fix: Limit error messages new line visualization (fixes #362)

[tomtau]

new_from_pos and new_from_span only visualize newline
characters if positions point to the newline characters.

[glyn]

Will conditional handling of newline characters violate the principle of least surprise?

Is it possible to use pest to parse a grammar in which newlines characters are not treated as whitespace? That may be an example where the principle of least surprise is most dramatically violated.

Maybe an example, before and after this fix, would help demonstrate that the behaviour is never surprising.

[CAD97]

This change (should) only impacts error rendering. When showing a snippet of source, the newlines are still visible as, well, newlines. The addition of ␊ is only needed for an error span to point at the newline character itself.

One potential source of confusion is if ␊ appears literally in the source. This PR (should) only make things better in this case, as it removes insertion of ␊ when it is not necessary in order to be able to point directly at the ␊.

[glyn]

Ok, sounds reasonable. Thanks. @tomtau feel free to add an example if you think it clarifies things.

[CAD97]

(I assigned @pest-parser/triage, which is set to automatically load-balance review requests. Feel free to reassign if you don't want to review. If we get a LGTM/Approve review from triage and CI is green (ignoring coverage, which is broken) I'll hit the merge button.)

[jhpratt]

Yeah, I haven't had a chance to look through any of the code yet, so I wouldn't know what I'm looking at at this point.

[CAD97]

In retrospect I'm not completely certain I agree with the choice to visualize LF in this manner, but this is clearly an improvement on the status quo.

[tomtau]

@glyn one of the error tests has an example where the ␊ character is visualized

pest/pest/src/error.rs

Line 774 in 96eace8

"1 | abcdef␊",

because the error position points to \n, while some of the other error tests were fixed not to visualize the newline characters in their examples, because their positions didn't point to \n or \r.