Add Polonius 2019/04/23 meeting notes

Author: lqd

working-groups/polonius/minutes/2019.04.23-meeting.md

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+# WG-Polonius meeting minutes: 2019/04/23
+---
+
+Relevant links:
+- The meeting happened in this [Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/186049-t-compiler.2Fwg-polonius/topic/meeting.202019.2E04.2E23)
+- A [Paper document](https://paper.dropbox.com/doc/Polonius-Roadmap--AY6C806s~AZK~e7wagmys2_wAg-hk3a9ynduUN2gk1A0NNTF) for the WIP roadmap
+
+---
+
+In this meeting, we started with a short review of the previous week's work:
+- [Albin] continued work on implementing liveness in Polonius, and finalized [#105](https://github.com/rust-lang/polonius/pull/105), which is now ready to be merged into a new `polonius-engine` release.
+- [Niko] wrote up [notes](https://github.com/rust-lang/polonius/issues/104#issuecomment-485011791) describing the second part of the liveness effort: modifying rustc to emit `var_used` and `var_defined` facts for the previous PR. [Albin] has already started working on these rustc steps.
+- [Rémy] landed the tiny [#106](https://github.com/rust-lang/polonius/pull/106).
+- As part of the `Hybrid` variant, [Rémy] started prototyping a low-hanging performance task: using the `LocationInsensitive` potential errors results to filter some of the input relations of the `DatafrogOpt` pass.
+
+We then welcomed a new contributor, [@lokalmatador], and spent some time discussing a good first task: using `rustc-perf`'s benchmarks, and [Albin]'s microbenchmarks, continuing the work towards an extended performance test suite. The goal would be to find crates, and functions, which would be representative along different dimensions, and incorporate their facts files in the Polonius repo to be used for tests and benchmarks. [Niko] described the basic steps in the [Paper document](https://paper.dropbox.com/doc/Polonius-Roadmap--Abw5txE2EuUEhL3E1CMrnzuWAg-hk3a9ynduUN2gk1A0NNTF#:uid=627651028890979090145392&h2=Benchmarking-suite).
+[Link to full discussion](https://rust-lang.zulipchat.com/#narrow/stream/186049-t-compiler.2Fwg-polonius/topic/meeting.202019.2E04.2E23/near/164021731).
+
+A useful task was mentioned for the near future: going through the Polonius failures in rustc's test suite (using the `--compare-mode polonius` test mode), categorizing the failures, and possibly making the tests pass, by `--bless`ing the expected polonius test output files. 
+
+----
+For the following week:
+- [Albin] will continue on the liveness fact generation in rustc.
+- [Rémy] will continue work on the `Hybrid` variant optimization.
+- [@lokalmatador] will familiarize with the steps [Niko] wrote, with available help from the rest of the WG.
+
+[Albin]: https://github.com/albins
+[Niko]: https://github.com/nikomatsakis
+[Rémy]: https://github.com/lqd
+[@lokalmatador]: https://github.com/lokalmatador