Rollup merge of #98576 - lcnr:region-stuff-cool-beans, r=jackh726

small regions refactoring

these commits should be fairly self-contained

r? rust-lang/types

21 files changed, 38 insertions, 59 deletions

diff --git a/compiler/rustc_borrowck/src/diagnostics/region_name.rs b/compiler/rustc_borrowck/src/diagnostics/region_name.rs
index 8f3699553d9..e60e11f11df 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_name.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_name.rs
@@ -879,7 +879,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
         }
 
         let mut found = false;
-        tcx.fold_regions(tcx.type_of(body_parent_did), &mut true, |r: ty::Region<'tcx>, _| {
+        tcx.fold_regions(tcx.type_of(body_parent_did), |r: ty::Region<'tcx>, _| {
             if *r == ty::ReEarlyBound(region) {
                 found = true;
             }
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 0fe3b45bc7c..f5c9392948b 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -1009,7 +1009,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
         debug!("try_promote_type_test_subject(ty = {:?})", ty);
 
-        let ty = tcx.fold_regions(ty, &mut false, |r, _depth| {
+        let ty = tcx.fold_regions(ty, |r, _depth| {
             let region_vid = self.to_region_vid(r);
 
             // The challenge if this. We have some region variable `r`
@@ -1289,7 +1289,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        tcx.fold_regions(value, &mut false, |r, _db| {
+        tcx.fold_regions(value, |r, _db| {
             let vid = self.to_region_vid(r);
             let scc = self.constraint_sccs.scc(vid);
             let repr = self.scc_representatives[scc];
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index 81073758791..d182c0cf4e8 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -59,7 +59,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             debug!(?concrete_type, ?substs);
 
             let mut subst_regions = vec![self.universal_regions.fr_static];
-            let universal_substs = infcx.tcx.fold_regions(substs, &mut false, |region, _| {
+            let universal_substs = infcx.tcx.fold_regions(substs, |region, _| {
                 if let ty::RePlaceholder(..) = region.kind() {
                     // Higher kinded regions don't need remapping, they don't refer to anything outside of this the substs.
                     return region;
@@ -91,7 +91,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             subst_regions.dedup();
 
             let universal_concrete_type =
-                infcx.tcx.fold_regions(concrete_type, &mut false, |region, _| match *region {
+                infcx.tcx.fold_regions(concrete_type, |region, _| match *region {
                     ty::ReVar(vid) => subst_regions
                         .iter()
                         .find(|ur_vid| self.eval_equal(vid, **ur_vid))
@@ -146,7 +146,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        tcx.fold_regions(ty, &mut false, |region, _| match *region {
+        tcx.fold_regions(ty, |region, _| match *region {
             ty::ReVar(vid) => {
                 // Find something that we can name
                 let upper_bound = self.approx_universal_upper_bound(vid);
diff --git a/compiler/rustc_borrowck/src/renumber.rs b/compiler/rustc_borrowck/src/renumber.rs
index 2876d60527f..7a8ce621c5d 100644
--- a/compiler/rustc_borrowck/src/renumber.rs
+++ b/compiler/rustc_borrowck/src/renumber.rs
@@ -31,7 +31,7 @@ pub fn renumber_regions<'tcx, T>(infcx: &InferCtxt<'_, 'tcx>, value: T) -> T
 where
     T: TypeFoldable<'tcx>,
 {
-    infcx.tcx.fold_regions(value, &mut false, |_region, _depth| {
+    infcx.tcx.fold_regions(value, |_region, _depth| {
         let origin = NllRegionVariableOrigin::Existential { from_forall: false };
         infcx.next_nll_region_var(origin)
     })
diff --git a/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs b/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
index 3c9e3870aea..5e33d9d25c2 100644
--- a/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
+++ b/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
@@ -23,7 +23,7 @@ pub(crate) struct ConstraintConversion<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     universal_regions: &'a UniversalRegions<'tcx>,
     region_bound_pairs: &'a RegionBoundPairs<'tcx>,
-    implicit_region_bound: Option<ty::Region<'tcx>>,
+    implicit_region_bound: ty::Region<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     locations: Locations,
     span: Span,
@@ -36,7 +36,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
         infcx: &'a InferCtxt<'a, 'tcx>,
         universal_regions: &'a UniversalRegions<'tcx>,
         region_bound_pairs: &'a RegionBoundPairs<'tcx>,
-        implicit_region_bound: Option<ty::Region<'tcx>>,
+        implicit_region_bound: ty::Region<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         locations: Locations,
         span: Span,
@@ -108,7 +108,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
                 // create new region variables, which can't be done later when
                 // verifying these bounds.
                 if t1.has_placeholders() {
-                    t1 = tcx.fold_regions(t1, &mut false, |r, _| match *r {
+                    t1 = tcx.fold_regions(t1, |r, _| match *r {
                         ty::RePlaceholder(placeholder) => {
                             self.constraints.placeholder_region(self.infcx, placeholder)
                         }
@@ -120,7 +120,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
                     &mut *self,
                     tcx,
                     region_bound_pairs,
-                    implicit_region_bound,
+                    Some(implicit_region_bound),
                     param_env,
                 )
                 .type_must_outlive(origin, t1, r2);
diff --git a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
index 813307356c4..421ef5be812 100644
--- a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
+++ b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
@@ -61,7 +61,7 @@ pub(crate) struct CreateResult<'tcx> {
 pub(crate) fn create<'tcx>(
     infcx: &InferCtxt<'_, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    implicit_region_bound: Option<ty::Region<'tcx>>,
+    implicit_region_bound: ty::Region<'tcx>,
     universal_regions: &Rc<UniversalRegions<'tcx>>,
     constraints: &mut MirTypeckRegionConstraints<'tcx>,
 ) -> CreateResult<'tcx> {
@@ -223,7 +223,7 @@ struct UniversalRegionRelationsBuilder<'this, 'tcx> {
     infcx: &'this InferCtxt<'this, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     universal_regions: Rc<UniversalRegions<'tcx>>,
-    implicit_region_bound: Option<ty::Region<'tcx>>,
+    implicit_region_bound: ty::Region<'tcx>,
     constraints: &'this mut MirTypeckRegionConstraints<'tcx>,
 
