3 files changed, 17 insertions, 12 deletions
diff --git a/compiler/rustc_next_trait_solver/src/canonicalizer.rs b/compiler/rustc_next_trait_solver/src/canonicalizer.rs
index 5c3c35b7b3e..0bf9d7b9249 100644
--- a/compiler/rustc_next_trait_solver/src/canonicalizer.rs
+++ b/compiler/rustc_next_trait_solver/src/canonicalizer.rs
@@ -42,17 +42,19 @@ pub enum CanonicalizeMode {
 
 pub struct Canonicalizer<'a, D: SolverDelegate<Interner = I>, I: Interner> {
     delegate: &'a D,
+
+    // Immutable field.
     canonicalize_mode: CanonicalizeMode,
 
+    // Mutable fields.
     variables: &'a mut Vec<I::GenericArg>,
-    variable_lookup_table: HashMap<I::GenericArg, usize>,
-
     primitive_var_infos: Vec<CanonicalVarInfo<I>>,
+    variable_lookup_table: HashMap<I::GenericArg, usize>,
     binder_index: ty::DebruijnIndex,
 
-    /// We only use the debruijn index during lookup as all other fields
-    /// should not be impacted by whether a type is folded once or multiple
-    /// times.
+    /// We only use the debruijn index during lookup. We don't need to
+    /// track the `variables` as each generic arg only results in a single
+    /// bound variable regardless of how many times it is encountered.
     cache: HashMap<(ty::DebruijnIndex, I::Ty), I::Ty>,
 }
 
diff --git a/compiler/rustc_next_trait_solver/src/resolve.rs b/compiler/rustc_next_trait_solver/src/resolve.rs
index a37066cec66..f2654f7534e 100644
--- a/compiler/rustc_next_trait_solver/src/resolve.rs
+++ b/compiler/rustc_next_trait_solver/src/resolve.rs
@@ -16,6 +16,8 @@ where
     I: Interner,
 {
     delegate: &'a D,
+    /// We're able to use a cache here as the folder does not have any
+    /// mutable state.
     cache: DelayedMap<I::Ty, I::Ty>,
 }
 
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index 12b4b3cb3a9..baa00465c0c 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -1057,16 +1057,17 @@ where
                 );
                 infer_ty
             }
-            _ if ty.has_aliases() => {
-                if let Some(&entry) = self.cache.get(&ty) {
+            _ => {
+                if !ty.has_aliases() {
+                    ty
+                } else if let Some(&entry) = self.cache.get(&ty) {
                     return entry;
+                } else {
+                    let res = ty.super_fold_with(self);
+                    assert!(self.cache.insert(ty, res).is_none());
+                    res
                 }
-
-                let res = ty.super_fold_with(self);
-                assert!(self.cache.insert(ty, res).is_none());
-                res
             }
-            _ => ty,
         }
     }