rust-lang
diff --git a/‎compiler/rustc_infer/src/infer/relate/combine.rs
+6-1 b/‎compiler/rustc_infer/src/infer/relate/combine.rs
+6-1
diff --git a/‎compiler/rustc_infer/src/infer/relate/type_relating.rs
+42-1 b/‎compiler/rustc_infer/src/infer/relate/type_relating.rs
+42-1
diff --git a/‎compiler/rustc_infer/src/infer/resolve.rs
+11-3 b/‎compiler/rustc_infer/src/infer/resolve.rs
+11-3
diff --git a/‎compiler/rustc_middle/src/ty/fold.rs
+25-3 b/‎compiler/rustc_middle/src/ty/fold.rs
+25-3
@@ -36,10 +36,15 @@ use crate::traits::{Obligation, PredicateObligation};
 #[derive(Clone)]
 pub struct CombineFields<'infcx, 'tcx> {
     pub infcx: &'infcx InferCtxt<'tcx>,
+    // Immutable fields
     pub trace: TypeTrace<'tcx>,
     pub param_env: ty::ParamEnv<'tcx>,
-    pub goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
     pub define_opaque_types: DefineOpaqueTypes,
+    // Mutable fields
+    //
+    // Adding any additional field likely requires
+    // changes to the cache of `TypeRelating`.
+    pub goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 }
 
 impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 
@@ -4,6 +4,7 @@ use rustc_middle::ty::relate::{
 };
 use rustc_middle::ty::{self, Ty, TyCtxt, TyVar};
 use rustc_span::Span;
+use rustc_type_ir::data_structures::DelayedSet;
 use tracing::{debug, instrument};
 
 use super::combine::CombineFields;
@@ -13,9 +14,38 @@ use crate::infer::{DefineOpaqueTypes, InferCtxt, SubregionOrigin};
 
 /// Enforce that `a` is equal to or a subtype of `b`.
 pub struct TypeRelating<'combine, 'a, 'tcx> {
+    // Immutable except for the `InferCtxt` and the
+    // resulting nested `goals`.
     fields: &'combine mut CombineFields<'a, 'tcx>,
+
+    // Immutable field.
     structurally_relate_aliases: StructurallyRelateAliases,
+    // Mutable field.
     ambient_variance: ty::Variance,
+
+    /// The cache only tracks the `ambient_variance` as it's the
+    /// only field which is mutable and which meaningfully changes
+    /// the result when relating types.
+    ///
+    /// The cache does not track whether the state of the
+    /// `InferCtxt` has been changed or whether we've added any
+    /// obligations to `self.fields.goals`. Whether a goal is added
+    /// once or multiple times is not really meaningful.
+    ///
+    /// Changes in the inference state may delay some type inference to
+    /// the next fulfillment loop. Given that this loop is already
+    /// necessary, this is also not a meaningful change. Consider
+    /// the following three relations:
+    /// ```text
+    /// Vec<?0> sub Vec<?1>
+    /// ?0 eq u32
+    /// Vec<?0> sub Vec<?1>
+    /// ```
+    /// Without a cache, the second `Vec<?0> sub Vec<?1>` would eagerly
+    /// constrain `?1` to `u32`. When using the cache entry from the
+    /// first time we've related these types, this only happens when
+    /// later proving the `Subtype(?0, ?1)` goal from the first relation.
+    cache: DelayedSet<(ty::Variance, Ty<'tcx>, Ty<'tcx>)>,
 }
 
 impl<'combine, 'infcx, 'tcx> TypeRelating<'combine, 'infcx, 'tcx> {
@@ -24,7 +54,12 @@ impl<'combine, 'infcx, 'tcx> TypeRelating<'combine, 'infcx, 'tcx> {
         structurally_relate_aliases: StructurallyRelateAliases,
         ambient_variance: ty::Variance,
     ) -> TypeRelating<'combine, 'infcx, 'tcx> {
-        TypeRelating { fields: f, structurally_relate_aliases, ambient_variance }
+        TypeRelating {
+            fields: f,
+            structurally_relate_aliases,
+            ambient_variance,
+            cache: Default::default(),
+        }
     }
 }
 
@@ -78,6 +113,10 @@ impl<'tcx> TypeRelation<TyCtxt<'tcx>> for TypeRelating<'_, '_, 'tcx> {
         let a = infcx.shallow_resolve(a);
         let b = infcx.shallow_resolve(b);
 
+        if self.cache.contains(&(self.ambient_variance, a, b)) {
+            return Ok(a);
+        }
+
         match (a.kind(), b.kind()) {
             (&ty::Infer(TyVar(a_id)), &ty::Infer(TyVar(b_id))) => {
                 match self.ambient_variance {
@@ -160,6 +199,8 @@ impl<'tcx> TypeRelation<TyCtxt<'tcx>> for TypeRelating<'_, '_, 'tcx> {
             }
         }
 
