[vala/wip/issue/987] parser: Avoid mitigation for missing token if it follows DOT or DOUBLE_COLON

[Rico Tzschichholz <ricotz src gnome org>]

commit aca11ebb702d67e641fd52f5c012749e2949297e
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Tue Jun 16 14:55:04 2020 +0200

    parser: Avoid mitigation for missing token if it follows DOT or DOUBLE_COLON
    
    Rolling back to a DOT/DOUBLE_COLON token will retrigger the attempt to parse
    an incomplete member access.
    
    Regression of 4368a4fa667148378dcdbd251a4ae4e00c9a8e5a
    
    Fixes https://gitlab.gnome.org/GNOME/vala/issues/987

 vala/valaparser.vala | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)
---
diff --git a/vala/valaparser.vala b/vala/valaparser.vala
index 191bc75d5..3adc6e71a 100644
--- a/vala/valaparser.vala
+++ b/vala/valaparser.vala
@@ -104,10 +104,25 @@ public class Vala.Parser : CodeVisitor {
                assert (size <= BUFFER_SIZE);
        }
 
+       inline void safe_prev () {
+               switch (previous ()) {
+               case TokenType.DOT:
+               case TokenType.DOUBLE_COLON:
+                       break;
+               default:
+                       prev ();
+                       break;
+               }
+       }
+
        inline TokenType current () {
                return tokens[index].type;
        }
 
+       inline TokenType previous () {
+               return tokens[(index - 1 + BUFFER_SIZE) % BUFFER_SIZE].type;
+       }
+
        inline bool accept (TokenType type) {
                if (current () == type) {
                        next ();
@@ -129,13 +144,13 @@ public class Vala.Parser : CodeVisitor {
 
                switch (type) {
                case TokenType.CLOSE_BRACE:
-                       prev ();
+                       safe_prev ();
                        report_parse_error (new ParseError.SYNTAX ("following block delimiter %s missing", 
type.to_string ()));
                        return true;
                case TokenType.CLOSE_BRACKET:
                case TokenType.CLOSE_PARENS:
                case TokenType.SEMICOLON:
-                       prev ();
+                       safe_prev ();
                        report_parse_error (new ParseError.SYNTAX ("following expression/statement delimiter 
%s missing", type.to_string ()));
                        return true;
                default:
@@ -1686,9 +1701,7 @@ public class Vala.Parser : CodeVisitor {
                try {
                        skip_type ();
                } catch (ParseError e) {
-                       prev ();
-                       var token = current ();
-                       next ();
+                       var token = previous ();
                        if (token == TokenType.DOT || token == TokenType.DOUBLE_COLON) {
                                rollback (begin);
                                return true;