Remove ReEmpty

18 files changed, 20 insertions, 199 deletions

diff --git a/compiler/rustc_borrowck/src/diagnostics/region_name.rs b/compiler/rustc_borrowck/src/diagnostics/region_name.rs
index 75fde53b6cd..ce7c2841662 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_name.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_name.rs
@@ -357,11 +357,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                 ty::BoundRegionKind::BrAnon(_) => None,
             },
 
-            ty::ReLateBound(..)
-            | ty::ReVar(..)
-            | ty::RePlaceholder(..)
-            | ty::ReEmpty(_)
-            | ty::ReErased => None,
+            ty::ReLateBound(..) | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReErased => None,
         }
     }
 
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index 0b8ed0fcf27..9d088642f77 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -486,9 +486,9 @@ impl<'tcx> TypeFolder<'tcx> for ReverseMapper<'tcx> {
             ty::ReErased => return r,
 
             // The regions that we expect from borrow checking.
-            ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReEmpty(ty::UniverseIndex::ROOT) => {}
+            ty::ReEarlyBound(_) | ty::ReFree(_) => {}
 
-            ty::ReEmpty(_) | ty::RePlaceholder(_) | ty::ReVar(_) => {
+            ty::RePlaceholder(_) | ty::ReVar(_) => {
                 // All of the regions in the type should either have been
                 // erased by writeback, or mapped back to named regions by
                 // borrow checking.
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 7c10047e9dc..f1b1c33a105 100644
--- a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
+++ b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
@@ -347,13 +347,6 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
 
             match outlives_bound {
                 OutlivesBound::RegionSubRegion(r1, r2) => {
-                    // `where Type:` is lowered to `where Type: 'empty` so that
-                    // we check `Type` is well formed, but there's no use for
-                    // this bound here.
-                    if r1.is_empty() {
-                        return;
-                    }
-
                     // The bound says that `r1 <= r2`; we store `r2: r1`.
                     let r1 = self.universal_regions.to_region_vid(r1);
                     let r2 = self.universal_regions.to_region_vid(r2);
diff --git a/compiler/rustc_borrowck/src/universal_regions.rs b/compiler/rustc_borrowck/src/universal_regions.rs
index b9b181681ec..8cf9ed53d39 100644
--- a/compiler/rustc_borrowck/src/universal_regions.rs
+++ b/compiler/rustc_borrowck/src/universal_regions.rs
@@ -54,13 +54,6 @@ pub struct UniversalRegions<'tcx> {
     /// The total number of universal region variables instantiated.
     num_universals: usize,
 
-    /// A special region variable created for the `'empty(U0)` region.
-    /// Note that this is **not** a "universal" region, as it doesn't
-    /// represent a universally bound placeholder or any such thing.
-    /// But we do create it here in this type because it's a useful region
-    /// to have around in a few limited cases.
-    pub root_empty: RegionVid,
-
     /// The "defining" type for this function, with all universal
     /// regions instantiated. For a closure or generator, this is the
     /// closure type, but for a top-level function it's the `FnDef`.
@@ -323,11 +316,7 @@ impl<'tcx> UniversalRegions<'tcx> {
 
     /// See `UniversalRegionIndices::to_region_vid`.
     pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid {
-        if let ty::ReEmpty(ty::UniverseIndex::ROOT) = *r {
-            self.root_empty
-        } else {
-            self.indices.to_region_vid(r)
-        }
+        self.indices.to_region_vid(r)
     }
 
     /// As part of the NLL unit tests, you can annotate a function with
@@ -501,16 +490,10 @@ impl<'cx, 'tcx> UniversalRegionsBuilder<'cx, 'tcx> {
             _ => None,
         };
 
-        let root_empty = self
-            .infcx
-            .next_nll_region_var(NllRegionVariableOrigin::Existential { from_forall: true })
-            .to_region_vid();
-
         UniversalRegions {
             indices,
             fr_static,
             fr_fn_body,
-            root_empty,
             first_extern_index,
             first_local_index,
             num_universals,
diff --git a/compiler/rustc_infer/src/errors/note_and_explain.rs b/compiler/rustc_infer/src/errors/note_and_explain.rs
index 6f1f9522c86..7e051835b4b 100644
--- a/compiler/rustc_infer/src/errors/note_and_explain.rs
+++ b/compiler/rustc_infer/src/errors/note_and_explain.rs
@@ -27,13 +27,6 @@ impl<'a> DescriptionCtx<'a> {
                 me.kind = "restatic";
             }
 
