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Elaborate trait obligations when typechecking impls #43786

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52 changes: 47 additions & 5 deletions src/librustc/ty/wf.rs
Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
-> Vec<traits::PredicateObligation<'tcx>>
{
let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
wf.compute_trait_ref(trait_ref);
wf.compute_trait_ref(trait_ref, Elaborate::All);
wf.normalize()
}

Expand All @@ -74,7 +74,7 @@ pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
// (*) ok to skip binders, because wf code is prepared for it
match *predicate {
ty::Predicate::Trait(ref t) => {
wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
}
ty::Predicate::Equate(ref t) => {
wf.compute(t.skip_binder().0);
Expand Down Expand Up @@ -114,6 +114,35 @@ struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
out: Vec<traits::PredicateObligation<'tcx>>,
}

/// Controls whether we "elaborate" supertraits and so forth on the WF
/// predicates. This is a kind of hack to address #43784. The
/// underlying problem in that issue was a trait structure like:
///
/// ```
/// trait Foo: Copy { }
/// trait Bar: Foo { }
/// impl<T: Bar> Foo for T { }
/// impl<T> Bar for T { }
/// ```
///
/// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
/// we decide that this is true because `T: Bar` is in the
/// where-clauses (and we can elaborate that to include `T:
/// Copy`). This wouldn't be a problem, except that when we check the
/// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
/// impl. And so nowhere did we check that `T: Copy` holds!
///
/// To resolve this, we elaborate the WF requirements that must be
/// proven when checking impls. This means that (e.g.) the `impl Bar
/// for T` will be forced to prove not only that `T: Foo` but also `T:
/// Copy` (which it won't be able to do, because there is no `Copy`
/// impl for `T`).
#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum Elaborate {
All,
None,
}

impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
traits::ObligationCause::new(self.span, self.body_id, code)
Expand All @@ -135,12 +164,25 @@ impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {

/// Pushes the obligations required for `trait_ref` to be WF into
/// `self.out`.
fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
self.out.extend(obligations);

let cause = self.cause(traits::MiscObligation);
let param_env = self.param_env;

if let Elaborate::All = elaborate {
let predicates = obligations.iter()
.map(|obligation| obligation.predicate.clone())
.collect();
let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates);
let implied_obligations = implied_obligations.map(|pred| {
traits::Obligation::new(cause.clone(), param_env, pred)
});
self.out.extend(implied_obligations);
}

self.out.extend(obligations);

self.out.extend(
trait_ref.substs.types()
.filter(|ty| !ty.has_escaping_regions())
Expand All @@ -156,7 +198,7 @@ impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
// WF and (b) the trait-ref holds. (It may also be
// normalizable and be WF that way.)
let trait_ref = data.trait_ref(self.infcx.tcx);
self.compute_trait_ref(&trait_ref);
self.compute_trait_ref(&trait_ref, Elaborate::None);

if !data.has_escaping_regions() {
let predicate = trait_ref.to_predicate();
Expand Down
25 changes: 25 additions & 0 deletions src/test/compile-fail/issue-43784-associated-type.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
// Copyright 2017 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.

pub trait Partial<X: ?Sized>: Copy {
}

pub trait Complete {
type Assoc: Partial<Self>;
}

impl<T> Partial<T> for T::Assoc where
T: Complete
{
}

impl<T> Complete for T { //~ ERROR the trait bound `T: std::marker::Copy` is not satisfied
type Assoc = T;
}
18 changes: 18 additions & 0 deletions src/test/compile-fail/issue-43784-supertrait.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
// Copyright 2017 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.

pub trait Partial: Copy {
}

pub trait Complete: Partial {
}

impl<T> Partial for T where T: Complete {}
impl<T> Complete for T {} //~ ERROR the trait bound `T: std::marker::Copy` is not satisfied