asm! macro failed to exhaustively parse all of options(), clobber_abis() and register operands

[badumbatish]

Current implementation of asm! macro will fail to exhaustively parse all of options(), clobber_abis() and register operands.

For example,

In parsing this,

fn main() {
    unsafe {
        asm!("nop", options(nomem), options(nomem));
    }
}

instead of

fn main() {
    unsafe {
        asm!("nop", options(nomem, nomem),);
    }
}

The asm! macro won't error out the duplication error since after the first options, it bubbles up to the first layer.

Here in parse_asm_arg(), when the parser successfully parses clobber_abi or options, it returns to the first layer right away instead of parsing until it have converged to a NON_COMMITTED after all paths have been taken.

token = parser.peek_current_token ();
      if (token->get_id () == COMMA && token->get_id () == last_token_id)
	{
	  parser.skip_token ();
	  break;
	}

      // Ok after the left paren is good, we better be parsing correctly
      // everything in here, which is operand in ABNF

      // TODO: Parse clobber abi, eat the identifier named "clobber_abi" if true
      if (check_identifier (parser, "clobber_abi"))
	{
	  auto expected = parse_clobber_abi (inline_asm_ctx);
	  if (expected || expected.error () == COMMITTED)
	    return expected;

	  continue;
	}

      // TODO: Parse options
      if (check_identifier (parser, "options"))
	{
	  auto expected = parse_options (inline_asm_ctx);
	  if (expected || expected.error () == COMMITTED)
	    return expected;

	  continue;
	}

      // Ok after we have check that neither clobber_abi nor options works, the
      // only other logical choice is reg_operand
      // std::cout << "reg_operand" << std::endl;

      // TODO: BUBBLE UP THIS EXPECTED(...)
      auto expected = parse_reg_operand (inline_asm_ctx);
      if (expected || expected.error () == COMMITTED)
	return expected;

To enlighten the second point, another case that might happen is that the parser have exhausted all the path, and since every path returns a NONCOMMITTED error, this means that no tokens have been consumed, and the while loop will keep on looping forever.

For the 2nd case it is advised to add an rust_unreachable() in fixing the parser.

The issue is first mentioned in #3053