-            ty::ReEmpty(ty::UniverseIndex::ROOT) => me.kind = "reempty",
-
-            ty::ReEmpty(ui) => {
-                me.kind = "reemptyuni";
-                me.arg = format!("{:?}", ui);
-            }
-
             ty::RePlaceholder(_) => return None,
 
             // FIXME(#13998) RePlaceholder should probably print like
diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index 46f8c0e8d8b..9488d0a6cbb 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -180,11 +180,7 @@ impl CanonicalizeMode for CanonicalizeQueryResponse {
         r: ty::Region<'tcx>,
     ) -> ty::Region<'tcx> {
         match *r {
-            ty::ReFree(_)
-            | ty::ReErased
-            | ty::ReStatic
-            | ty::ReEmpty(ty::UniverseIndex::ROOT)
-            | ty::ReEarlyBound(..) => r,
+            ty::ReFree(_) | ty::ReErased | ty::ReStatic | ty::ReEarlyBound(..) => r,
 
             ty::RePlaceholder(placeholder) => canonicalizer.canonical_var_for_region(
                 CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderRegion(placeholder) },
@@ -199,10 +195,6 @@ impl CanonicalizeMode for CanonicalizeQueryResponse {
                 )
             }
 
-            ty::ReEmpty(ui) => {
-                bug!("canonicalizing 'empty in universe {:?}", ui) // FIXME
-            }
-
             _ => {
                 // Other than `'static` or `'empty`, the query
                 // response should be executing in a fully
@@ -381,7 +373,6 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
             ty::ReStatic
             | ty::ReEarlyBound(..)
             | ty::ReFree(_)
-            | ty::ReEmpty(_)
             | ty::RePlaceholder(..)
             | ty::ReErased => self.canonicalize_mode.canonicalize_free_region(self, r),
         }
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index c1fb59009d3..524383e381f 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -688,7 +688,6 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 
             ty::RePlaceholder(..)
             | ty::ReVar(..)
-            | ty::ReEmpty(_)
             | ty::ReStatic
             | ty::ReEarlyBound(..)
             | ty::ReFree(..) => {
@@ -900,7 +899,6 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
 
             ty::RePlaceholder(..)
             | ty::ReVar(..)
-            | ty::ReEmpty(_)
             | ty::ReStatic
             | ty::ReEarlyBound(..)
             | ty::ReFree(..) => {
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index ed85c753360..720d74b1ce6 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -96,11 +96,6 @@ pub(super) fn note_and_explain_region<'tcx>(
             msg_span_from_free_region(tcx, region, alt_span)
         }
 
-        ty::ReEmpty(ty::UniverseIndex::ROOT) => ("the empty lifetime".to_owned(), alt_span),
-
-        // uh oh, hope no user ever sees THIS
-        ty::ReEmpty(ui) => (format!("the empty lifetime in universe {:?}", ui), alt_span),
-
         ty::RePlaceholder(_) => return,
 
         // FIXME(#13998) RePlaceholder should probably print like
@@ -139,8 +134,6 @@ fn msg_span_from_free_region<'tcx>(
             (msg, Some(span))
         }
         ty::ReStatic => ("the static lifetime".to_owned(), alt_span),
-        ty::ReEmpty(ty::UniverseIndex::ROOT) => ("an empty lifetime".to_owned(), alt_span),
-        ty::ReEmpty(ui) => (format!("an empty lifetime in universe {:?}", ui), alt_span),
         _ => bug!("{:?}", region),
     }
 }
@@ -250,17 +243,7 @@ pub fn unexpected_hidden_region_diagnostic<'tcx>(
 