     // outputs:
diff --git a/compiler/rustc_borrowck/src/type_check/input_output.rs b/compiler/rustc_borrowck/src/type_check/input_output.rs
index 2259a59e195..2a6ca5246da 100644
--- a/compiler/rustc_borrowck/src/type_check/input_output.rs
+++ b/compiler/rustc_borrowck/src/type_check/input_output.rs
@@ -230,7 +230,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 self.infcx,
                 &self.borrowck_context.universal_regions,
                 &self.region_bound_pairs,
-                Some(self.implicit_region_bound),
+                self.implicit_region_bound,
                 self.param_env,
                 Locations::All(DUMMY_SP),
                 DUMMY_SP,
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 355254fe942..542fc6b0f48 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -157,7 +157,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
     } = free_region_relations::create(
         infcx,
         param_env,
-        Some(implicit_region_bound),
+        implicit_region_bound,
         universal_regions,
         &mut constraints,
     );
@@ -1142,7 +1142,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             self.infcx,
             self.borrowck_context.universal_regions,
             self.region_bound_pairs,
-            Some(self.implicit_region_bound),
+            self.implicit_region_bound,
             self.param_env,
             locations,
             locations.span(self.body),
diff --git a/compiler/rustc_borrowck/src/universal_regions.rs b/compiler/rustc_borrowck/src/universal_regions.rs
index c2c093f9f2f..89d84fcf09c 100644
--- a/compiler/rustc_borrowck/src/universal_regions.rs
+++ b/compiler/rustc_borrowck/src/universal_regions.rs
@@ -725,7 +725,7 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        self.tcx.fold_regions(value, &mut false, |_region, _depth| self.next_nll_region_var(origin))
+        self.tcx.fold_regions(value, |_region, _depth| self.next_nll_region_var(origin))
     }
 
     #[instrument(level = "debug", skip(self, indices))]
@@ -817,9 +817,7 @@ impl<'tcx> UniversalRegionIndices<'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        tcx.fold_regions(value, &mut false, |region, _| {
-            tcx.mk_region(ty::ReVar(self.to_region_vid(region)))
-        })
+        tcx.fold_regions(value, |region, _| tcx.mk_region(ty::ReVar(self.to_region_vid(region))))
     }
 }
 
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
index b9596cd10ed..42d52446ab6 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/util.rs
@@ -79,7 +79,7 @@ pub fn find_param_with_region<'tcx>(
             // May return None; sometimes the tables are not yet populated.
             let ty = fn_sig.inputs()[index];
             let mut found_anon_region = false;
-            let new_param_ty = tcx.fold_regions(ty, &mut false, |r, _| {
+            let new_param_ty = tcx.fold_regions(ty, |r, _| {
                 if r == anon_region {
                     found_anon_region = true;
                     replace_region
diff --git a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
index 455de47acef..68c709a2e24 100644
--- a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
+++ b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
@@ -868,7 +868,7 @@ impl<'tcx> LexicalRegionResolutions<'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        tcx.fold_regions(value, &mut false, |r, _db| match *r {
+        tcx.fold_regions(value, |r, _db| match *r {
             ty::ReVar(rid) => self.resolve_var(rid),
             _ => r,
         })
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index a268493b28f..1c1906f3375 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -141,9 +141,6 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     ///   `('a, K)` in this list tells us that the bounds in scope
     ///   indicate that `K: 'a`, where `K` is either a generic
     ///   parameter like `T` or a projection like `T::Item`.
-    /// - `implicit_region_bound`: if some, this is a region bound
-    ///   that is considered to hold for all type parameters (the
-    ///   function body).
     /// - `param_env` is the parameter environment for the enclosing function.
     /// - `body_id` is the body-id whose region obligations are being
     ///   processed.
@@ -151,7 +148,6 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     pub fn process_registered_region_obligations(
         &self,
         region_bound_pairs_map: &FxHashMap<hir::HirId, RegionBoundPairs<'tcx>>,
-        implicit_region_bound: Option<ty::Region<'tcx>>,
         param_env: ty::ParamEnv<'tcx>,
     ) {
         assert!(
@@ -170,13 +166,8 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
             let sup_type = self.resolve_vars_if_possible(sup_type);
 
