

Task-number: QTCREATORBUG-12890
Change-Id: I6637daeb840dd549d669080775228fa91fc932eb
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/libs/3rdparty/cplusplus/Parser.cpp

@@ -66,6 +66,8 @@ public:
 
 int DebugRule::depth = 0;
 
+const int declarationsInRowAllowedToFail = 2;
+
 inline bool lookAtAssignmentOperator(int tokenKind)
 {
     switch (tokenKind) {
@@ -310,6 +312,20 @@ bool Parser::skipUntil(int token)
     return false;
 }
 
+void Parser::skipUntilAfterSemicolonOrRightBrace()
+{
+    while (int tk = LA()) {
+        switch (tk) {
+            case T_SEMICOLON:
+            case T_RBRACE:
+                consumeToken();
+                return;
+            default:
+                consumeToken();
+        }
+    }
+}
+
 void Parser::skipUntilDeclaration()
 {
     for (; ; consumeToken()) {
@@ -626,18 +642,24 @@ bool Parser::parseTranslationUnit(TranslationUnitAST *&node)
     TranslationUnitAST *ast = new (_pool) TranslationUnitAST;
     DeclarationListAST **decl = &ast->declaration_list;
 
+    int declarationsInRowFailedToParse = 0;
+
     while (LA()) {
         unsigned start_declaration = cursor();
 
         DeclarationAST *declaration = 0;
 
         if (parseDeclaration(declaration)) {
+            declarationsInRowFailedToParse = 0;
             *decl = new (_pool) DeclarationListAST;
             (*decl)->value = declaration;
             decl = &(*decl)->next;
         } else {
             error(start_declaration, "expected a declaration");
             rewind(start_declaration + 1);
+            if (++declarationsInRowFailedToParse == declarationsInRowAllowedToFail)
+                skipUntilAfterSemicolonOrRightBrace();
+            else
                 skipUntilDeclaration();
         }
 
@@ -787,6 +809,8 @@ bool Parser::parseLinkageBody(DeclarationAST *&node)
         ast->lbrace_token = consumeToken();
         DeclarationListAST **declaration_ptr = &ast->declaration_list;
 
+        int declarationsInRowFailedToParse = 0;
+
         while (int tk = LA()) {
             if (tk == T_RBRACE)
                 break;
@@ -794,12 +818,16 @@ bool Parser::parseLinkageBody(DeclarationAST *&node)
             unsigned start_declaration = cursor();
             DeclarationAST *declaration = 0;
             if (parseDeclaration(declaration)) {
+                declarationsInRowFailedToParse = 0;
                 *declaration_ptr = new (_pool) DeclarationListAST;
                 (*declaration_ptr)->value = declaration;
                 declaration_ptr = &(*declaration_ptr)->next;
             } else {
                 error(start_declaration, "expected a declaration");
                 rewind(start_declaration + 1);
+                if (++declarationsInRowFailedToParse == declarationsInRowAllowedToFail)
+                    skipUntilAfterSemicolonOrRightBrace();
+                else
                     skipUntilDeclaration();
             }
 

src/libs/3rdparty/cplusplus/Parser.h

@@ -254,6 +254,7 @@ public:
     bool parseDesignator(DesignatorAST *&node);
 
     bool skipUntil(int token);
+    void skipUntilAfterSemicolonOrRightBrace();
     void skipUntilDeclaration();
     bool skipUntilStatement();
     bool skip(int l, int r);