     // Explain the region we are capturing.
     match *hidden_region {
-        ty::ReEmpty(ty::UniverseIndex::ROOT) => {
-            // All lifetimes shorter than the function body are `empty` in
-            // lexical region resolution. The default explanation of "an empty
-            // lifetime" isn't really accurate here.
-            let message = format!(
-                "hidden type `{}` captures lifetime smaller than the function body",
-                hidden_ty
-            );
-            err.span_note(span, &message);
-        }
-        ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic | ty::ReEmpty(_) => {
+        ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
             // Assuming regionck succeeded (*), we ought to always be
             // capturing *some* region from the fn header, and hence it
             // ought to be free. So under normal circumstances, we will go
diff --git a/compiler/rustc_infer/src/infer/freshen.rs b/compiler/rustc_infer/src/infer/freshen.rs
index 84004d2b21f..fee15afc7b3 100644
--- a/compiler/rustc_infer/src/infer/freshen.rs
+++ b/compiler/rustc_infer/src/infer/freshen.rs
@@ -126,7 +126,6 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
             | ty::ReFree(_)
             | ty::ReVar(_)
             | ty::RePlaceholder(..)
-            | ty::ReEmpty(_)
             | ty::ReErased => {
                 // replace all free regions with 'erased
                 self.tcx().lifetimes.re_erased
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 99f956bede7..0ed57706790 100644
--- a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
+++ b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
@@ -16,7 +16,7 @@ use rustc_data_structures::intern::Interned;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_middle::ty::{ReEarlyBound, ReEmpty, ReErased, ReFree, ReStatic};
+use rustc_middle::ty::{ReEarlyBound, ReErased, ReFree, ReStatic};
 use rustc_middle::ty::{ReLateBound, RePlaceholder, ReVar};
 use rustc_middle::ty::{Region, RegionVid};
 use rustc_span::Span;
@@ -261,13 +261,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                                         cur_region
                                     }
 
-                                    ReEmpty(b_ui) => {
-                                        // Empty regions are ordered according to the universe
-                                        // they are associated with.
-                                        let ui = a_universe.min(b_ui);
-                                        self.tcx().mk_region(ReEmpty(ui))
-                                    }
-
                                     RePlaceholder(placeholder) => {
                                         // If the empty and placeholder regions are in the same universe,
                                         // then the LUB is the Placeholder region (which is the cur_region).
@@ -399,13 +392,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                         a_region
                     }
 
-                    ReEmpty(a_ui) => {
-                        // Empty regions are ordered according to the universe
-                        // they are associated with.
-                        let ui = a_ui.min(empty_ui);
-                        self.tcx().mk_region(ReEmpty(ui))
-                    }
-
                     RePlaceholder(placeholder) => {
                         // If this empty region is from a universe that can
                         // name the placeholder, then the placeholder is
@@ -428,9 +414,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 // check below for a common case, here purely as an
                 // optimization.
                 let b_universe = self.var_infos[b_vid].universe;
-                if let ReEmpty(a_universe) = *a_region && a_universe == b_universe {
-                    return false;
-                }
 
                 let mut lub = self.lub_concrete_regions(a_region, cur_region);
                 if lub == cur_region {
@@ -470,7 +453,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 // they are associated with.
                 a_ui.min(b_ui) == b_ui
             }
-            (VarValue::Value(a), VarValue::Empty(b_ui)) => {
+            (VarValue::Value(a), VarValue::Empty(_)) => {
                 match *a {
                     ReLateBound(..) | ReErased => {
                         bug!("cannot relate region: {:?}", a);
@@ -493,12 +476,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                         false
                     }
 
-                    ReEmpty(a_ui) => {
-                        // Empty regions are ordered according to the universe
-                        // they are associated with.
-                        a_ui.min(b_ui) == b_ui
-                    }
-
                     RePlaceholder(_) => {
                         // The LUB is either `a` or `'static`
                         false
@@ -526,12 +503,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                         true
                     }
 
-                    ReEmpty(b_ui) => {
-                        // Empty regions are ordered according to the universe
-                        // they are associated with.
-                        a_ui.min(b_ui) == b_ui
-                    }
-
                     RePlaceholder(placeholder) => {
                         // If this empty region is from a universe that can
                         // name the placeholder, then the placeholder is
@@ -599,37 +570,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 self.tcx().lifetimes.re_static
             }
 
