gccrs: Changes made to ensure that '=>' token is parsed properly

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_expr): made change to parsing logic so that it will stop when encountered a '=>' or ',' while having restrictions.stop_on_token turned on.
	(Parser::left_denotations): Same as above stopping parsing on above special tokens.
	(Parser::null_denotation): Same as above stopping parsing on above special tokens.
	* parse/rust-parse.h (struct ParseRestrictions): Made a field stop_on_token (default set to false) so that the above functionality is maintained.

gcc/rust/parse/rust-parse-impl.h

@@ -12023,6 +12023,11 @@ Parser<ManagedTokenSource>::parse_expr (int right_binding_power,
   if (restrictions.expr_can_be_null)
     {
       TokenId id = current_token->get_id ();
+      if (restrictions.stop_on_token && (id == MATCH_ARROW || id == COMMA))
+        {
+          rust_debug ("Stopping Parsing at special token");
+          return nullptr;
+        }
       if (id == SEMICOLON || id == RIGHT_PAREN || id == RIGHT_CURLY
 	  || id == RIGHT_SQUARE || id == COMMA || id == LEFT_CURLY)
 	return nullptr;
@@ -12078,6 +12083,13 @@ Parser<ManagedTokenSource>::left_denotations (std::unique_ptr<AST::Expr> expr,
   while (right_binding_power < left_binding_power (current_token))
     {
       lexer.skip_token ();
+      if (restrictions.stop_on_token
+          && (current_token->get_id() == MATCH_ARROW 
+              || current_token->get_id() == COMMA))
+        {
+            rust_debug ("Stopping parsing at restricted token in left_denotations");
+            return expr;
+        }
 
       // FIXME attributes should generally be applied to the null denotation.
       expr = left_denotation (current_token, std::move (expr),
@@ -12115,6 +12127,13 @@ Parser<ManagedTokenSource>::null_denotation (const_TokenPtr tok,
 {
   /* note: tok is previous character in input stream, not current one, as
    * parse_expr skips it before passing it in */
+
+  if (restrictions.stop_on_token 
+      && (tok->get_id() == MATCH_ARROW || tok->get_id() == COMMA))
+    {
+        rust_debug("Stopping parsing at restricted token in null_denotation");
+        return nullptr;
+    }
 
   /* as a Pratt parser (which works by decomposing expressions into a null
    * denotation and then a left denotation), null denotations handle primaries

gcc/rust/parse/rust-parse.h

@@ -89,6 +89,8 @@ struct ParseRestrictions
   bool entered_from_unary = false;
   bool expr_can_be_null = false;
   bool expr_can_be_stmt = false;
+  bool stop_on_token 
+    = false; // This is for stopping parsing at a specific token
   bool consume_semi = true;
   /* Macro invocations that are statements can expand without a semicolon after
    * the final statement, if it's an expression statement. */