diff --git a/compiler/rustc_data_structures/src/vec_cache.rs b/compiler/rustc_data_structures/src/vec_cache.rs
index 70524bae624a..f9f5871c8d61 100644
--- a/compiler/rustc_data_structures/src/vec_cache.rs
+++ b/compiler/rustc_data_structures/src/vec_cache.rs
@@ -359,6 +359,10 @@ where
             }
         }
     }
+
+    pub fn len(&self) -> usize {
+        self.len.load(Ordering::Acquire)
+    }
 }
 
 #[cfg(test)]
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index 48e2ee8e9abd..449dc95ea372 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -508,6 +508,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         // Unresolved macros produce dummy outputs as a recovery measure.
         invocations.reverse();
         let mut expanded_fragments = Vec::new();
+        let mut expanded_fragments_len = 0;
         let mut undetermined_invocations = Vec::new();
         let (mut progress, mut force) = (false, !self.monotonic);
         loop {
@@ -602,6 +603,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                         expanded_fragments.push(Vec::new());
                     }
                     expanded_fragments[depth - 1].push((expn_id, expanded_fragment));
+                    expanded_fragments_len += 1;
                     invocations.extend(derive_invocations.into_iter().rev());
                 }
                 ExpandResult::Retry(invoc) => {
@@ -622,7 +624,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         self.cx.force_mode = orig_force_mode;
 
         // Finally incorporate all the expanded macros into the input AST fragment.
-        let mut placeholder_expander = PlaceholderExpander::default();
+        let mut placeholder_expander = PlaceholderExpander::with_capacity(expanded_fragments_len);
         while let Some(expanded_fragments) = expanded_fragments.pop() {
             for (expn_id, expanded_fragment) in expanded_fragments.into_iter().rev() {
                 placeholder_expander
diff --git a/compiler/rustc_expand/src/placeholders.rs b/compiler/rustc_expand/src/placeholders.rs
index 05f9a5aa43f7..2db18429a521 100644
--- a/compiler/rustc_expand/src/placeholders.rs
+++ b/compiler/rustc_expand/src/placeholders.rs
@@ -218,12 +218,17 @@ pub(crate) fn placeholder(
     }
 }
 
-#[derive(Default)]
 pub(crate) struct PlaceholderExpander {
     expanded_fragments: FxHashMap<ast::NodeId, AstFragment>,
 }
 
 impl PlaceholderExpander {
+    pub(crate) fn with_capacity(capacity: usize) -> Self {
+        PlaceholderExpander {
+            expanded_fragments: FxHashMap::with_capacity_and_hasher(capacity, Default::default()),
+        }
+    }
+
     pub(crate) fn add(&mut self, id: ast::NodeId, mut fragment: AstFragment) {
         fragment.mut_visit_with(self);
         self.expanded_fragments.insert(id, fragment);
diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index d1721f1f455f..3835f3f557a6 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -410,9 +410,8 @@ 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 @@ where
             }
         }
     }
+
+    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 @@ where
             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 @@ where
         });
         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 @@ where
     fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
         self.iter(f)
     }
+
+    fn len(&self) -> usize {
+        self.len()
+    }
 }