-            (ReEmpty(_), ReEarlyBound(_) | ReFree(_)) => {
-                // All empty regions are less than early-bound, free,
-                // and scope regions.
-                b
-            }
-
-            (ReEarlyBound(_) | ReFree(_), ReEmpty(_)) => {
-                // All empty regions are less than early-bound, free,
-                // and scope regions.
-                a
-            }
-
-            (ReEmpty(a_ui), ReEmpty(b_ui)) => {
-                // Empty regions are ordered according to the universe
-                // they are associated with.
-                let ui = a_ui.min(b_ui);
-                self.tcx().mk_region(ReEmpty(ui))
-            }
-
-            (ReEmpty(empty_ui), RePlaceholder(placeholder))
-            | (RePlaceholder(placeholder), ReEmpty(empty_ui)) => {
-                // If this empty region is from a universe that can
-                // name the placeholder, then the placeholder is
-                // larger; otherwise, the only ancestor is `'static`.
-                if empty_ui.can_name(placeholder.universe) {
-                    self.tcx().mk_region(RePlaceholder(placeholder))
-                } else {
-                    self.tcx().lifetimes.re_static
-                }
-            }
-
             (ReEarlyBound(_) | ReFree(_), ReEarlyBound(_) | ReFree(_)) => {
                 self.region_rels.lub_free_regions(a, b)
             }
@@ -1088,9 +1028,9 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 ty::ReVar(rid) => match var_values.values[rid] {
                     VarValue::ErrorValue => false,
                     VarValue::Empty(_) => true,
-                    VarValue::Value(min) => matches!(*min, ty::ReEmpty(_)),
+                    VarValue::Value(_) => false,
                 },
-                _ => matches!(*min, ty::ReEmpty(_)),
+                _ => false,
             },
 
             VerifyBound::AnyBound(bs) => {
diff --git a/compiler/rustc_infer/src/infer/region_constraints/mod.rs b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
index 780e6ead10e..764ef8fa4bb 100644
--- a/compiler/rustc_infer/src/infer/region_constraints/mod.rs
+++ b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
@@ -699,7 +699,6 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
             ty::ReStatic | ty::ReErased | ty::ReFree(..) | ty::ReEarlyBound(..) => {
                 ty::UniverseIndex::ROOT
             }
-            ty::ReEmpty(ui) => ui,
             ty::RePlaceholder(placeholder) => placeholder.universe,
             ty::ReVar(vid) => self.var_universe(vid),
             ty::ReLateBound(..) => bug!("universe(): encountered bound region {:?}", region),
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 1ae3063dae4..9dbefe33772 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1978,7 +1978,7 @@ impl<'tcx> PrettyPrinter<'tcx> for FmtPrinter<'_, 'tcx> {
 
             ty::ReVar(_) | ty::ReErased => false,
 
-            ty::ReStatic | ty::ReEmpty(_) => true,
+            ty::ReStatic => true,
         }
     }
 
@@ -2062,14 +2062,6 @@ impl<'tcx> FmtPrinter<'_, 'tcx> {
                 p!("'static");
                 return Ok(self);
             }
-            ty::ReEmpty(ty::UniverseIndex::ROOT) => {
-                p!("'<empty>");
-                return Ok(self);
-            }
-            ty::ReEmpty(ui) => {
-                p!(write("'<empty:{:?}>", ui));
-                return Ok(self);
-            }
         }
 
         p!("'_");
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 9fb91b5fe87..a8d2ca4a24e 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1511,7 +1511,6 @@ impl<'tcx> Region<'tcx> {
             ty::ReStatic => true,
             ty::ReVar(..) => false,
             ty::RePlaceholder(placeholder) => placeholder.name.is_named(),
-            ty::ReEmpty(_) => false,
             ty::ReErased => false,
         }
     }
@@ -1537,11 +1536,6 @@ impl<'tcx> Region<'tcx> {
     }
 
