parse and handle `where T = U` predicate #87471

JulianKnodt · 2021-07-26T07:47:01Z

Updates #20041.

I took the most low-budget approach to adding equality checks (2 subtype checks in each direction), if it makes more sense to add in one check that does just equality, please let me know.

r? @varkor

compiler/rustc_feature/src/active.rs

fee1-dead · 2021-07-26T11:57:25Z

I think making it an actual PredicateKind would be better. You can use rustc_infer::infer::equate to do this i think

varkor · 2021-07-27T10:26:03Z

What's the status on the previous blockers? Have these been addressed now?

JulianKnodt · 2021-07-27T17:29:50Z

It's not so clear to me whether the previous blockers are completed, but given the time that passed since then, I wouldn't be surprised if they have. I still need to clean up some of the branches (since I just threw them all in last night), and will explore if this is feasible.

bors · 2021-07-28T09:17:58Z

☔ The latest upstream changes (presumably #86735) made this pull request unmergeable. Please resolve the merge conflicts.

varkor

This could do with more tests (both successful and error cases). Also, could you add tests for the cases @nikomatsakis mentions in #22074 (comment)?

compiler/rustc_feature/src/active.rs

compiler/rustc_parse/src/parser/generics.rs

varkor · 2021-07-28T17:52:14Z

Since @nikomatsakis reviewed the original PR (#22074) that was closed, I'm going to reassign to them.

r? @nikomatsakis

compiler/rustc_trait_selection/src/traits/object_safety.rs

JulianKnodt · 2021-07-28T19:53:52Z

compiler/rustc_trait_selection/src/traits/fulfill.rs

+                    match self.selcx.infcx().can_eq(obligation.param_env, lhs, rhs) {
+                        Ok(()) => ProcessResult::Changed(vec![]),
+                        Err(e) => ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
+                            ExpectedFound::new(false, lhs, rhs),


TODO create a new fulfillment error code

JulianKnodt · 2021-07-28T19:54:13Z

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

+                            span,
+                            "type-equate requirement gave wrong error: `{:?}`",
+                            obligation
+                        )


Not entirely sure what this was referencing, I suspect it's fine to leave this?

compiler/rustc_typeck/src/collect.rs

JulianKnodt · 2021-07-28T19:55:16Z

src/librustdoc/clean/mod.rs

@@ -327,6 +327,7 @@ impl<'a> Clean<Option<WherePredicate>> for ty::Predicate<'a> {
            ty::PredicateKind::RegionOutlives(pred) => pred.clean(cx),
            ty::PredicateKind::TypeOutlives(pred) => pred.clean(cx),
            ty::PredicateKind::Projection(pred) => Some(pred.clean(cx)),
+            ty::PredicateKind::TypeEquate(..) => todo!(),


TODO not entirely sure whether to make this None, or have something else

nikomatsakis · 2021-07-30T11:31:19Z

I don't think we really want this sort of where-clause. As I've come to learn more about the limits of various logics, introducing equality checks implies the ability to say things that we probably don't want to say (although I would like to support them someday, but only after we've made a transition to chalk and decided to do it). For example, being able to write fn foo<A, B>() where A = B would be really nifty, but we don't have the infrastructure to support that kind of equality properly, and it moves the logic from one class of logics to another (in particular we would no longer map so directly to 1st order hereditary harrop clauses -- as they don't permit equality in "assumptions", only in goals). I don't think that's a move we should make at this time.

bors · 2021-07-30T14:57:09Z

☔ The latest upstream changes (presumably #87237) made this pull request unmergeable. Please resolve the merge conflicts.

JulianKnodt · 2021-07-30T17:48:37Z

That's certainly fair, equality tends to imply != which seems to be quite controversial.
I was hoping to add this to improve specialization, so that you can provide impl<T: Iterator> Foo for T where T = Map/Other<...>, but given the additional burdens it's understandable.
In the future if/when this future is desired, this PR can be revived but I'll close it for now.

Please let me know if there is additional work elsewhere I can do to help move specialization along!

rust-highfive assigned varkor Jul 26, 2021

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 26, 2021

JulianKnodt changed the title ~~add in where equality predicate~~ parse and handle where T = U predicate Jul 26, 2021

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Tracking Issue for where T = U #87533

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add in where equality predicate

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parse and handle `where T = U` predicate #87471

parse and handle `where T = U` predicate #87471

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parse and handle where T = U predicate #87471

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parse and handle `where T = U` predicate #87471

parse and handle `where T = U` predicate #87471

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