parser: shrink the size of WithSpan to one register

[Kijewski]

To be rebased after #214 is merged.

[GuillaumeGomez]

To ensure the size doesn't grow up, might be worth adding a compile-time size_of check of WithSpan and of Span.

[Kijewski]

I don't think it matters too much, as the PR fixes another problem, too:

If the input span is shrunk at the end of the input, e.g. let i = i.trim(), then the error messages will be broken afterward, because only the lengths are compared. This PR fixes that, too. This was actually my original intention with, because I had to refactor some code a few times in order not to break the error messages. That the Span is smaller afterwards is just a happy co-incidence. :)

[Kijewski]

I added a check, anyway. :)

[Kijewski]

Oh, I did not expect a fuzzer error, but it makes me happy that it is actually working!

Offset::offset_to only accepts slices of self

Oops, I should have read the documentation of the method more carefully. :)

The failed scenario:

Parser(
    Scenario {
        syntax: Syntax {
            block_start: "{%",
            block_end: "%}",
            expr_start: "{{",
            expr_end: "}}",
            comment_start: "{#",
            comment_end: "#}",
        },
        src: "FFFFF\u{1a}FFFFFFFD& {%\r\rset\rnnnnnnnnnnnyA%}\u{11}\rr\ti-0)mmm..%}68-iioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFforn\r\rmmm%}68-0)mmm%}68-iioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rLLLLLL& {%\r\rset\rnnnnnnnnnnnyA%}\u{11}\rr\ti-0)mmm%}68-iioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFfom%}68-iioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFforn\r\rmmm%}68-0)mmm%}68-iioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14} s\0\0\0{%-\r\relse+%}|\rLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}LL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFforn\r\rmmm%F\rnnnnnnnnLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rLLLLLLLL1i2{%for\r\rrr\tin\rif*W%}\u{14}LL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFforn\r\rmmm%F\rnnnnnnnnLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\riioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14} s\0\0\0{%-\r\relse+%}|\rLLLLLLLL1i2{%for\r\rr\tin\r\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\riioirLLLLLLLLcLLLLLLLLLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14} s\0\0\0{%-\r\relse+%}|\rLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}LL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFforn\r\rmmm%F\rnnnnnnnnLLLLLL1i2{%for\r\rr\tin\tif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rLLLLLLLL1i2{%for\r\rr\tin\rif*W%}\u{14}  !\u{19} {%-\r\relse+%}|\rFFforn\r\rmmm%F\rnnnnnnnnnnnyA%\r}yyyA%}",
    },
)

[Kijewski]

Well, the changes in the parser are not too big. No reason to wait.

[GuillaumeGomez]

You even added the fuzzing error, nice!

Well, the changes in the parser are not too big.

As the reviewer, I disagree. :p

[GuillaumeGomez]

Please merge once fuzzer passed.