Polonius review

What do we want to accomplish?

• EOY 2024 goal and the milestones along the way
• Resolution to the placeholder question (if possible)

Goals

• EOY 2023 is we have a location-sensitive prototype that is landed on nightly
  • accepts the tests we want (or important ones)
  • performance is not important
  • don't crash
• EOY 2024 is "stable on nightly"

• Rémy's thesis:
  • Most code is simple and doesn't require complex borrow checking w/ location sensitivity
    • or perhaps even NLL
    • evidence: AST borrow check was mostly ok :)

Counterexample (sort of): This is probably fairly common, and was not accepted by AST borrow check, but we don't have to be that dumb

let p = &foo;
if *p {
    foo += 1;
}

Question: how much of a performance regression would we accept?

Jack: How "fast" is NLL today? Is it "as fast as it could be"?

lqd: No, there's room to be faster.

QOTD: "Precise and fast" – said nobody ever. -lqd

Less precise analyses

• Location insensitive is a max of 3% slower
• Jack: This would be a reasonable floor, we could adopt this as the main solution
• lqd: our benchmark suite is not complete, though
• Amanda: could gather data on the cost of increasingly complex analysis

Roadmap

• Add Niko's graph to the dev guide for onboarding
• Land PR (location-insensitive analysis behind flag)
• Gather data on how much sensitivity we need
• Prototype location sensitive variant (milestone: EOY 2023)
  • Most naive version
• … (a miracle happens)
• SHIP IT