chalkify: Implement lowering rule Implied-Bound-From-Trait #49435

tmandry · 2018-03-28T03:44:27Z

TODO:

Implement where clauses besides trait and projection predicates
Is the output of the lower_trait_higher_rank test correct?
Remove Self::Trait from the query tcx.predicates_of(<trait_id>).predicates
Consider moving tests to compile-fail to make them more manageable

tmandry · 2018-03-28T03:46:12Z

scalexm · 2018-03-28T08:17:26Z

src/librustc_traits/lowering.rs

@@ -58,6 +58,8 @@ impl<'tcx> Lower<DomainGoal<'tcx>> for ty::TypeOutlivesPredicate<'tcx> {
    }
 }

+/// Lowers a trait or projection predicate to a `FromEnv(...)` clause.


What does this comment refer to actually?

Nothing, I added it by accident!

scalexm · 2018-03-28T08:29:31Z

src/librustc_traits/lowering.rs

@@ -99,6 +101,38 @@ impl<'tcx> Lower<Goal<'tcx>> for ty::Predicate<'tcx> {
    }
 }

+/// For a given `where_clause`, returns a clause `FromEnv(WC) :- Implemented(Self: Trait<P1..Pn>)`.


So actually the rule we want is FromEnv(WC) :- FromEnv(Self: Trait<P1..Pn>). But there is a slight subtlety: the FromEnv in the left hand side is not exactly a real FromEnv predicate, it's rather some notation (maybe not very appropriate, I agree):

if WC == TraitRef, then indeed it means FromEnv(TraitRef)

if WC == ProjectionPredicate, then again it means FromEnv(ProjectionPredicate)

in any other case, WC is left unchanged, e.g. in the above FromEnv(T: 'a) just means T: 'a
see this section in the book.

As an example, given this trait:

trait Foo<'a, T> where T: Copy, T: 'a, T: Iterator<Item = i32> { }

we would like the following clauses:

forall<'a, Self, T> { FromEnv(T: Copy) :- FromEnv(Self: Foo<'a, T>). FromEnv(T: Iterator<Item = i32>) :- FromEnv(Self: Foo<'a, T>). (T: 'a) :- FromEnv(Self: Foo<'a, T>). }

Oops, I really don't know how I mixed that up >.>

Doing the rest in a match clause really ought to be a matter of using Lower IMO, but right now it eagerly lowers straight to a Goal when DomainGoal is needed. I want to play around with this when I get some time.

Yeah, I think I'm going to issue a fix for the eager lowering soon, it should benefit the ergonomics of your PR

scalexm · 2018-03-28T08:40:38Z

src/librustc_traits/lowering.rs

@@ -99,6 +101,38 @@ impl<'tcx> Lower<Goal<'tcx>> for ty::Predicate<'tcx> {
    }
 }

+/// For a given `where_clause`, returns a clause `FromEnv(WC) :- Implemented(Self: Trait<P1..Pn>)`.


As an aside, to avoid writing again code about binders, I'd rather see something like:

impl<'tcx> DomainGoal<'tcx> { fn into_from_env_goal(self) -> DomainGoal<'tcx> { match self { DomainGoal::Holds(wca) => DomainGoal::FromEnv(wca), other => other // unchanged } } }

then the same thing recursively for Goal:

impl<'tcx> Goal<'tcx> { fn into_from_env_goal(self) -> Goal<'tcx> { match self { Goal::DomainGoal(dg) => Goal::DomainGoal(dg.into_from_env_goal()), Goal::Quantified(q, g) => Goal::Quantified(q, g.into_from_env_goal()), /* ... */ } } }

and then you can just do something like:

let predicates = tcx.predicates_of(def_id).predicates.lower(); /* do appropriate things to remove the `Self: Trait` predicate */ let predicates = predicates.into_iter().map(|p| p.into_from_env_goal());

Actually I was mistaken this would not quite work since we only can have DomainGoal as a consequence. Ok so do as you want 😀

scalexm · 2018-03-28T08:41:39Z

src/librustc_traits/lowering.rs

+    // forall<Self, P1..Pn> {
+    //   FromEnv(WC) :- FromEnv(Self: Trait<P1..Pn)
+    // }
+


You might want to keep this comment just above your code which generates the clauses just below.

scalexm · 2018-03-28T08:44:19Z

src/test/ui/chalkify/lower_trait_higher_rank.rs

+#[rustc_dump_program_clauses] //~ ERROR Implemented(Self: Foo<F>) :-
+                              //~| ERROR FromEnv
+                              //~| ERROR forall
+                              //~| ERROR forall


The output looks good, just need to change to FromEnv(...) :- FromEnv(Self: Foo<F>) instead of FromEnv(...) :- Implemented(Self: Foo<F>) 😃

tmandry · 2018-03-31T05:53:33Z

Rebased on #49497 and it definitely makes things nicer! 🏂

Note that the test output for HRTBs changed.