             if let Some(region_bound_pairs) = region_bound_pairs_map.get(&body_id) {
-                let outlives = &mut TypeOutlives::new(
-                    self,
-                    self.tcx,
-                    &region_bound_pairs,
-                    implicit_region_bound,
-                    param_env,
-                );
+                let outlives =
+                    &mut TypeOutlives::new(self, self.tcx, &region_bound_pairs, None, param_env);
                 outlives.type_must_outlive(origin, sup_type, sub_region);
             } else {
                 self.tcx.sess.delay_span_bug(
diff --git a/compiler/rustc_infer/src/infer/outlives/verify.rs b/compiler/rustc_infer/src/infer/outlives/verify.rs
index 191f5f18ec2..86b025dce5e 100644
--- a/compiler/rustc_infer/src/infer/outlives/verify.rs
+++ b/compiler/rustc_infer/src/infer/outlives/verify.rs
@@ -16,6 +16,11 @@ use rustc_middle::ty::{self, EarlyBinder, Ty, TyCtxt};
 pub struct VerifyBoundCx<'cx, 'tcx> {
     tcx: TyCtxt<'tcx>,
     region_bound_pairs: &'cx RegionBoundPairs<'tcx>,
+    /// During borrowck, if there are no outlives bounds on a generic
+    /// parameter `T`, we assume that `T: 'in_fn_body` holds.
+    ///
+    /// Outside of borrowck the only way to prove `T: '?0` is by
+    /// setting  `'?0` to `'empty`.
     implicit_region_bound: Option<ty::Region<'tcx>>,
     param_env: ty::ParamEnv<'tcx>,
 }
@@ -263,8 +268,8 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         //     fn foo<'a, A>(x: &'a A) { x.bar() }
         //
         // The problem is that the type of `x` is `&'a A`. To be
-        // well-formed, then, A must be lower-generic by `'a`, but we
-        // don't know that this holds from first principles.
+        // well-formed, then, A must outlive `'a`, but we don't know that
+        // this holds from first principles.
         let from_region_bound_pairs = self.region_bound_pairs.iter().filter_map(|&(r, p)| {
             debug!(
                 "declared_generic_bounds_from_env_for_erased_ty: region_bound_pair = {:?}",
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index b1b8bc13e2f..a6310ae5e66 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -465,13 +465,12 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn fold_regions<T>(
         self,
         value: T,
-        skipped_regions: &mut bool,
         mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
     ) -> T
     where
         T: TypeFoldable<'tcx>,
     {
-        value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
+        value.fold_with(&mut RegionFolder::new(self, &mut f))
     }
 
     /// Invoke `callback` on every region appearing free in `value`.
@@ -579,7 +578,6 @@ impl<'tcx> TyCtxt<'tcx> {
 
 pub struct RegionFolder<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
-    skipped_regions: &'a mut bool,
 
     /// Stores the index of a binder *just outside* the stuff we have
     /// visited.  So this begins as INNERMOST; when we pass through a
@@ -597,10 +595,9 @@ impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
     #[inline]
     pub fn new(
         tcx: TyCtxt<'tcx>,
-        skipped_regions: &'a mut bool,
         fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
     ) -> RegionFolder<'a, 'tcx> {
-        RegionFolder { tcx, skipped_regions, current_index: ty::INNERMOST, fold_region_fn }
+        RegionFolder { tcx, current_index: ty::INNERMOST, fold_region_fn }
     }
 }
 
@@ -624,7 +621,6 @@ impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
         match *r {
             ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
                 debug!(?self.current_index, "skipped bound region");
-                *self.skipped_regions = true;
                 r
             }
             _ => {
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index a63790b594d..90ff07cba02 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -220,7 +220,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 .map(|&(id, _)| (id, vec![]))
                 .collect();
 
