Neither require nor imply lifetime bounds on opaque type for well formedness

Author: oli-obk

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -362,6 +362,11 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
                     self.region_bound_pairs
                         .insert(ty::OutlivesPredicate(GenericKind::Projection(projection_b), r_a));
                 }
+
+                OutlivesBound::RegionSubOpaque(r_a, def_id, substs) => {
+                    self.region_bound_pairs
+                        .insert(ty::OutlivesPredicate(GenericKind::Opaque(def_id, substs), r_a));
+                }
             }
         }
     }

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2374,6 +2374,9 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let labeled_user_string = match bound_kind {
             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
+            GenericKind::Opaque(def_id, substs) => {
+                format!("the opaque type `{}`", self.tcx.def_path_str_with_substs(def_id, substs))
+            }
         };
 
         if let Some(SubregionOrigin::CompareImplItemObligation {

compiler/rustc_infer/src/infer/outlives/components.rs

@@ -3,8 +3,9 @@
 // RFC for reference.
 
 use rustc_data_structures::sso::SsoHashSet;
+use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
+use rustc_middle::ty::{self, SubstsRef, Ty, TyCtxt, TypeVisitable};
 use smallvec::{smallvec, SmallVec};
 
 #[derive(Debug)]
@@ -45,6 +46,8 @@ pub enum Component<'tcx> {
     // them. This gives us room to improve the regionck reasoning in
     // the future without breaking backwards compat.
     EscapingProjection(Vec<Component<'tcx>>),
+
+    Opaque(DefId, SubstsRef<'tcx>),
 }
 
 /// Push onto `out` all the things that must outlive `'a` for the condition
@@ -120,6 +123,17 @@ fn compute_components<'tcx>(
                 out.push(Component::Param(p));
             }
 
+            // Ignore lifetimes found in opaque types. Opaque types can
+            // have lifetimes in their substs which their hidden type doesn't
+            // actually use. If we inferred that an opaque type is outlived by
+            // its parameter lifetimes, then we could prove that any lifetime
+            // outlives any other lifetime, which is unsound.
+            // See https://github.com/rust-lang/rust/issues/84305 for
+            // more details.
+            ty::Opaque(def_id, substs) => {
+                out.push(Component::Opaque(def_id, substs));
+            },
+
             // For projections, we prefer to generate an obligation like
             // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the
             // regionck more ways to prove that it holds. However,
@@ -168,7 +182,6 @@ fn compute_components<'tcx>(
             ty::Float(..) |       // OutlivesScalar
             ty::Never |           // ...
             ty::Adt(..) |         // OutlivesNominalType
-            ty::Opaque(..) |      // OutlivesNominalType (ish)
             ty::Foreign(..) |     // OutlivesNominalType
             ty::Str |             // OutlivesScalar (ish)
             ty::Slice(..) |       // ...

compiler/rustc_infer/src/infer/outlives/env.rs

@@ -140,6 +140,10 @@ impl<'a, 'tcx> OutlivesEnvironmentBuilder<'tcx> {
                     self.region_bound_pairs
                         .insert(ty::OutlivesPredicate(GenericKind::Projection(projection_b), r_a));
                 }
+                OutlivesBound::RegionSubOpaque(r_a, def_id, substs) => {
+                    self.region_bound_pairs
+                        .insert(ty::OutlivesPredicate(GenericKind::Opaque(def_id, substs), r_a));
+                }
                 OutlivesBound::RegionSubRegion(r_a, r_b) => {
                     if let (ReEarlyBound(_) | ReFree(_), ReVar(vid_b)) = (r_a.kind(), r_b.kind()) {
                         infcx

compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -284,6 +284,9 @@ where
                 Component::Param(param_ty) => {
                     self.param_ty_must_outlive(origin, region, *param_ty);
                 }
+                Component::Opaque(def_id, substs) => {
+                    self.opaque_must_outlive(*def_id, substs, origin, region)
+                }
                 Component::Projection(projection_ty) => {
                     self.projection_must_outlive(origin, region, *projection_ty);
                 }
@@ -319,6 +322,27 @@ where
         self.delegate.push_verify(origin, generic, region, verify_bound);
     }
 
+    #[instrument(level = "debug", skip(self))]
+    fn opaque_must_outlive(
+        &mut self,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+        origin: infer::SubregionOrigin<'tcx>,
+        region: ty::Region<'tcx>,
+    ) {
+        self.generic_must_outlive(
+            origin,
+            region,
+            GenericKind::Opaque(def_id, substs),
+            def_id,
+            substs,
+            |ty| match *ty.kind() {
+                ty::Opaque(def_id, substs) => (def_id, substs),
+                _ => bug!("expected only projection types from env, not {:?}", ty),
+            },
+        );
+    }
+
     #[instrument(level = "debug", skip(self))]
     fn projection_must_outlive(
         &mut self,

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -47,6 +47,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
             GenericKind::Projection(projection_ty) => {
                 self.projection_bound(projection_ty, &mut visited)
             }
+            GenericKind::Opaque(def_id, substs) => self.opaque_bound(def_id, substs),
         }
     }
 
@@ -155,6 +156,20 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         VerifyBound::AnyBound(env_bounds.chain(trait_bounds).collect()).or(recursive_bound)
     }
 
+    fn opaque_bound(&self, def_id: DefId, substs: SubstsRef<'tcx>) -> VerifyBound<'tcx> {
+        let bounds: Vec<_> =
+            self.bounds(def_id, substs).map(|r| VerifyBound::OutlivedBy(r)).collect();
+        trace!("{:#?}", bounds);
+        if bounds.is_empty() {
+            // No bounds means the value must not have any lifetimes.
+            // FIXME: should we implicitly add 'static to `tcx.item_bounds` for opaque types, just
+            // like we add `Sized`?
+            VerifyBound::OutlivedBy(self.tcx.lifetimes.re_static)
+        } else {
+            VerifyBound::AnyBound(bounds)
+        }
+    }
+
     fn bound_from_components(
         &self,
         components: &[Component<'tcx>],
@@ -184,6 +199,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         match *component {
             Component::Region(lt) => VerifyBound::OutlivedBy(lt),
             Component::Param(param_ty) => self.param_bound(param_ty),
+            Component::Opaque(did, substs) => self.opaque_bound(did, substs),
             Component::Projection(projection_ty) => self.projection_bound(projection_ty, visited),
             Component::EscapingProjection(ref components) => {
                 self.bound_from_components(components, visited)

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -12,8 +12,10 @@ use rustc_data_structures::intern::Interned;
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::undo_log::UndoLogs;
 use rustc_data_structures::unify as ut;
+use rustc_hir::def_id::DefId;
 use rustc_index::vec::IndexVec;
 use rustc_middle::infer::unify_key::{RegionVidKey, UnifiedRegion};
+use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::ReStatic;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{ReLateBound, ReVar};
@@ -168,6 +170,7 @@ pub struct Verify<'tcx> {
 pub enum GenericKind<'tcx> {
     Param(ty::ParamTy),
     Projection(ty::ProjectionTy<'tcx>),
+    Opaque(DefId, SubstsRef<'tcx>),
 }
 
 /// Describes the things that some `GenericKind` value `G` is known to
@@ -747,6 +750,9 @@ impl<'tcx> fmt::Debug for GenericKind<'tcx> {
         match *self {
             GenericKind::Param(ref p) => write!(f, "{:?}", p),
             GenericKind::Projection(ref p) => write!(f, "{:?}", p),
+            GenericKind::Opaque(def_id, substs) => ty::tls::with(|tcx| {
+                write!(f, "{}", tcx.def_path_str_with_substs(def_id, tcx.lift(substs).unwrap()))
+            }),
         }
     }
 }
@@ -756,6 +762,9 @@ impl<'tcx> fmt::Display for GenericKind<'tcx> {
         match *self {
             GenericKind::Param(ref p) => write!(f, "{}", p),
             GenericKind::Projection(ref p) => write!(f, "{}", p),
+            GenericKind::Opaque(def_id, substs) => ty::tls::with(|tcx| {
+                write!(f, "{}", tcx.def_path_str_with_substs(def_id, tcx.lift(substs).unwrap()))
+            }),
         }
     }
 }
@@ -765,6 +774,7 @@ impl<'tcx> GenericKind<'tcx> {
         match *self {
             GenericKind::Param(ref p) => p.to_ty(tcx),
             GenericKind::Projection(ref p) => tcx.mk_projection(p.item_def_id, p.substs),
+            GenericKind::Opaque(def_id, substs) => tcx.mk_opaque(def_id, substs),
         }
     }
 }

compiler/rustc_infer/src/traits/util.rs

@@ -246,6 +246,13 @@ impl<'tcx> Elaborator<'tcx> {
 
                             Component::UnresolvedInferenceVariable(_) => None,
 
+                            Component::Opaque(def_id, substs) => {
+                                let ty = tcx.mk_opaque(def_id, substs);
+                                Some(ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(
+                                    ty, r_min,
+                                )))
+                            }
+
                             Component::Projection(projection) => {
                                 // We might end up here if we have `Foo<<Bar as Baz>::Assoc>: 'a`.
                                 // With this, we can deduce that `<Bar as Baz>::Assoc: 'a`.
@@ -262,8 +269,9 @@ impl<'tcx> Elaborator<'tcx> {
                                 None
                             }
                         })
-                        .map(ty::Binder::dummy)
-                        .map(|predicate_kind| predicate_kind.to_predicate(tcx))
+                        .map(|predicate_kind| {
+                            bound_predicate.rebind(predicate_kind).to_predicate(tcx)
+                        })
                         .filter(|&predicate| visited.insert(predicate))
                         .map(|predicate| {
                             predicate_obligation(

compiler/rustc_middle/src/traits/query.rs

@@ -8,8 +8,9 @@
 use crate::error::DropCheckOverflow;
 use crate::infer::canonical::{Canonical, QueryResponse};
 use crate::ty::error::TypeError;
-use crate::ty::subst::GenericArg;
+use crate::ty::subst::{GenericArg, SubstsRef};
 use crate::ty::{self, Ty, TyCtxt};
+use rustc_hir::def_id::DefId;
 use rustc_span::source_map::Span;
 use std::iter::FromIterator;
 
@@ -219,4 +220,5 @@ pub enum OutlivesBound<'tcx> {
     RegionSubRegion(ty::Region<'tcx>, ty::Region<'tcx>),
     RegionSubParam(ty::Region<'tcx>, ty::ParamTy),
     RegionSubProjection(ty::Region<'tcx>, ty::ProjectionTy<'tcx>),
+    RegionSubOpaque(ty::Region<'tcx>, DefId, SubstsRef<'tcx>),
 }

compiler/rustc_traits/src/implied_outlives_bounds.rs

@@ -153,6 +153,9 @@ fn implied_bounds_from_components<'tcx>(
                 Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
                 Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
                 Component::Projection(p) => Some(OutlivesBound::RegionSubProjection(sub_region, p)),
+                Component::Opaque(def_id, substs) => {
+                    Some(OutlivesBound::RegionSubOpaque(sub_region, def_id, substs))
+                }
                 Component::EscapingProjection(_) =>
                 // If the projection has escaping regions, don't
                 // try to infer any implied bounds even for its