     #[inline]
-    pub fn is_empty(self) -> bool {
-        matches!(*self, ty::ReEmpty(..))
-    }
-
-    #[inline]
     pub fn bound_at_or_above_binder(self, index: ty::DebruijnIndex) -> bool {
         match *self {
             ty::ReLateBound(debruijn, _) => debruijn >= index,
@@ -1572,7 +1566,7 @@ impl<'tcx> Region<'tcx> {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
             }
-            ty::ReEmpty(_) | ty::ReStatic => {
+            ty::ReStatic => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
             }
             ty::ReLateBound(..) => {
diff --git a/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs b/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs
index b1de979e8f8..bd8126f6177 100644
--- a/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs
+++ b/compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs
@@ -305,8 +305,7 @@ fn encode_region<'tcx>(
         | RegionKind::ReFree(..)
         | RegionKind::ReStatic
         | RegionKind::ReVar(..)
-        | RegionKind::RePlaceholder(..)
-        | RegionKind::ReEmpty(..) => {
+        | RegionKind::RePlaceholder(..) => {
             bug!("encode_region: unexpected `{:?}`", region.kind());
         }
     }
diff --git a/compiler/rustc_traits/src/chalk/lowering.rs b/compiler/rustc_traits/src/chalk/lowering.rs
index a166371fed1..a6a098ce73f 100644
--- a/compiler/rustc_traits/src/chalk/lowering.rs
+++ b/compiler/rustc_traits/src/chalk/lowering.rs
@@ -485,10 +485,6 @@ impl<'tcx> LowerInto<'tcx, chalk_ir::Lifetime<RustInterner<'tcx>>> for Region<'t
                 })
                 .intern(interner)
             }
-            ty::ReEmpty(ui) => {
-                chalk_ir::LifetimeData::Empty(chalk_ir::UniverseIndex { counter: ui.index() })
-                    .intern(interner)
-            }
             ty::ReErased => chalk_ir::LifetimeData::Erased.intern(interner),
         }
     }
@@ -510,8 +506,8 @@ impl<'tcx> LowerInto<'tcx, Region<'tcx>> for &chalk_ir::Lifetime<RustInterner<'t
                 name: ty::BoundRegionKind::BrAnon(p.idx as u32),
             }),
             chalk_ir::LifetimeData::Static => return interner.tcx.lifetimes.re_static,
-            chalk_ir::LifetimeData::Empty(ui) => {
-                ty::ReEmpty(ty::UniverseIndex::from_usize(ui.counter))
+            chalk_ir::LifetimeData::Empty(_) => {
+                bug!("Chalk should not have been passed an empty lifetime.")
             }
             chalk_ir::LifetimeData::Erased => return interner.tcx.lifetimes.re_erased,
             chalk_ir::LifetimeData::Phantom(void, _) => match *void {},
diff --git a/compiler/rustc_type_ir/src/sty.rs b/compiler/rustc_type_ir/src/sty.rs
index 26e48d2d214..e164aaed6b4 100644
--- a/compiler/rustc_type_ir/src/sty.rs
+++ b/compiler/rustc_type_ir/src/sty.rs
@@ -3,14 +3,14 @@
 use std::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
 use std::{fmt, hash};
 
+use crate::DebruijnIndex;
 use crate::FloatTy;
+use crate::HashStableContext;
 use crate::IntTy;
 use crate::Interner;
 use crate::TyDecoder;
 use crate::TyEncoder;
 use crate::UintTy;
-use crate::UniverseIndex;
-use crate::{DebruijnIndex, HashStableContext};
 
 use self::RegionKind::*;
 use self::TyKind::*;
@@ -1023,14 +1023,6 @@ pub enum RegionKind<I: Interner> {
     /// Should not exist outside of type inference.
     RePlaceholder(I::PlaceholderRegion),
 
-    /// Empty lifetime is for data that is never accessed.  We tag the
-    /// empty lifetime with a universe -- the idea is that we don't
-    /// want `exists<'a> { forall<'b> { 'b: 'a } }` to be satisfiable.
-    /// Therefore, the `'empty` in a universe `U` is less than all
-    /// regions visible from `U`, but not less than regions not visible
-    /// from `U`.
-    ReEmpty(UniverseIndex),
-
     /// Erased region, used by trait selection, in MIR and during codegen.
     ReErased,
 }
@@ -1046,8 +1038,7 @@ const fn regionkind_discriminant<I: Interner>(value: &RegionKind<I>) -> usize {
         ReStatic => 3,
         ReVar(_) => 4,
         RePlaceholder(_) => 5,
-        ReEmpty(_) => 6,
-        ReErased => 7,
+        ReErased => 6,
     }
 }
 
