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‎minutes/triage-2019-02-24.md‎

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# Triage meeting on 2019-02-24
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## Info
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[Zulip thread.](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-02-25)
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## Summary
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The goal of the meeting was to try and figure out, in somewhat more
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detail, how we want to organize the traits working group itself. We've
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been doing [work on enumerating and triaging the set of things we hope
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to do][roadmap], but in this meeting we were basically trying to come
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up with some concrete next steps.
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[roadmap]: https://paper.dropbox.com/doc/Traits-Roadmap-sketch--AYTQPvAVQ~p4lyElpr24zoAQAg-m6hucUslp7GihsY3SjNN2
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Ultimately we identified the following next steps:
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- Schedule a meeting to dig into some of the **lifetime problems around
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async-await**. This is kind of an "intersecting" issue between async await
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and the traigs WG, but seems high priority. (nikomatsakis)
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- **Explore the use cases for lazy normalization**, trying to create a summary
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of exactly which test cases are causing problems and why, and what it might
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take to fix them (can it be done without a full chalk transition?). (nikomatsakis)
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- **Explore the use cases for GATs**, trying to find a set of specific examples
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from async-await and other contexts. It seems likely that we could do an initial
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impl of GATs in rustc without chalk, but would it be enough? (aturon, centril)
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- Schedule a meeting to **dig into the chalk transition** -- what is
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current status and what are the possible future routes, what should
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we be exploring? (nikomatsakis, scalexm)
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- Get tests plus FCP around `type Foo = impl Bar` (centril)
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In general, we're trying to get a sense for what the [initial set of
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active 'subprojects' will
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be](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-02-25/near/159365173)
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(basically, the things that the WG is focusing on). Ideally, each of
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them will have a distinct "lead" (or, better, leads!) who is driving
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the effort (in collaboration with the rest of us).
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A **likely** set of initial subprojects will contain:
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- some amount of hacking on rustc (GATs, lazy norm) in an effort to "relieve the pressure"
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- some amount of hacking on chalk itself and on chalk integration
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- some amount of work on defining the semantics of other features like
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specialization by lowering them to chalk predicates
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but that is not certain. A key drive is both the set of people we have
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involved and also what seems to be highest priority in terms of
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unblocking other things within Rust.

‎minutes/triage-2019-03-04.md‎

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# Triage meeting on 2019-03-04
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## Info
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[Zulip thread.](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04)
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## Summary
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We followed up on the goals from last time.
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### Lifetime problems around async-await
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We
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[discussed](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159939976)
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how we found a way to resolve the `async fn` complications by changing
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the async fn desugaring, and hence don't need any deep trait
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action. We are [still
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interested](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940028)
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in trying to find a way to solve the "captures problem" for impl
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Trait, however, and alexreg and nikomatsakis may do some follow-up
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there.
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### Explore use cases for lazy normalization
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Some investigation occurred. [Current plan](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940188)
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to continue documenting the way that the current normalization code
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plays out and to hold some sort of meeting to document that and walk
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through it. That meeting will hopefully happen this week and will be
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recorded.
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### Explorse use cases for GATs
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[Not a lot of progress here.](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940294) (Yet!)
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Once we **do** make progress, though, the [logical next
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step](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940943)
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is probably to try and model those use cases some in Chalk, as well as
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to investigate more deeply how rustc could be extended to handle the
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cases natively (without blocking on full chalk integration).
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### Dig into the chalk transition
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We [have a meeting scheduled for tomorrow](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940496) with the RLS 2.0 folks, planning to talk about how the type checker is working in that context.
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[**We are thinking that integrating Chalk first into the RLS 2.0
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project may be a better idea than starting with
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rustc**](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940552),
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particularly as it can help get RLS 2.0 to a "working product" faster,
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but also because we may be able to use that to pursue the idea of
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sharing more code between chalk + compiler than we could readily do
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with rustc. Specifically, we should be able to have:
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- a library to share the definition of types between chalk + RLS 2.0
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- shared code for lowering traits + impls into logical rules
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We also talked about other possible work on chalk, in particular the
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idea of chalk-specific refactorings but also building a [benchmarking
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harness](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159940894)
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for the `chalk-engine` crate.
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## Miscellaneous discussion
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There are a few other things we talked about this week:
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- [the idea of a research intern working on chalk and what they might
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do](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159941464)
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- [how we might support `async fn` in traits without full GAT support](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weekly.20meeting.202019-03-04/near/159942222)
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## Plans for the coming week
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We settled on the following plans for the coming week:
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So plans for this week are:
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- Describe how normalization works today and hold a (recorded) call to
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try and explain it to others. Trace through some of the examples
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where lazy norm would be helpful. (nikomatsakis)
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- Explore the use cases for GATs (aturon, centril)
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- Discuss RLS 2.0 type checker and contemplate how to integrate chalk
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into it (nikomatsakis, scalexm)
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- Maybe discuss other chalk improvements? (e.g., benchmarking harness) (nikomatsakis)
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- Maybe discuss the "lifetime capture" problem in impl Trait? (nikomatsakis, alexreg)
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‎minutes/triage-2019-03-11.md‎

