Get line containing the point of an span.

[kaj]

A method to get the textual line containing the (start of) a LocatedSpan would be nice.

I want to present parse errors in a user-friendly way, like this:

  ,
2 |     div.selector { property: } /* No value */
  |                              ^ Parse error: Expected value.
  '

As LocatedSpan has methods get_column, get_utf8_column and location_line to get line-relative information, and fragment to get the source text from the span, I think it would be reasonable to also provide a method line_fragment (or some better name) to get the line text.

I currently do this with the following unsafe and abstraction-level-breaking code, adapted from the private get_columns_and_bytes_before method.

https://github.com/kaj/rsass/blob/580121abdf1367384109afaa0c18b88b99e4c959/src/error.rs#L125-L143

[pitaj]

This is implemented and should be closed, unless we also want to provide some way to get the full line given the original input span.