Proposal to refactor Span.

[otrho]

Why Span?

• For error reporting.
• For source mapping.

What does Span look like now?

• A Pest Span:
  • Shared source string.
  • Start and end indices into string.
• An optional source file path.

What is this structure currently good for?

• Error reporting. Everything is very handy. Even without the path.

Bad for?

• Source mapping:
  • Requires path, and for start and end to be file offsets.
• IR:
  • Not every error producing value has a known span.
  • (De)serialisation prefers path + indices, does not want to (de)serialise source strings.

A solution?

• Remove Pest span and make the path mandatory:
  • Source mapping is happy.
  • If no path is applicable then one cannot create a Span, so the whole Span must then be Option, or we have an 'official' or 'default' empty/null Span.
  • IR (de)serialisation is simplified.
• Make spans optional for error reporting:
  • IR can create errors without Spans. Note, this would hopefully be pretty rare. The IR would try very hard to provide a Span when asked but can't guarantee a Span, and therefore must be optional.
  • Source mapping doesn't care.