2 files changed, 51 insertions, 10 deletions

diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
index 7f8fc1774..b5b712c36 100644
--- a/compiler/rustc_middle/src/infer/canonical.rs
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -35,9 +35,9 @@ use std::ops::Index;
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable)]
 #[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
 pub struct Canonical<'tcx, V> {
+    pub value: V,
     pub max_universe: ty::UniverseIndex,
     pub variables: CanonicalVarInfos<'tcx>,
-    pub value: V,
 }
 
 pub type CanonicalVarInfos<'tcx> = &'tcx List<CanonicalVarInfo<'tcx>>;
@@ -80,6 +80,18 @@ impl CanonicalVarValues<'_> {
             }
         })
     }
+
+    pub fn is_identity_modulo_regions(&self) -> bool {
+        self.var_values.iter().enumerate().all(|(bv, arg)| match arg.unpack() {
+            ty::GenericArgKind::Lifetime(_) => true,
+            ty::GenericArgKind::Type(ty) => {
+                matches!(*ty.kind(), ty::Bound(ty::INNERMOST, bt) if bt.var.as_usize() == bv)
+            }
+            ty::GenericArgKind::Const(ct) => {
+                matches!(ct.kind(), ty::ConstKind::Bound(ty::INNERMOST, bc) if bc.as_usize() == bv)
+            }
+        })
+    }
 }
 
 /// When we canonicalize a value to form a query, we wind up replacing
@@ -149,15 +161,15 @@ impl<'tcx> CanonicalVarInfo<'tcx> {
         }
     }
 
-    pub fn expect_anon_placeholder(self) -> u32 {
+    pub fn expect_placeholder_index(self) -> usize {
         match self.kind {
             CanonicalVarKind::Ty(_)
             | CanonicalVarKind::Region(_)
             | CanonicalVarKind::Const(_, _) => bug!("expected placeholder: {self:?}"),
 
-            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.name.expect_anon(),
-            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.name.expect_anon(),
-            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.name.as_u32(),
+            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.bound.var.as_usize(),
+            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.bound.var.as_usize(),
+            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.bound.as_usize(),
         }
     }
 }
@@ -411,7 +423,7 @@ impl<'tcx> CanonicalVarValues<'tcx> {
                         CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => {
                             let br = ty::BoundRegion {
                                 var: ty::BoundVar::from_usize(i),
-                                kind: ty::BrAnon(i as u32, None),
+                                kind: ty::BrAnon(None),
                             };
                             tcx.mk_re_late_bound(ty::INNERMOST, br).into()
                         }
diff --git a/compiler/rustc_middle/src/infer/unify_key.rs b/compiler/rustc_middle/src/infer/unify_key.rs
index 41d8c7ffd..a873854f0 100644
--- a/compiler/rustc_middle/src/infer/unify_key.rs
+++ b/compiler/rustc_middle/src/infer/unify_key.rs
@@ -1,4 +1,4 @@
-use crate::ty::{self, Ty, TyCtxt};
+use crate::ty::{self, Region, Ty, TyCtxt};
 use rustc_data_structures::unify::{NoError, UnifyKey, UnifyValue};
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::Symbol;
@@ -11,7 +11,20 @@ pub trait ToType {
 }
 
 #[derive(PartialEq, Copy, Clone, Debug)]
-pub struct UnifiedRegion<'tcx>(pub Option<ty::Region<'tcx>>);
+pub struct UnifiedRegion<'tcx> {
+    value: Option<ty::Region<'tcx>>,
+}
+
+impl<'tcx> UnifiedRegion<'tcx> {
+    pub fn new(value: Option<Region<'tcx>>) -> Self {
+        Self { value }
+    }
+
+    /// The caller is responsible for checking universe compatibility before using this value.
+    pub fn get_value_ignoring_universes(self) -> Option<Region<'tcx>> {
+        self.value
+    }
+}
 
 #[derive(PartialEq, Copy, Clone, Debug)]
 pub struct RegionVidKey<'tcx> {
@@ -44,11 +57,27 @@ impl<'tcx> UnifyValue for UnifiedRegion<'tcx> {
     type Error = NoError;
 
     fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
-        Ok(match (value1.0, value2.0) {
+        // We pick the value of the least universe because it is compatible with more variables.
+        // This is *not* necessary for soundness, but it allows more region variables to be
+        // resolved to the said value.
+        #[cold]
+        fn min_universe<'tcx>(r1: Region<'tcx>, r2: Region<'tcx>) -> Region<'tcx> {
+            cmp::min_by_key(r1, r2, |r| match r.kind() {
+                ty::ReStatic
+                | ty::ReErased
+                | ty::ReFree(..)
+                | ty::ReEarlyBound(..)
+                | ty::ReError(_) => ty::UniverseIndex::ROOT,
+                ty::RePlaceholder(placeholder) => placeholder.universe,
+                ty::ReVar(..) | ty::ReLateBound(..) => bug!("not a universal region"),
+            })
+        }
+
+        Ok(match (value1.value, value2.value) {
             // Here we can just pick one value, because the full constraints graph
             // will be handled later. Ideally, we might want a `MultipleValues`
             // variant or something. For now though, this is fine.
-            (Some(_), Some(_)) => *value1,
+            (Some(val1), Some(val2)) => Self { value: Some(min_universe(val1, val2)) },
 
             (Some(_), _) => *value1,
             (_, Some(_)) => *value2,