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

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_expr): Updated parsing logic to stop on
	encountering '=>' or ',' while having restrictions.stop_on_token turned on.
	(Parser::left_denotations): Updated to stop parsing on the special tokens mentioned above.
	(Parser::null_denotation): Updated to stop parsing on the special tokens mentioned above.
	* parse/rust-parse.h (struct ParseRestrictions): Added a field stop_on_token (default false)
	to maintain the above functionality.

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 +12083,14 @@ 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),
@@ -12116,6 +12129,13 @@ 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
    * and unary operands (but only prefix unary operands) */

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. */