Tracking issue for 'Allow Self to appear in the where clause of trait impls'

RFC: https://github.com/rust-lang/rfcs/blob/master/text/1647-allow-self-in-where-clauses.md

Proposes allowing the Self type to be used in every position in trait implementations, including where clauses and other parameters to the trait being implemented.

Auto merge of #38920 - petrochenkov:selfimpl, r=eddyb

Auto merge of #40008 - eddyb:lazy-12, r=nikomatsakis

The RFC still isn't fully implemented. In particular, this example from the RFC text still fails to compile:

impl SomeTrait for SomeType where Self::AssocType: SomeOtherTrait {
    type AssocType = SomeOtherType;
}

@sgrif

@sgrif What's the error?

https://is.gd/amqdSq:

struct SomeType;
struct SomeOtherType;

trait SomeTrait {
    type AssocType;
}
trait SomeOtherTrait {}

impl SomeTrait for SomeType where Self::AssocType: SomeOtherTrait {
    type AssocType = SomeOtherType;
}
fn main() {}

rustc 1.19.0-nightly (afa1240e5 2017-04-29)
error[E0275]: overflow evaluating the requirement `<SomeType as SomeTrait>::AssocType`
 --> <anon>:9:6
  |
9 | impl SomeTrait for SomeType where Self::AssocType: SomeOtherTrait {
  |      ^^^^^^^^^
  |
  = note: required because of the requirements on the impl of `SomeTrait` for `SomeType`

error: aborting due to previous error

Ah that's not part of the actual syntactical support (which works!), but rather the trait system.
You can reproduce by expanding Self::AssocType to <SomeType as SomeTrait>::AssocType.
This looks like it was overlooked in the RFC, and I'd suggest opening a separate issue.
I suppose we can keep this one open and depend on that issue but they're unrelated IMO.
cc @rust-lang/lang

According to the last comment here, it seems like maybe this is finished?

The issue I raised still exists. I'm not sure how it was overlooked in the RFC, the example given from the RFC itself, and the text explicitly states that we'd like to be able to reference associated types from the trait being implemented. I agree that this is a fix that needs to happen in the trait system, not just syntactic support, but it is something the RFC calls out explicitly multiple times.

Could you open a separate ticket please, focused on that specific bug?
This is a tracking issue for the feature in general, and the feature in general was implemented and stabilized.

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Tracking issue for 'Allow `Self` to appear in the where clause of trait impls' #38864

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Tracking issue for 'Allow Self to appear in the where clause of trait impls' #38864

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sgrif commented on Apr 30, 2017

eddyb commented on Apr 30, 2017

Mark-Simulacrum commented on Apr 30, 2017

eddyb commented on Apr 30, 2017

steveklabnik commented on Mar 12, 2019

sgrif commented on Mar 12, 2019

petrochenkov commented on Mar 12, 2019

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Tracking issue for 'Allow `Self` to appear in the where clause of trait impls' #38864