nikomatsakis · 2018-04-04T20:19:55Z

Blocked on #49497

scalexm · 2018-04-04T20:21:17Z

I’d like to review one more time once #49497 lands.

nikomatsakis

Few nits -- seems roughly right? I've been doing reviews all day though so my "attention to detail" is starting to flag. I'll try and take another look tomorrow.

nikomatsakis · 2018-04-04T20:20:27Z

src/librustc_traits/lowering.rs

+    where_clause: &ty::Predicate<'tcx>,
+) -> Clause<'tcx> {
+    // We can't use the full `Lower` chain here, as it'll wrap our where clause in `Holds` instead
+    // of `FromEnv`. We also handle ty::Binder similarly to `Lower`, but at the Clause level.


I don't understand this comment actually

Neither do I, honestly. I was tired when I wrote this.

nikomatsakis · 2018-04-04T20:22:05Z

src/librustc_traits/lowering.rs

+            Trait(binder) => binder.lower_from_env(),
+            Projection(binder) => binder.lower_from_env(),
+            RegionOutlives(p) => p.lower(),
+            TypeOutlives(p) => p.lower(),


Can we make where_clause implement LowerFromEnv? So we can just do where_clause.lower_from_env()?

I was going to suggest, instead of the LowerFromEnv machinery:

trait IntoFromEnvGoal { fn into_from_env_goal(self) -> Self; } impl<'tcx> IntoFromEnvGoal for DomainGoal<'tcx> { fn into_from_env_goal(self) -> DomainGoal<'tcx> { /* ... */ } }

and then a straightforward:

ProgramClause::ForAll( // we might map a dummy binder here, ok for now where_clause.lower().map_bound(|goal| ProgramClause { goal: goal.into_from_env_goal(), hypotheses }) )

Yep, IntoFromEnvGoal turns out to be much simpler. Thanks!

tmandry · 2018-04-06T18:26:20Z

Changed implementation to use IntoFromEnvGoal as suggested by @scalexm, rebased, and squashed. Also, added some Outlives predicates to the test.

This should be ready, modulo the predicates_of query, which I can do as part of this PR or separately. I'll be working on that this afternoon.

nikomatsakis · 2018-04-09T17:53:45Z

@bors r+

bors · 2018-04-09T17:53:46Z

📌 Commit 7ac35ea has been approved by nikomatsakis

bors · 2018-04-10T00:33:01Z

⌛ Testing commit 7ac35ea with merge a8a8d6b...

…atsakis chalkify: Implement lowering rule Implied-Bound-From-Trait For #49177. TODO: - [x] Implement where clauses besides trait and projection predicates - [x] Is the output of the `lower_trait_higher_rank` test correct? - [ ] Remove `Self::Trait` from the query `tcx.predicates_of(<trait_id>).predicates` - [ ] Consider moving tests to compile-fail to make them more manageable

bors · 2018-04-10T03:27:40Z

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing a8a8d6b to master...

rust-highfive assigned nikomatsakis Mar 28, 2018

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nikomatsakis mentioned this pull request Mar 28, 2018

🛠 Extend the "chalk lowering" pass with remaining rules #49177

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shepmaster added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 30, 2018

tmandry added WG-traits Working group: Traits, https://internals.rust-lang.org/t/announcing-traits-working-group/6804 A-traits Area: Trait system labels Mar 30, 2018

tmandry force-pushed the rule-implied-bound-from-trait branch from 5c67da7 to 287a1ac Compare March 31, 2018 05:49

tmandry force-pushed the rule-implied-bound-from-trait branch from 287a1ac to 4ef87c6 Compare March 31, 2018 15:50

nikomatsakis added S-blocked Status: Marked as blocked ❌ on something else such as an RFC or other implementation work. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 4, 2018

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chalkify: Implement Rule Implied-Bound-From-Trait

7ac35ea

tmandry force-pushed the rule-implied-bound-from-trait branch from 4ef87c6 to 7ac35ea Compare April 6, 2018 18:22

tmandry changed the title ~~[WIP] chalkify: Implement lowering rule Implied-Bound-From-Trait~~ chalkify: Implement lowering rule Implied-Bound-From-Trait Apr 6, 2018

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-blocked Status: Marked as blocked ❌ on something else such as an RFC or other implementation work. labels Apr 9, 2018

bors merged commit 7ac35ea into rust-lang:master Apr 10, 2018

bors mentioned this pull request Apr 10, 2018

traits: Implement interning for Goal and Clause #49800

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tmandry deleted the rule-implied-bound-from-trait branch April 14, 2018 03:50

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chalkify: Implement lowering rule Implied-Bound-From-Trait #49435

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chalkify: Implement lowering rule Implied-Bound-From-Trait #49435

chalkify: Implement lowering rule Implied-Bound-From-Trait #49435

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