diff --git a/compiler/rustc_data_structures/src/vec_cache.rs b/compiler/rustc_data_structures/src/vec_cache.rs
index 599970663db8..c3baae204f66 100644
--- a/compiler/rustc_data_structures/src/vec_cache.rs
+++ b/compiler/rustc_data_structures/src/vec_cache.rs
@@ -331,6 +331,10 @@ pub fn iter(&self, f: &mut dyn FnMut(&K, &V, I)) {
             }
         }
     }
+
+    pub fn len(&self) -> usize {
+        self.len.load(Ordering::Acquire)
+    }
 }
 
 #[cfg(test)]
diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index 2d4e10a0380c..55c5ff911c31 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -415,9 +415,8 @@ pub(crate) fn query_key_hash_verify<'tcx>(
 ) {
     let _timer = qcx.tcx.prof.generic_activity_with_arg("query_key_hash_verify_for", query.name());
 
-    let mut map = UnordMap::default();
-
     let cache = query.query_cache(qcx);
+    let mut map = UnordMap::with_capacity(cache.len());
     cache.iter(&mut |key, _, _| {
         let node = DepNode::construct(qcx.tcx, query.dep_kind(), key);
         if let Some(other_key) = map.insert(node, *key) {
diff --git a/compiler/rustc_query_system/src/query/caches.rs b/compiler/rustc_query_system/src/query/caches.rs
index 30b5d7e59549..67ad767d4d31 100644
--- a/compiler/rustc_query_system/src/query/caches.rs
+++ b/compiler/rustc_query_system/src/query/caches.rs
@@ -30,6 +30,8 @@ pub trait QueryCache: Sized {
     fn complete(&self, key: Self::Key, value: Self::Value, index: DepNodeIndex);
 
     fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex));
+
+    fn len(&self) -> usize;
 }
 
 /// In-memory cache for queries whose keys aren't suitable for any of the
@@ -71,6 +73,10 @@ fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
             }
         }
     }
+
+    fn len(&self) -> usize {
+        self.cache.len()
+    }
 }
 
 /// In-memory cache for queries whose key type only has one value (e.g. `()`).
@@ -107,6 +113,10 @@ fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
             f(&(), &value.0, value.1)
         }
     }
+
+    fn len(&self) -> usize {
+        self.cache.get().is_some().into()
+    }
 }
 
 /// In-memory cache for queries whose key is a [`DefId`].
@@ -157,6 +167,10 @@ fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
         });
         self.foreign.iter(f);
     }
+
+    fn len(&self) -> usize {
+        self.local.len() + self.foreign.len()
+    }
 }
 
 impl<K, V> QueryCache for VecCache<K, V, DepNodeIndex>
@@ -180,4 +194,8 @@ fn complete(&self, key: K, value: V, index: DepNodeIndex) {
     fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
         self.iter(f)
     }
+
+    fn len(&self) -> usize {
+        self.len()
+    }
 }