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Avoid cycle with projections from object types
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Normalizing `<dyn Iterator<Item = ()> as Iterator>::Item` no longer
requires selecting `dyn Iterator<Item = ()>: Iterator`. This was
previously worked around by using a special type-folder to normalize
things.
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@matthewjasper
matthewjasper committed Oct 6, 2020
1 parent 34e5a49 commit 596d6c4
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Showing 11 changed files with 193 additions and 350 deletions.
207 changes: 77 additions & 130 deletions compiler/rustc_trait_selection/src/traits/project.rs
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@@ -1,6 +1,5 @@
//! Code for projecting associated types out of trait references.

use super::elaborate_predicates;
use super::specialization_graph;
use super::translate_substs;
use super::util;
Expand Down Expand Up @@ -53,13 +52,16 @@ pub enum ProjectionTyError<'tcx> {

#[derive(PartialEq, Eq, Debug)]
enum ProjectionTyCandidate<'tcx> {
// from a where-clause in the env or object type
/// From a where-clause in the env or object type
ParamEnv(ty::PolyProjectionPredicate<'tcx>),

// from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
/// From the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
TraitDef(ty::PolyProjectionPredicate<'tcx>),

// from a "impl" (or a "pseudo-impl" returned by select)
/// Bounds specified on an object type
Object(ty::PolyProjectionPredicate<'tcx>),

/// From a "impl" (or a "pseudo-impl" returned by select)
Select(Selection<'tcx>),
}

Expand Down Expand Up @@ -561,14 +563,6 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
} else {
obligations.extend(ty.obligations);
}

obligations.push(get_paranoid_cache_value_obligation(
infcx,
param_env,
projection_ty,
cause,
depth,
));
return Ok(Some(ty.value));
}
Err(ProjectionCacheEntry::Error) => {
Expand Down Expand Up @@ -703,45 +697,6 @@ fn prune_cache_value_obligations<'a, 'tcx>(
NormalizedTy { value: result.value, obligations }
}

/// Whenever we give back a cache result for a projection like `<T as
/// Trait>::Item ==> X`, we *always* include the obligation to prove
/// that `T: Trait` (we may also include some other obligations). This
/// may or may not be necessary -- in principle, all the obligations
/// that must be proven to show that `T: Trait` were also returned
/// when the cache was first populated. But there are some vague concerns,
/// and so we take the precautionary measure of including `T: Trait` in
/// the result:
///
/// Concern #1. The current setup is fragile. Perhaps someone could
/// have failed to prove the concerns from when the cache was
/// populated, but also not have used a snapshot, in which case the
/// cache could remain populated even though `T: Trait` has not been
/// shown. In this case, the "other code" is at fault -- when you
/// project something, you are supposed to either have a snapshot or
/// else prove all the resulting obligations -- but it's still easy to
/// get wrong.
///
/// Concern #2. Even within the snapshot, if those original
/// obligations are not yet proven, then we are able to do projections
/// that may yet turn out to be wrong. This *may* lead to some sort
/// of trouble, though we don't have a concrete example of how that
/// can occur yet. But it seems risky at best.
fn get_paranoid_cache_value_obligation<'a, 'tcx>(
infcx: &'a InferCtxt<'a, 'tcx>,
param_env: ty::ParamEnv<'tcx>,
projection_ty: ty::ProjectionTy<'tcx>,
cause: ObligationCause<'tcx>,
depth: usize,
) -> PredicateObligation<'tcx> {
let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
Obligation {
cause,
recursion_depth: depth,
param_env,
predicate: trait_ref.without_const().to_predicate(infcx.tcx),
}
}

/// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
/// hold. In various error cases, we cannot generate a valid
/// normalized projection. Therefore, we create an inference variable
Expand Down Expand Up @@ -848,12 +803,21 @@ fn project_type<'cx, 'tcx>(

assemble_candidates_from_trait_def(selcx, obligation, &obligation_trait_ref, &mut candidates);

assemble_candidates_from_impls(selcx, obligation, &obligation_trait_ref, &mut candidates);
assemble_candidates_from_object_ty(selcx, obligation, &obligation_trait_ref, &mut candidates);

if let ProjectionTyCandidateSet::Single(ProjectionTyCandidate::Object(_)) = candidates {
// Avoid normalization cycle from selection (see
// `assemble_candidates_from_object_ty`).
// FIXME(lazy_normalization): Lazy normalization should save us from
// having to do special case this.
} else {
assemble_candidates_from_impls(selcx, obligation, &obligation_trait_ref, &mut candidates);
};

match candidates {
ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
confirm_candidate(selcx, obligation, &obligation_trait_ref, candidate),
)),
ProjectionTyCandidateSet::Single(candidate) => {
Ok(ProjectedTy::Progress(confirm_candidate(selcx, obligation, candidate)))
}
ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
selcx
.tcx()
Expand Down Expand Up @@ -932,6 +896,53 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
)
}