-            infcx.process_registered_region_obligations(&body_id_map, None, full_env);
+            infcx.process_registered_region_obligations(&body_id_map, full_env);
 
             let region_data = infcx
                 .inner
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index b37db4b9e18..2b26b916d32 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -407,11 +407,7 @@ fn resolve_negative_obligation<'cx, 'tcx>(
     // function bodies with closures).
     outlives_env.save_implied_bounds(CRATE_HIR_ID);
 
-    infcx.process_registered_region_obligations(
-        outlives_env.region_bound_pairs_map(),
-        Some(tcx.lifetimes.re_root_empty),
-        param_env,
-    );
+    infcx.process_registered_region_obligations(outlives_env.region_bound_pairs_map(), param_env);
 
     let errors = infcx.resolve_regions(region_context, &outlives_env);
 
diff --git a/compiler/rustc_typeck/src/check/generator_interior.rs b/compiler/rustc_typeck/src/check/generator_interior.rs
index f09c7f51f47..6ee989070b4 100644
--- a/compiler/rustc_typeck/src/check/generator_interior.rs
+++ b/compiler/rustc_typeck/src/check/generator_interior.rs
@@ -225,7 +225,7 @@ pub fn resolve_interior<'a, 'tcx>(
                 // Note that each region slot in the types gets a new fresh late bound region,
                 // which means that none of the regions inside relate to any other, even if
                 // typeck had previously found constraints that would cause them to be related.
-                let folded = fcx.tcx.fold_regions(erased, &mut false, |_, current_depth| {
+                let folded = fcx.tcx.fold_regions(erased, |_, current_depth| {
                     let br = ty::BoundRegion {
                         var: ty::BoundVar::from_u32(counter),
                         kind: ty::BrAnon(counter),
diff --git a/compiler/rustc_typeck/src/check/regionck.rs b/compiler/rustc_typeck/src/check/regionck.rs
index 161ec31793d..0ce63922098 100644
--- a/compiler/rustc_typeck/src/check/regionck.rs
+++ b/compiler/rustc_typeck/src/check/regionck.rs
@@ -366,7 +366,6 @@ impl<'a, 'tcx> RegionCtxt<'a, 'tcx> {
     fn resolve_regions_and_report_errors(&self) {
         self.infcx.process_registered_region_obligations(
             self.outlives_environment.region_bound_pairs_map(),
-            Some(self.tcx.lifetimes.re_root_empty),
             self.param_env,
         );
 
diff --git a/compiler/rustc_typeck/src/check/wfcheck.rs b/compiler/rustc_typeck/src/check/wfcheck.rs
index e78d58dc210..40344b2ed60 100644
--- a/compiler/rustc_typeck/src/check/wfcheck.rs
+++ b/compiler/rustc_typeck/src/check/wfcheck.rs
@@ -596,13 +596,7 @@ fn ty_known_to_outlive<'tcx>(
 ) -> bool {
     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |infcx, region_bound_pairs| {
         let origin = infer::RelateParamBound(DUMMY_SP, ty, None);
-        let outlives = &mut TypeOutlives::new(
-            infcx,
-            tcx,
-            region_bound_pairs,
-            Some(infcx.tcx.lifetimes.re_root_empty),
-            param_env,
-        );
+        let outlives = &mut TypeOutlives::new(infcx, tcx, region_bound_pairs, None, param_env);
         outlives.type_must_outlive(origin, ty, region);
     })
 }
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 1f2e6ad86bd..2a52167c597 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -393,7 +393,7 @@ impl<'tcx> AstConv<'tcx> for ItemCtxt<'tcx> {
     }
 
     fn ct_infer(&self, ty: Ty<'tcx>, _: Option<&ty::GenericParamDef>, span: Span) -> Const<'tcx> {
-        let ty = self.tcx.fold_regions(ty, &mut false, |r, _| match *r {
+        let ty = self.tcx.fold_regions(ty, |r, _| match *r {
             ty::ReErased => self.tcx.lifetimes.re_static,
             _ => r,
         });
@@ -1917,7 +1917,7 @@ fn infer_return_ty_for_fn_sig<'tcx>(
         Some(ty) => {
             let fn_sig = tcx.typeck(def_id).liberated_fn_sigs()[hir_id];
             // Typeck doesn't expect erased regions to be returned from `type_of`.
-            let fn_sig = tcx.fold_regions(fn_sig, &mut false, |r, _| match *r {
+            let fn_sig = tcx.fold_regions(fn_sig, |r, _| match *r {
                 ty::ReErased => tcx.lifetimes.re_static,
                 _ => r,
             });
diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index 7011dd6e15c..6ee2b544916 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -772,7 +772,7 @@ fn infer_placeholder_type<'a>(
     }
 
     // Typeck doesn't expect erased regions to be returned from `type_of`.
-    tcx.fold_regions(ty, &mut false, |r, _| match *r {
+    tcx.fold_regions(ty, |r, _| match *r {
         ty::ReErased => tcx.lifetimes.re_static,
         _ => r,
     })