This is a tracking issue for the work items remaining for the NLL team before we can consider NLL stabilized across all editions (see also #43234).

Here is an outline of the steps we have identified.

• Stage 1: Move all editions to NLL (NLL: turn on borrowck=migrate by default on 2015 edition #57804)
  • Move Rust 2015 to borrowck=migrate
  • Potential blockers:
    • NLL-sound bugs that affect migration mode (vs ones that only arise with #![feature(nll)] today)
    • Major diagnostic regressions that affect migration mode
• Stage 2: Transition fully to NLL (NLL: turn off migration mode #58781)
  • one currently opts into this via #![feature(nll)] or -Z borrowck=mir -Z two-phase-borrows)
  • this causes NLL warnings (downgraded from errors) become errors again
    • possible intermediate steps
      • move to deny-by-default lints for the warnings
      • move 2018 to hard error on its own, leaving 2015 with downgrade-to-warnings
  • It also gets rid of AST-region inference, causing some code to be accepted that is rejected by migrate mode.
  • Potential blockers:
    • NLL-sound bugs that affect NLL mode without migration
    • Major diagnostic regressions
    • Two-phase borrows (2PB) question or other "unsettled questions" must have some conservative answer in place
    • Crater run showing no major crates fail in practice
• Stage 3: remove old AST-borrowck (done in Rest In Peace, AST borrowck (2012-2019) #64790)

Blocking issues

• coercion becomes order dependent for higher-ranked functions #73154
• NLL accepts higher-ranked subtype that non-NLL rejects #57642
• Tracking issue for mutable_borrow_reservation_conflict compatibility lint #59159