@@ -1072,7 +1063,6 @@ impl<I: Interner> Clone for RegionKind<I> {
             ReStatic => ReStatic,
             ReVar(a) => ReVar(a.clone()),
             RePlaceholder(a) => RePlaceholder(a.clone()),
-            ReEmpty(a) => ReEmpty(a.clone()),
             ReErased => ReErased,
         }
     }
@@ -1099,7 +1089,6 @@ impl<I: Interner> PartialEq for RegionKind<I> {
                 (&RePlaceholder(ref __self_0), &RePlaceholder(ref __arg_1_0)) => {
                     __self_0 == __arg_1_0
                 }
-                (&ReEmpty(ref __self_0), &ReEmpty(ref __arg_1_0)) => __self_0 == __arg_1_0,
                 (&ReErased, &ReErased) => true,
                 _ => true,
             }
@@ -1144,7 +1133,6 @@ impl<I: Interner> Ord for RegionKind<I> {
                 (&RePlaceholder(ref __self_0), &RePlaceholder(ref __arg_1_0)) => {
                     Ord::cmp(__self_0, __arg_1_0)
                 }
-                (&ReEmpty(ref __self_0), &ReEmpty(ref __arg_1_0)) => Ord::cmp(__self_0, __arg_1_0),
                 (&ReErased, &ReErased) => Ordering::Equal,
                 _ => Ordering::Equal,
             }
@@ -1182,10 +1170,6 @@ impl<I: Interner> hash::Hash for RegionKind<I> {
                 hash::Hash::hash(&regionkind_discriminant(self), state);
                 hash::Hash::hash(__self_0, state)
             }
-            (&ReEmpty(ref __self_0),) => {
-                hash::Hash::hash(&regionkind_discriminant(self), state);
-                hash::Hash::hash(__self_0, state)
-            }
             (&ReErased,) => {
                 hash::Hash::hash(&regionkind_discriminant(self), state);
             }
@@ -1211,8 +1195,6 @@ impl<I: Interner> fmt::Debug for RegionKind<I> {
 
             RePlaceholder(placeholder) => write!(f, "RePlaceholder({:?})", placeholder),
 
-            ReEmpty(ui) => write!(f, "ReEmpty({:?})", ui),
-
             ReErased => write!(f, "ReErased"),
         }
     }
@@ -1247,9 +1229,6 @@ where
             RePlaceholder(a) => e.emit_enum_variant(disc, |e| {
                 a.encode(e);
             }),
-            ReEmpty(a) => e.emit_enum_variant(disc, |e| {
-                a.encode(e);
-            }),
             ReErased => e.emit_enum_variant(disc, |_| {}),
         }
     }
@@ -1272,8 +1251,7 @@ where
             3 => ReStatic,
             4 => ReVar(Decodable::decode(d)),
             5 => RePlaceholder(Decodable::decode(d)),
-            6 => ReEmpty(Decodable::decode(d)),
-            7 => ReErased,
+            6 => ReErased,
             _ => panic!(
                 "{}",
                 format!(
@@ -1305,9 +1283,6 @@ where
             ReErased | ReStatic => {
                 // No variant fields to hash for these ...
             }
-            ReEmpty(universe) => {
-                universe.hash_stable(hcx, hasher);
-            }
             ReLateBound(db, br) => {
                 db.hash_stable(hcx, hasher);
                 br.hash_stable(hcx, hasher);
diff --git a/compiler/rustc_typeck/src/outlives/utils.rs b/compiler/rustc_typeck/src/outlives/utils.rs
index b718ca94213..3e8d023fb55 100644
--- a/compiler/rustc_typeck/src/outlives/utils.rs
+++ b/compiler/rustc_typeck/src/outlives/utils.rs
@@ -161,12 +161,6 @@ fn is_free_region(region: Region<'_>) -> bool {
         // ignore it.  We can't put it on the struct header anyway.
         ty::ReLateBound(..) => false,
 
-        // This can appear in `where Self: ` bounds (#64855):
-        //
-        //     struct Bar<T>(<Self as Foo>::Type) where Self: ;
-        //     struct Baz<'a>(&'a Self) where Self: ;
-        ty::ReEmpty(_) => false,
-
         // These regions don't appear in types from type declarations:
         ty::ReErased | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReFree(..) => {
             bug!("unexpected region in outlives inference: {:?}", region);
diff --git a/compiler/rustc_typeck/src/variance/constraints.rs b/compiler/rustc_typeck/src/variance/constraints.rs
index 4fe213ffeea..840f0c97848 100644
--- a/compiler/rustc_typeck/src/variance/constraints.rs
+++ b/compiler/rustc_typeck/src/variance/constraints.rs
@@ -411,11 +411,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
                 // way early-bound regions do, so we skip them here.
             }
 
-            ty::ReFree(..)
-            | ty::ReVar(..)
-            | ty::RePlaceholder(..)
-            | ty::ReEmpty(_)
-            | ty::ReErased => {
+            ty::ReFree(..) | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReErased => {
                 // We don't expect to see anything but 'static or bound
                 // regions when visiting member types or method types.
                 bug!(