"The parser"

It'd be worth covering

• where the lexer is
• how the parser works
• conventions around error recovery
• - What is this eat function?
  • It consumes a token, and queues up the next token as lookahead
• How do you handle identifiers and keywords?
• What about constructing spans?
  • (quoting Niko from below) I have to bring this back into cache, but in general there is this last_span thing that stores the span of the last token, and many things follow the idiom of saving the "lo" point of the span, parsing some stuff, then extracting the "hi" point and combining them. This would be used to make a span that encompasses, for example, an entire trait definition (the lo point would come from the trait keyword, but the end point comes after having parsed a bunch of other things).
• What is this crazy pretty printer?
  • It dumps out the AST in human readable form.
  • We have tests that will parse each file in run-pass, pretty print it, and then test that the resulting pretty printed file type checks, so you have to implement this.
• Something like this: https://github.com/rust-lang-nursery/rustc-guide/issues/13#issuecomment-361848936 and https://github.com/rust-lang-nursery/rustc-guide/issues/13#issuecomment-361992949

Start by reviewing the rust-lang/rust parser.rs reference linked in the issue and the existing run-pass pretty-print tests. Document the lexer, parser flow, error recovery, eat, identifiers and keywords, span construction, and AST pretty printer, including how the tests validate printed output.】【。