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chalkify: Fix lowering of traits with supertraits #49971

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chalkify: Fix lowering of traits with supertraits #49971

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12 changes: 10 additions & 2 deletions src/librustc_traits/lowering.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -172,11 +172,19 @@ fn program_clauses_for_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefI
// ```

// `FromEnv(WC) :- FromEnv(Self: Trait<P1..Pn>)`, for each where clause WC
// FIXME: Remove the [1..] slice; this is a hack because the query
// FIXME: Remove the filter; this is a hack because the query
// predicates_of currently includes the trait itself (`Self: Trait<P1..Pn>`).
let where_clauses = &tcx.predicates_of(def_id).predicates;
let implied_bound_clauses =
where_clauses[1..].into_iter()
where_clauses.into_iter()
.filter(|wc| {
if let ty::Predicate::Trait(pred) = wc {
if pred.skip_binder().def_id() == def_id {
return false;
}
}
true
})
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It seems like this would also skip a predicate of the form:

trait A where <Self as A>::Item: A {
    type Item;
}

or equivalently:

trait A {
    type Item: A;
}

since you are only relying on the def id. Actually maybe it would be better to just wait for your PR which removes the Self: Trait predicate...

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Ah, good catch. I'll try and make progress on that PR today. A patch that might possibly still work here is to filter out only the first predicate that matches this condition, but it still feels quite brittle!

.map(|wc| implied_bound_from_trait(tcx, trait_pred, wc));

Lrc::new(tcx.mk_clauses(clauses.chain(implied_bound_clauses)))
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21 changes: 21 additions & 0 deletions src/test/ui/chalkify/lower_trait_supertrait.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![feature(rustc_attrs)]

trait Bar {}

#[rustc_dump_program_clauses] //~ ERROR Implemented
//~^ ERROR FromEnv
trait Foo: Bar {}

fn main() {
println!("hello");
}
14 changes: 14 additions & 0 deletions src/test/ui/chalkify/lower_trait_supertrait.stderr
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@@ -0,0 +1,14 @@
error: Implemented(Self: Foo) :- FromEnv(Self: Foo).
--> $DIR/lower_trait_supertrait.rs:15:1
|
LL | #[rustc_dump_program_clauses] //~ ERROR Implemented
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: FromEnv(Self: Bar) :- FromEnv(Self: Foo).
--> $DIR/lower_trait_supertrait.rs:15:1
|
LL | #[rustc_dump_program_clauses] //~ ERROR Implemented
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors