simplify applying closure requirements

Don't use `ConstraintCategory::ClosureBounds`!
Set the category and the span for the promoted constraints to that of
the original constraint earlier than before.
This eliminates the need for `closure_bounds_mapping`.

compiler/rustc_borrowck/src/constraints/graph.rs

@@ -163,6 +163,7 @@ impl<'s, 'tcx, D: ConstraintGraphDirecton> Iterator for Edges<'s, 'tcx, D> {
                 span: DUMMY_SP,
                 category: ConstraintCategory::Internal,
                 variance_info: VarianceDiagInfo::default(),
+                from_closure: false,
             })
         } else {
             None

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -96,6 +96,9 @@ pub struct OutlivesConstraint<'tcx> {
 
     /// Variance diagnostic information
     pub variance_info: VarianceDiagInfo<'tcx>,
+
+    /// If this constraint is promoted from closure requirements.
+    pub from_closure: bool,
 }
 
 impl<'tcx> fmt::Debug for OutlivesConstraint<'tcx> {

compiler/rustc_borrowck/src/nll.rs

@@ -242,7 +242,6 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
         mut liveness_constraints,
         outlives_constraints,
         member_constraints,
-        closure_bounds_mapping,
         universe_causes,
         type_tests,
     } = constraints;
@@ -264,7 +263,6 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
         universal_region_relations,
         outlives_constraints,
         member_constraints,
-        closure_bounds_mapping,
         universe_causes,
         type_tests,
         liveness_constraints,

compiler/rustc_borrowck/src/region_infer/dump_mir.rs

@@ -74,8 +74,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
         constraints.sort_by_key(|c| (c.sup, c.sub));
         for constraint in &constraints {
-            let OutlivesConstraint { sup, sub, locations, category, span, variance_info: _ } =
+            let OutlivesConstraint { sup, sub, locations, category, span, .. } = constraint;
-                constraint;
             let (name, arg) = match locations {
                 Locations::All(span) => {
                     ("All", tcx.sess.source_map().span_to_embeddable_string(*span))

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -6,10 +6,9 @@ use rustc_data_structures::frozen::Frozen;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::graph::scc::Sccs;
 use rustc_errors::Diagnostic;
-use rustc_hir::def_id::{DefId, CRATE_DEF_ID};
+use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_hir::CRATE_HIR_ID;
 use rustc_index::vec::IndexVec;
-use rustc_infer::infer::canonical::QueryOutlivesConstraint;
 use rustc_infer::infer::outlives::test_type_match;
 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
 use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
@@ -19,9 +18,7 @@ use rustc_middle::mir::{
 };
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::traits::ObligationCauseCode;
-use rustc_middle::ty::{
+use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitable};
-    self, subst::SubstsRef, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitable,
-};
 use rustc_span::Span;
 
 use crate::{
@@ -89,10 +86,6 @@ pub struct RegionInferenceContext<'tcx> {
     /// `member_region_scc`.
     member_constraints_applied: Vec<AppliedMemberConstraint>,
 
-    /// Map closure bounds to a `Span` that should be used for error reporting.
-    closure_bounds_mapping:
-        FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>>,
-
     /// Map universe indexes to information on why we created it.
     universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
 
@@ -265,10 +258,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
         outlives_constraints: OutlivesConstraintSet<'tcx>,
         member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
-        closure_bounds_mapping: FxHashMap<
-            Location,
-            FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>,
-        >,
         universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
         type_tests: Vec<TypeTest<'tcx>>,
         liveness_constraints: LivenessValues<RegionVid>,
@@ -310,7 +299,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             rev_scc_graph: None,
             member_constraints,
             member_constraints_applied: Vec::new(),
-            closure_bounds_mapping,
             universe_causes,
             scc_universes,
             scc_representatives,
@@ -1804,18 +1792,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         }
     }
 
-    pub(crate) fn retrieve_closure_constraint_info(
-        &self,
-        constraint: OutlivesConstraint<'tcx>,
-    ) -> Option<(ConstraintCategory<'tcx>, Span)> {
-        match constraint.locations {
-            Locations::All(_) => None,
-            Locations::Single(loc) => {
-                self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub)).copied()
-            }
-        }
-    }
-
     /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`.
     pub(crate) fn find_outlives_blame_span(
         &self,
@@ -1921,6 +1897,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     span: p_c.definition_span,
                     category: ConstraintCategory::OpaqueType,
                     variance_info: ty::VarianceDiagInfo::default(),
+                    from_closure: false,
                 };
                 handle_constraint(constraint);
             }
@@ -2066,31 +2043,12 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // Classify each of the constraints along the path.
         let mut categorized_path: Vec<BlameConstraint<'tcx>> = path
             .iter()
-            .map(|constraint| {
+            .map(|constraint| BlameConstraint {
-                let (category, span, from_closure, cause_code) =
+                category: constraint.category,
-                    if constraint.category == ConstraintCategory::ClosureBounds {
+                from_closure: constraint.from_closure,
-                        if let Some((category, span)) =
+                cause: ObligationCause::new(constraint.span, CRATE_HIR_ID, cause_code.clone()),
-                            self.retrieve_closure_constraint_info(*constraint)
-                        {
-                            (category, span, true, ObligationCauseCode::MiscObligation)
-                        } else {
-                            (
-                                constraint.category,
-                                constraint.span,
-                                false,
-                                ObligationCauseCode::MiscObligation,
-                            )
-                        }
-                    } else {
-                        (constraint.category, constraint.span, false, cause_code.clone())
-                    };
-                BlameConstraint {
-                    category,
-                    from_closure,
-                    cause: ObligationCause::new(span, CRATE_HIR_ID, cause_code),
                 variance_info: constraint.variance_info,
                 outlives_constraint: *constraint,
-                }
             })
             .collect();
         debug!("categorized_path={:#?}", categorized_path);
@@ -2274,92 +2232,6 @@ impl<'tcx> RegionDefinition<'tcx> {
     }
 }
 
-pub trait ClosureRegionRequirementsExt<'tcx> {
-    fn apply_requirements(
-        &self,
-        tcx: TyCtxt<'tcx>,
-        closure_def_id: DefId,
-        closure_substs: SubstsRef<'tcx>,
-    ) -> Vec<QueryOutlivesConstraint<'tcx>>;
-}
-
-impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
-    /// Given an instance T of the closure type, this method
-    /// instantiates the "extra" requirements that we computed for the
-    /// closure into the inference context. This has the effect of
-    /// adding new outlives obligations to existing variables.
-    ///
-    /// As described on `ClosureRegionRequirements`, the extra
-    /// requirements are expressed in terms of regionvids that index
-    /// into the free regions that appear on the closure type. So, to
-    /// do this, we first copy those regions out from the type T into
-    /// a vector. Then we can just index into that vector to extract
-    /// out the corresponding region from T and apply the
-    /// requirements.
-    fn apply_requirements(
-        &self,
-        tcx: TyCtxt<'tcx>,
-        closure_def_id: DefId,
-        closure_substs: SubstsRef<'tcx>,
-    ) -> Vec<QueryOutlivesConstraint<'tcx>> {
-        debug!(
-            "apply_requirements(closure_def_id={:?}, closure_substs={:?})",
-            closure_def_id, closure_substs
-        );
-
-        // Extract the values of the free regions in `closure_substs`
-        // into a vector.  These are the regions that we will be
-        // relating to one another.
-        let closure_mapping = &UniversalRegions::closure_mapping(
-            tcx,
-            closure_substs,
-            self.num_external_vids,
-            closure_def_id.expect_local(),
-        );
-        debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
-
-        // Create the predicates.
-        self.outlives_requirements
-            .iter()
-            .map(|outlives_requirement| {
-                let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
-
-                match outlives_requirement.subject {
-                    ClosureOutlivesSubject::Region(region) => {
-                        let region = closure_mapping[region];
-                        debug!(
-                            "apply_requirements: region={:?} \
-                             outlived_region={:?} \
-                             outlives_requirement={:?}",
-                            region, outlived_region, outlives_requirement,
-                        );
-                        (
-                            ty::Binder::dummy(ty::OutlivesPredicate(
-                                region.into(),
-                                outlived_region,
-                            )),
-                            ConstraintCategory::BoringNoLocation,
-                        )
-                    }
-
-                    ClosureOutlivesSubject::Ty(ty) => {
-                        debug!(
-                            "apply_requirements: ty={:?} \
-                             outlived_region={:?} \
-                             outlives_requirement={:?}",
-                            ty, outlived_region, outlives_requirement,
-                        );
-                        (
-                            ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region)),
-                            ConstraintCategory::BoringNoLocation,
-                        )
-                    }
-                }
-            })
-            .collect()
-    }
-}
-
 #[derive(Clone, Debug)]
 pub struct BlameConstraint<'tcx> {
     pub category: ConstraintCategory<'tcx>,

compiler/rustc_borrowck/src/type_check/constraint_conversion.rs

@@ -1,10 +1,10 @@
-use rustc_infer::infer::canonical::QueryOutlivesConstraint;
+use rustc_hir::def_id::DefId;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::env::RegionBoundPairs;
 use rustc_infer::infer::outlives::obligations::{TypeOutlives, TypeOutlivesDelegate};
 use rustc_infer::infer::region_constraints::{GenericKind, VerifyBound};
 use rustc_infer::infer::{self, InferCtxt, SubregionOrigin};
-use rustc_middle::mir::ConstraintCategory;
+use rustc_middle::mir::{ClosureOutlivesSubject, ClosureRegionRequirements, ConstraintCategory};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::TypeFoldable;
 use rustc_middle::ty::{self, TyCtxt};
@@ -38,6 +38,7 @@ pub(crate) struct ConstraintConversion<'a, 'tcx> {
     locations: Locations,
     span: Span,
     category: ConstraintCategory<'tcx>,
+    from_closure: bool,
     constraints: &'a mut MirTypeckRegionConstraints<'tcx>,
 }
 
@@ -64,6 +65,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
             span,
             category,
             constraints,
+            from_closure: false,
         }
     }
 
@@ -81,12 +83,62 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         }
         self.constraints.member_constraints = tmp;
 
-        for query_constraint in outlives {
+        for (predicate, constraint_category) in outlives {
-            self.convert(query_constraint);
+            // At the moment, we never generate any "higher-ranked"
+            // region constraints like `for<'a> 'a: 'b`. At some point
+            // when we move to universes, we will, and this assertion
+            // will start to fail.
+            let predicate = predicate.no_bound_vars().unwrap_or_else(|| {
+                bug!("query_constraint {:?} contained bound vars", predicate,);
+            });
+
+            self.convert(predicate, *constraint_category);
         }
     }
 
-    fn convert(&mut self, query_constraint: &QueryOutlivesConstraint<'tcx>) {
+    /// Given an instance of the closure type, this method instantiates the "extra" requirements
+    /// that we computed for the closure. This has the effect of adding new outlives obligations
+    /// to existing region variables in `closure_substs`.
+    #[instrument(skip(self), level = "debug")]
+    pub fn apply_closure_requirements(
+        &mut self,
+        closure_requirements: &ClosureRegionRequirements<'tcx>,
+        closure_def_id: DefId,
+        closure_substs: ty::SubstsRef<'tcx>,
+    ) {
+        // Extract the values of the free regions in `closure_substs`
+        // into a vector.  These are the regions that we will be
+        // relating to one another.
+        let closure_mapping = &UniversalRegions::closure_mapping(
+            self.tcx,
+            closure_substs,
+            closure_requirements.num_external_vids,
+            closure_def_id.expect_local(),
+        );
+        debug!(?closure_mapping);
+
+        // Create the predicates.
+        let backup = (self.category, self.span, self.from_closure);
+        self.from_closure = true;
+        for outlives_requirement in &closure_requirements.outlives_requirements {
+            let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
+            let subject = match outlives_requirement.subject {
+                ClosureOutlivesSubject::Region(re) => closure_mapping[re].into(),
+                ClosureOutlivesSubject::Ty(ty) => ty.into(),
+            };
+
+            self.category = outlives_requirement.category;
+            self.span = outlives_requirement.blame_span;
+            self.convert(ty::OutlivesPredicate(subject, outlived_region), self.category);
+        }
+        (self.category, self.span, self.from_closure) = backup;
+    }
+
+    fn convert(
+        &mut self,
+        predicate: ty::OutlivesPredicate<ty::GenericArg<'tcx>, ty::Region<'tcx>>,
+        constraint_category: ConstraintCategory<'tcx>,
+    ) {
         debug!("generate: constraints at: {:#?}", self.locations);
 
         // Extract out various useful fields we'll need below.
@@ -94,17 +146,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
             tcx, region_bound_pairs, implicit_region_bound, param_env, ..
         } = *self;
 
-        // At the moment, we never generate any "higher-ranked"
+        let ty::OutlivesPredicate(k1, r2) = predicate;
-        // region constraints like `for<'a> 'a: 'b`. At some point
-        // when we move to universes, we will, and this assertion
-        // will start to fail.
-        let ty::OutlivesPredicate(k1, r2) =
-            query_constraint.0.no_bound_vars().unwrap_or_else(|| {
-                bug!("query_constraint {:?} contained bound vars", query_constraint,);
-            });
-
-        let constraint_category = query_constraint.1;
-
         match k1.unpack() {
             GenericArgKind::Lifetime(r1) => {
                 let r1_vid = self.to_region_vid(r1);
@@ -188,6 +230,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
             sub,
             sup,
             variance_info: ty::VarianceDiagInfo::default(),
+            from_closure: self.from_closure,
         });
     }
 

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -27,7 +27,7 @@ use rustc_middle::mir::AssertKind;
 use rustc_middle::mir::*;
 use rustc_middle::ty::adjustment::PointerCast;
 use rustc_middle::ty::cast::CastTy;
-use rustc_middle::ty::subst::{GenericArgKind, SubstsRef, UserSubsts};
+use rustc_middle::ty::subst::{SubstsRef, UserSubsts};
 use rustc_middle::ty::visit::TypeVisitable;
 use rustc_middle::ty::{
     self, Binder, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, Dynamic,
@@ -61,7 +61,7 @@ use crate::{
     region_infer::values::{
         LivenessValues, PlaceholderIndex, PlaceholderIndices, RegionValueElements,
     },
-    region_infer::{ClosureRegionRequirementsExt, TypeTest},
+    region_infer::TypeTest,
     type_check::free_region_relations::{CreateResult, UniversalRegionRelations},
     universal_regions::{DefiningTy, UniversalRegions},
     Upvar,
@@ -144,7 +144,6 @@ pub(crate) fn type_check<'mir, 'tcx>(
         liveness_constraints: LivenessValues::new(elements.clone()),
         outlives_constraints: OutlivesConstraintSet::default(),
         member_constraints: MemberConstraintSet::default(),
-        closure_bounds_mapping: Default::default(),
         type_tests: Vec::default(),
         universe_causes: FxHashMap::default(),
     };
@@ -585,7 +584,6 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
         let all_facts = &mut None;
         let mut constraints = Default::default();
         let mut type_tests = Default::default();
-        let mut closure_bounds = Default::default();
         let mut liveness_constraints =
             LivenessValues::new(Rc::new(RegionValueElements::new(&promoted_body)));
         // Don't try to add borrow_region facts for the promoted MIR
@@ -597,10 +595,6 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
                 &mut constraints,
             );
             mem::swap(&mut this.cx.borrowck_context.constraints.type_tests, &mut type_tests);
-            mem::swap(
-                &mut this.cx.borrowck_context.constraints.closure_bounds_mapping,
-                &mut closure_bounds,
-            );
             mem::swap(
                 &mut this.cx.borrowck_context.constraints.liveness_constraints,
                 &mut liveness_constraints,
@@ -653,18 +647,6 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
                     .add_element(region, location);
             }
         }
-
-        if !closure_bounds.is_empty() {
-            let combined_bounds_mapping =
-                closure_bounds.into_iter().flat_map(|(_, value)| value).collect();
-            let existing = self
-                .cx
-                .borrowck_context
-                .constraints
-                .closure_bounds_mapping
-                .insert(location, combined_bounds_mapping);
-            assert!(existing.is_none(), "Multiple promoteds/closures at the same location.");
-        }
     }
 
     fn sanitize_projection(
@@ -941,9 +923,6 @@ pub(crate) struct MirTypeckRegionConstraints<'tcx> {
 
     pub(crate) member_constraints: MemberConstraintSet<'tcx, RegionVid>,
 
-    pub(crate) closure_bounds_mapping:
-        FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>>,
-
     pub(crate) universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
 
     pub(crate) type_tests: Vec<TypeTest<'tcx>>,
@@ -2562,6 +2541,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 span: location.to_locations().span(body),
                                 category,
                                 variance_info: ty::VarianceDiagInfo::default(),
+                                from_closure: false,
                             });
 
                             match mutbl {
@@ -2679,62 +2659,22 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         substs: SubstsRef<'tcx>,
         location: Location,
     ) -> ty::InstantiatedPredicates<'tcx> {
-        if let Some(ref closure_region_requirements) = tcx.mir_borrowck(def_id).closure_requirements
+        if let Some(ref closure_requirements) = tcx.mir_borrowck(def_id).closure_requirements {
-        {
+            constraint_conversion::ConstraintConversion::new(
-            let closure_constraints = QueryRegionConstraints {
+                self.infcx,
-                outlives: closure_region_requirements.apply_requirements(
+                self.borrowck_context.universal_regions,
-                    tcx,
+                self.region_bound_pairs,
+                self.implicit_region_bound,
+                self.param_env,
+                location.to_locations(),
+                DUMMY_SP,                   // irrelevant; will be overrided.
+                ConstraintCategory::Boring, // same as above.
+                &mut self.borrowck_context.constraints,
+            )
+            .apply_closure_requirements(
+                &closure_requirements,
                 def_id.to_def_id(),
                 substs,
-                ),
-
-                // Presently, closures never propagate member
-                // constraints to their parents -- they are enforced
-                // locally.  This is largely a non-issue as member
-                // constraints only come from `-> impl Trait` and
-                // friends which don't appear (thus far...) in
-                // closures.
-                member_constraints: vec![],
-            };
-
-            let bounds_mapping = closure_constraints
-                .outlives
-                .iter()
-                .enumerate()
-                .filter_map(|(idx, constraint)| {
-                    let ty::OutlivesPredicate(k1, r2) =
-                        constraint.0.no_bound_vars().unwrap_or_else(|| {
-                            bug!("query_constraint {:?} contained bound vars", constraint,);
-                        });
-
-                    match k1.unpack() {
-                        GenericArgKind::Lifetime(r1) => {
-                            // constraint is r1: r2
-                            let r1_vid = self.borrowck_context.universal_regions.to_region_vid(r1);
-                            let r2_vid = self.borrowck_context.universal_regions.to_region_vid(r2);
-                            let outlives_requirements =
-                                &closure_region_requirements.outlives_requirements[idx];
-                            Some((
-                                (r1_vid, r2_vid),
-                                (outlives_requirements.category, outlives_requirements.blame_span),
-                            ))
-                        }
-                        GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
-                    }
-                })
-                .collect();
-
-            let existing = self
-                .borrowck_context
-                .constraints
-                .closure_bounds_mapping
-                .insert(location, bounds_mapping);
-            assert!(existing.is_none(), "Multiple closures at the same location.");
-
-            self.push_region_constraints(
-                location.to_locations(),
-                ConstraintCategory::ClosureBounds,
-                &closure_constraints,
             );
         }
 

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -136,6 +136,7 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx>
                 span: self.locations.span(self.type_checker.body),
                 category: self.category,
                 variance_info: info,
+                from_closure: false,
             },
         );
     }

src/test/ui/nll/ty-outlives/projection-two-region-trait-bound-closure.stderr

@@ -167,7 +167,7 @@ LL | |     T: Anything<'b, 'b>,
    = note: defining type: two_regions::<'_#1r, T>
 
 error: lifetime may not live long enough
-  --> $DIR/projection-two-region-trait-bound-closure.rs:87:29
+  --> $DIR/projection-two-region-trait-bound-closure.rs:87:5
    |
 LL | fn two_regions<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
    |                --  -- lifetime `'b` defined here
@@ -175,9 +175,12 @@ LL | fn two_regions<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
    |                lifetime `'a` defined here
 ...
 LL |     with_signature(cell, t, |cell, t| require(cell, t));
-   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^ closure body requires that `'b` must outlive `'a`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must outlive `'a`
    |
    = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of the type `Cell<&'_#8r ()>`, which makes the generic argument `&'_#8r ()` invariant
+   = note: the struct `Cell<T>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 note: external requirements
   --> $DIR/projection-two-region-trait-bound-closure.rs:97:29