Combining HRTBs with associated types

[alecmocatta]

I'm working on data-parallelism library, heavily inspired by Rayon but focused on describing certain computations on a dataset in an SQL-like manner, somewhat akin to Diesel.

One part of this combines HRTBs with associated types. I've bumped into a few issues that have made things a bit tricky (#30472, #30867, #53943, and similar), the hardest to work around however has been that I don't believe it's currently possible to write this:

type Task = for<'a> <B as Abc<&'a A::Item>>::Task;

Currently I've resorted to a dummy trait, manually reimplemented on various structs without the reference, such that I can do:

type Task = <B as AbcDummy<A::Item>>::Task;

and (horribly) transmute between the two.

I'd be very interested to discuss

1. issues I've bumped into combining HRTBs and associated types;
2. how to get rid of this transmute and the dummy trait;
3. any feedback on the library before I publish and promote it, given it's heavily inspired by your work on Rayon!

I can do any of the Monday or Friday times listed, though I have a preference for the 16 - 16.30 slots.

Again, much appreciation for you doing this, I think it's awesome!

[nikomatsakis]

Hmm, this doesn't sound like a beginner topic to me, and the two Friday slots I laid out are already taken. Let me take a look, maybe I'll add another slot.

The library sounds awesome!

The limitations around HRTB can be pretty annoying and there aren't always good workarounds. This is one of the things I hope to fix in the Chalk work.

[alecmocatta]

Thanks! No problem, I thought I might as well express interest, and I can update if any other slots or 2018/10.md come into existence.

From what I've seen I'm really excited about Chalk, looking forward to seeing it progress!

[nikomatsakis]

@alecmocatta october slots now available :)

[alecmocatta]

@nikomatsakis Thanks!