+        assert!(self.cache.insert((self.ambient_variance, a, b)));
+
         Ok(a)
     }
 
 
@@ -2,6 +2,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, Const, InferConst, Ty, TyCtxt, TypeFoldable};
+use rustc_type_ir::data_structures::DelayedMap;
 
 use super::{FixupError, FixupResult, InferCtxt};
 
@@ -15,12 +16,15 @@ use super::{FixupError, FixupResult, InferCtxt};
 /// points for correctness.
 pub struct OpportunisticVarResolver<'a, 'tcx> {
     infcx: &'a InferCtxt<'tcx>,
+    /// We're able to use a cache here as the folder does
+    /// not have any mutable state.
+    cache: DelayedMap<Ty<'tcx>, Ty<'tcx>>,
 }
 
 impl<'a, 'tcx> OpportunisticVarResolver<'a, 'tcx> {
     #[inline]
     pub fn new(infcx: &'a InferCtxt<'tcx>) -> Self {
-        OpportunisticVarResolver { infcx }
+        OpportunisticVarResolver { infcx, cache: Default::default() }
     }
 }
 
@@ -33,9 +37,13 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for OpportunisticVarResolver<'a, 'tcx> {
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         if !t.has_non_region_infer() {
             t // micro-optimize -- if there is nothing in this type that this fold affects...
+        } else if let Some(&ty) = self.cache.get(&t) {
+            return ty;
         } else {
-            let t = self.infcx.shallow_resolve(t);
-            t.super_fold_with(self)
+            let shallow = self.infcx.shallow_resolve(t);
+            let res = shallow.super_fold_with(self);
+            assert!(self.cache.insert(t, res));
+            res
         }
     }
 
 
@@ -1,5 +1,6 @@
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir::def_id::DefId;
+use rustc_type_ir::data_structures::DelayedMap;
 pub use rustc_type_ir::fold::{
     FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable, shift_region, shift_vars,
 };
@@ -131,12 +132,20 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for RegionFolder<'a, 'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 // Bound vars replacer
 
+/// A delegate used when instantiating bound vars.
+///
+/// Any implementation must make sure that each bound variable always
+/// gets mapped to the same result. `BoundVarReplacer` caches by using
+/// a `DelayedMap` which does not cache the first few types it encounters.
 pub trait BoundVarReplacerDelegate<'tcx> {
     fn replace_region(&mut self, br: ty::BoundRegion) -> ty::Region<'tcx>;
     fn replace_ty(&mut self, bt: ty::BoundTy) -> Ty<'tcx>;
     fn replace_const(&mut self, bv: ty::BoundVar) -> ty::Const<'tcx>;
 }
 
+/// A simple delegate taking 3 mutable functions. The used functions must
+/// always return the same result for each bound variable, no matter how
+/// frequently they are called.
 pub struct FnMutDelegate<'a, 'tcx> {
     pub regions: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
     pub types: &'a mut (dyn FnMut(ty::BoundTy) -> Ty<'tcx> + 'a),
@@ -164,11 +173,15 @@ struct BoundVarReplacer<'tcx, D> {
     current_index: ty::DebruijnIndex,
 
     delegate: D,
+
+    /// This cache only tracks the `DebruijnIndex` and assumes that it does not matter
+    /// for the delegate how often its methods get used.
+    cache: DelayedMap<(ty::DebruijnIndex, Ty<'tcx>), Ty<'tcx>>,
 }
 
 impl<'tcx, D: BoundVarReplacerDelegate<'tcx>> BoundVarReplacer<'tcx, D> {
     fn new(tcx: TyCtxt<'tcx>, delegate: D) -> Self {
-        BoundVarReplacer { tcx, current_index: ty::INNERMOST, delegate }
+        BoundVarReplacer { tcx, current_index: ty::INNERMOST, delegate, cache: Default::default() }
     }
 }
 
@@ -197,8 +210,17 @@ where
                 debug_assert!(!ty.has_vars_bound_above(ty::INNERMOST));
                 ty::fold::shift_vars(self.tcx, ty, self.current_index.as_u32())
             }
-            _ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
-            _ => t,
+            _ => {
+                if !t.has_vars_bound_at_or_above(self.current_index) {
+                    t
+                } else if let Some(&t) = self.cache.get(&(self.current_index, t)) {
+                    t
+                } else {
+                    let res = t.super_fold_with(self);
+                    assert!(self.cache.insert((self.current_index, t), res));
+                    res
+                }
+            }
         }
     }