/// In the case of a trait object like
/// `<dyn Iterator<Item = ()> as Iterator>::Item` we can use the existential
/// predicate in the trait object.
///
/// We don't go through the select candidate for these bounds to avoid cycles:
/// In the above case, `dyn Iterator<Item = ()>: Iterator` would create a
/// nested obligation of `<dyn Iterator<Item = ()> as Iterator>::Item: Sized`,
/// this then has to be normalized without having to prove
/// `dyn Iterator<Item = ()>: Iterator` again.
fn assemble_candidates_from_object_ty<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
obligation_trait_ref: &ty::TraitRef<'tcx>,
candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
) {
debug!("assemble_candidates_from_object_ty(..)");

let tcx = selcx.tcx();

let self_ty = obligation_trait_ref.self_ty();
let object_ty = selcx.infcx().shallow_resolve(self_ty);
let data = match object_ty.kind {
ty::Dynamic(ref data, ..) => data,
ty::Infer(ty::TyVar(_)) => {
// If the self-type is an inference variable, then it MAY wind up
// being an object type, so induce an ambiguity.
candidate_set.mark_ambiguous();
return;
}
_ => return,
};
let env_predicates = data
.projection_bounds()
.filter(|bound| bound.item_def_id() == obligation.predicate.item_def_id)
.map(|p| p.with_self_ty(tcx, object_ty).to_predicate(tcx));

assemble_candidates_from_predicates(
selcx,
obligation,
obligation_trait_ref,
candidate_set,
ProjectionTyCandidate::Object,
env_predicates,
false,
);
}

fn assemble_candidates_from_predicates<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
Expand Down Expand Up @@ -1000,7 +1011,6 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
super::ImplSource::Closure(_)
| super::ImplSource::Generator(_)
| super::ImplSource::FnPointer(_)
| super::ImplSource::Object(_)
| super::ImplSource::TraitAlias(_) => {
debug!("assemble_candidates_from_impls: impl_source={:?}", impl_source);
true
Expand Down Expand Up @@ -1125,6 +1135,12 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
// in `assemble_candidates_from_param_env`.
false
}
super::ImplSource::Object(_) => {
// Handled by the `Object` projection candidate. See
// `assemble_candidates_from_object_ty` for an explanation of
// why we special case object types.
false
}
super::ImplSource::AutoImpl(..) | super::ImplSource::Builtin(..) => {
// These traits have no associated types.
selcx.tcx().sess.delay_span_bug(
Expand All @@ -1150,13 +1166,13 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
fn confirm_candidate<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
obligation_trait_ref: &ty::TraitRef<'tcx>,
candidate: ProjectionTyCandidate<'tcx>,
) -> Progress<'tcx> {
debug!("confirm_candidate(candidate={:?}, obligation={:?})", candidate, obligation);

let mut progress = match candidate {
ProjectionTyCandidate::ParamEnv(poly_projection) => {
ProjectionTyCandidate::ParamEnv(poly_projection)
| ProjectionTyCandidate::Object(poly_projection) => {
confirm_param_env_candidate(selcx, obligation, poly_projection, false)
}

Expand All @@ -1165,7 +1181,7 @@ fn confirm_candidate<'cx, 'tcx>(
}

ProjectionTyCandidate::Select(impl_source) => {
confirm_select_candidate(selcx, obligation, obligation_trait_ref, impl_source)
confirm_select_candidate(selcx, obligation, impl_source)
}
};
// When checking for cycle during evaluation, we compare predicates with
Expand All @@ -1182,7 +1198,6 @@ fn confirm_candidate<'cx, 'tcx>(
fn confirm_select_candidate<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
obligation_trait_ref: &ty::TraitRef<'tcx>,
impl_source: Selection<'tcx>,
) -> Progress<'tcx> {
match impl_source {
Expand All @@ -1193,10 +1208,8 @@ fn confirm_select_candidate<'cx, 'tcx>(
super::ImplSource::DiscriminantKind(data) => {
confirm_discriminant_kind_candidate(selcx, obligation, data)
}
super::ImplSource::Object(_) => {
confirm_object_candidate(selcx, obligation, obligation_trait_ref)
}
super::ImplSource::AutoImpl(..)
super::ImplSource::Object(_)
| super::ImplSource::AutoImpl(..)
| super::ImplSource::Param(..)
| super::ImplSource::Builtin(..)
| super::ImplSource::TraitAlias(..) =>
Expand All @@ -1211,72 +1224,6 @@ fn confirm_select_candidate<'cx, 'tcx>(
}
}

fn confirm_object_candidate<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
obligation_trait_ref: &ty::TraitRef<'tcx>,
) -> Progress<'tcx> {
let self_ty = obligation_trait_ref.self_ty();
let object_ty = selcx.infcx().shallow_resolve(self_ty);
debug!("confirm_object_candidate(object_ty={:?})", object_ty);
let data = match object_ty.kind() {
ty::Dynamic(data, ..) => data,
_ => span_bug!(
obligation.cause.span,
"confirm_object_candidate called with non-object: {:?}",
object_ty
),
};
let env_predicates = data
.projection_bounds()
.map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate(selcx.tcx()));
let env_predicate = {
let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);

// select only those projections that are actually projecting an
// item with the correct name

let env_predicates = env_predicates.filter_map(|o| match o.predicate.skip_binders() {
ty::PredicateAtom::Projection(data)
if data.projection_ty.item_def_id == obligation.predicate.item_def_id =>
{
Some(ty::Binder::bind(data))
}
_ => None,
});

// select those with a relevant trait-ref
let mut env_predicates = env_predicates.filter(|data| {
let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
selcx.infcx().probe(|_| {
selcx
.infcx()
.at(&obligation.cause, obligation.param_env)
.sup(obligation_poly_trait_ref, data_poly_trait_ref)
.is_ok()
})
});

// select the first matching one; there really ought to be one or
// else the object type is not WF, since an object type should
// include all of its projections explicitly
match env_predicates.next() {
Some(env_predicate) => env_predicate,
None => {
debug!(
"confirm_object_candidate: no env-predicate \
found in object type `{:?}`; ill-formed",
object_ty
);
return Progress::error(selcx.tcx());
}
}
};

confirm_param_env_candidate(selcx, obligation, env_predicate, false)
}

fn confirm_generator_candidate<'cx, 'tcx>(
selcx: &mut SelectionContext<'cx, 'tcx>,
obligation: &ProjectionTyObligation<'tcx>,
Expand Down

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