never type stabilization

Summary

Long term goal is to stabilize the ! type and in particular to alter fallback for “diverging type variables” to use ! and not ().

Current status

Working towards a lint that catches the problems associated with changing fallback (https://github.com/rust-lang/rust/issues/66173). Have a draft PR https://github.com/rust-lang/rust/pull/74535 that is being developed by @blitzerr.

Info

• Lead: @nikomatsakis
• Lang-team liaison: @nikomatsakis
• Charter – not yet written
• Zulip stream #t-lang/project-never-type
• Relevant rust tracking issues: https://github.com/rust-lang/rust/issues/65992, https://github.com/rust-lang/rust/issues/66173, https://github.com/rust-lang/rust/issues/35121

What is this issue?

This issue represents an active project group. It is meant to be used for the group to post updates to the lang team (and others) in a lightweight fashion. Please do not use the comments here for discussion, that should be kept in the Zulip stream (discussion comments here will be marked as off-topic).

[nikomatsakis]

Status update 2021-04-06:

• @Mark-Simulacrum wrote up a detailed and useful report after doing some investigation. He and I also discussed the results and came to a preliminary consensus on the path forward.
• The upshot was:
  • We can’t really leverage editions to avoid breakage so long as we want to update Infallible to !.
  • We can extend the hybrid proposal for never type fallback to cover cases where trait selection would cause further errors. This should allow us to land the never type fallback change without significant breakage.
  • The problem then is that the fallback proposal is kind of complex and still accepts code we arguably should not. We can deprecate that over time using warnings.
  • Once we’ve come to an algorithm or subset of fallback we are happy with, we can use an edition if desired to make a harder change.

[Mark-Simulacrum]

The initial set of patches landed on master in September. For October, the plan is to:

• Create an initiative repository - https://github.com/rust-lang/never-type-initiative/
• Prepare a written explainer, or two, with T-lang and community audience
  • RFC draft (https://github.com/rust-lang/never-type-initiative/blob/master/RFC.md) is written up, likely needs some more iteration
• Prepare a draft stabilization PR to assess current master’s breakage to determine if there are blockers to stabilization
  • Crater results: https://github.com/rust-lang/rust/pull/89630#issuecomment-939547646

It’s a little unlikely, but the hope is that we will also get a chance to:

• Prepare a blog post – though likely not for immediate publication – with the content from the community explainer, with a goal of getting feedback on our design.
• Start work on a “N year plan” for the future of the inference rules around ! (and perhaps more generally). Niko and I agreed that the current proposal for ! is not ideal, and we’d like to see something simpler – and perhaps more easily understandable/implementable under different schemes than currently in the compiler.

I also discussed with @nikomatsakis whether an RFC makes sense as something to aim towards. We noted that the vast majority of changes related to never type that we’ve been iterating on are likely a little better suited to initiative and/or more internal discussion, since they’re mostly just implementation details of the larger goal laid out in the RFC. We’ve already accepted RFC 1216 for the motivation and general shape of the feature, so that part is largely taken care of. It’s also true that such an RFC – or indeed any documentation in this area – quickly is confronted with the relative lack of documentation for type inference as a whole as implemented in Rust today, so it’s hard to describe ! inference as an “edit”.