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# Triage meeting on 2019-03-11
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## Info
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[Zulip thread][]
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[Zulip thread]: https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weeky.20meeting.202019-03-11
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## Help wanted!
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- nikomatsakis is looking for someone to help with investigating lazy
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normatlization. The task would be to experimentally modify rustc and
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is best suited to someone already vaguely familiar with the Rust query
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system etc. See below.
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## Summary
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We followed up on the goals from last time.
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### Lazy normalization and const generics
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[Link](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weeky.20meeting.202019-03-11/near/160502973)
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Last week, nikomatsakis did more investigation, taking notes in [this
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dropbox paper document][lnpaper]. In general, it's still not entirely
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clear if the "cycle" that gives rise to the need for lazy
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normalization is something we can easily circumvent. One thing is that
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the `generics_of` and other queries for "anonymous constants"
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currently use the incorrect parent def-id precisely to avoid a cycle,
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so it's hard to see how the cycle would manifest.
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[lnpaper]: https://paper.dropbox.com/doc/Lazy-normalization-Ryv4YfpIcGAl6R3ZtWrWs
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**Next step:** re-establish the cycle in those queries so it can be observed.
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### GATs
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[Link](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weeky.20meeting.202019-03-11/near/160503327)
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centril + aturon drew up a [paper document containing GAT use
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casts][gat] and there are some notes from the conversation in [this
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Zulip thread][gatthread].
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**Next step:** unclear, but probably to investigate the use cases and
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make some comments.
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[gat]: https://paper.dropbox.com/doc/GAT-use-case-analysis--AY1Ck74Fgk1Ztq1kHrethI8BAg-xFJQMxHXTOUekCyweukU1
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[gatthread]: https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/GAT.20use-case.20analysis
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### Integrating into RLS 2.0
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[Link](https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/weeky.20meeting.202019-03-11/near/160503711)
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We had a chat ([with
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video](https://www.youtube.com/watch?v=Lmp3P9WNL8o)) about how the RLS
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2.0 type checker works. We decided we'd rather try to do the chalk
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integration into RLS 2.0 "correct", meaning that we can share a lot
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more code with chalk.
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**Next step:** nikomatsakis to schedule some time to sketch out what chalk integration would look like.
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### Other chalk improvements
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> Maybe discuss other chalk improvements? (e.g., benchmarking harness) (nikomatsakis)
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This didn't happen and we'll probably shelve it for now in favor of the previous point.
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### "lifetime capture"
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> Maybe discuss the "lifetime capture" problem in impl Trait?
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This didn't happen and we'll probably shelve it for now in favor of the next point.
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### Draft RFCs
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centril plans to (at minimum) upload some draft RFCs to a new wg-traits repository.
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### Associated type bounds
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alexreg has been working on associated type bounds in [this Zulip
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thread][atb] and will likely continue to do so.
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[atb]: https://rust-lang.zulipchat.com/#narrow/stream/144729-t-compiler.2Fwg-traits/topic/associate.20type.20bounds
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## Plans for the coming week
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- Try to correct the `generics_of` query etc for constants so we can
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observe what results.
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- **Help wanted!** nikomatsakis is looking for someone to help
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with investigating lazy normatlization. The task would be to
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experimentally modify rustc and is best suited to someone
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already vaguely familiar with the Rust query system etc.
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- Read-over GAT use cases and try to theorize about what it would take to
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support them in rustc etc (nikomatsakis)
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- Schedule a call to talk over what chalk integration into RLS 2.0 might look like (nikomatsakis)
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- Create a wg-traits repository to house draft RFCs and the like. (nikomatsakis)
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- Upload drafts of various traits-related RFCs to wg-traits (centril)
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- Continue work on associated type bounds (alexreg)

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