​​NLL: Migration + Retrospective
​​
​​Audience
​​NLL developers primarily
​​Key People
​​RalfJung, MatthewJasper, pnkfelix
​​When
​​Tuesday, 14:00-15:00
​​Where
​​Gainsboro
​​Meeting Style
​​Discussion
​​Deliverables
  • ​​Review of migration plan
  • ​​Short retrospective on the process
​​Homework to do before meeting
​​Agenda
  • ​​Migration plan
  • ​​Current plan is “migrate mode” on 2015 for 1.34 (beta cut on W/C 2018-02-25), 167636
  • ​​
  • ​​Open issues blocking enabling migrate mode
  • ​​Matches (PR #57609)  - change in match lowering to MIR, could be a breaking change.
  • ​​#57170 - promoted aren’t type-checked currently. Has open PR.
  • ​​#58127 - migrate mode is not applying to “nice region errors”. The error is also a completeness issue.
  • ​​#58178 - ~80x perf regression on code generated by GLL.
  • ​​Plan for migration:
  • ​​Fix above issues
  • ​​Make -Zborrowck=migrate -Ztwo-phase-borrows​ the default on all editions, remove -Zborrowck=ast​
  • ​​Cleanup PRs to remove -Zborrowck=compare​, -Ztwo-phase-borrows​ and running MIR type checking outside of NLL.
  • ​​Change compare mode to use -Zborrowck=mir​ - will show differences in “free region errors”.
  • ​​Triage and fix issues blocking changing to full NLL
  • ​​Enable -Zborrowck=mir​ on both editions.
  • ​​Possible additional step: change -Zborrowck=migrate​ warnings to be a lint, make that lint deny-by-default.
  • ​​Potential additional step: fix other peoples projects to address the future-compatibility issues
  • ​​Retrospective
​​

​​Notes

​​Migration Plan
  • ​​migration mode:
  • ​​run MIR checker, run also the AST checker:
  • ​​if MIR checker thinks code is ok, accept the code
  • ​​if MIR checker gives error, run AST checker:
  • ​​if AST checker is happy: report warnings
  • ​​else: return MIR errors
  • ​​currently 2018 only
  • ​​first step is to enable for 2015
  • ​​eventually the plan is to go from migration to “full NLL”
  • ​​unknown whether to enable for all editions or not
  • ​​this is a “future compatibility warning” situation, so we would decide to do it based on crater runs etc
  • ​​issues blocking migration
  • ​​(see agenda)
  • ​​how to migrate
  • ​​(see agenda)
  • ​​question mark:
  • ​​would it make sense to have 2018 be “full NLL” before 2015?
  • ​​just for more limited exposure