NFC: Introduce Lexer.Cursor.Position

[hamishknight]

This just holds the position info, which is useful for clients that don't care about the extra state in Cursor (e.g the state stack). LexingDiagnostic is one such client, and regex literal lexing will eventually be another.

[hamishknight]

@swift-ci please test

[ahoppen]

Nice. Just to be clear. Changing the state stack to use Lexer.Cursor.Position will be a follow-up PR?

Also out of curiosity: Did you check if changing the sate stack to use Lexer.Cursor.Postion improves performance?

[hamishknight]

@ahoppen Sorry, I realize now the PR description is ambiguously worded, when I mentioned the state stack, I'm referring to it as state that certain clients don't care about (and so should use Cursor.Position instead). AFAIK the state stack does not currently store Cursors, so there shouldn't be any work to do there. That being said, one of the reasons I'm adding this is to avoid storing a whole Cursor as the endpoint for a regex literal lexeme, which will need to be stored in the state stack (I'm planning on doing some benchmarks to see whether storing that state inline vs indirectly is better).

If you're interested in where I am currently with the regex work, I have a WIP branch https://github.com/hamishknight/swift-syntax/compare/lets-go-back-to-the-island

[ahoppen]

Ah, I see. Makes sense 👍🏽