Improve error prompt information when there is insufficient input

[A4-Tacks]

error output:

error: proc macro panicked
 --> src/main.rs:1:1
  |
1 | / peg::parser!(
2 | |     grammar parser() for str
3 | | );
  | |_^
  |
  = help: message: index out of bounds: the len is 6 but the index is 6

from source

rust-peg/peg-macros/tokens.rs

Lines 124 to 126 in ffcead2

	fn position_repr(&self, pos: usize) -> Sp {
	Sp(self.tokens[pos].span(), pos)
	}

rust-peg/peg-runtime/error.rs

Lines 128 to 133 in ffcead2

	pub fn into_parse_error<I: Parse + ?Sized>(self, input: &I) -> ParseError<I::PositionRepr> {
	ParseError {
	location: Parse::position_repr(input, self.max_err_pos.into()),
	expected: self.expected,
	}
	}

I tried to improve, but couldn't find a good way to obtain the Location of the input eof

[kevinmehall]

Fixed in d888952.

I don't know of a way to get a span pointing at the closing } of the macro, and also want to handle the case of an empty peg::parser!() with no other span available, so the span is just the full Span::call_site(), but at least it's not a panic.

error: expected one of "::", "<", "{" at end of input
 --> tests/compile-fail/incomplete_grammar.rs:4:1
  |
4 | / peg::parser!(
5 | |     grammar parser() for str
6 | | );
  | |_^
  |
  = note: this error originates in the macro `peg::parser` (in Nightly builds, run with -Z macro-backtrace for more info)