From c6d1849411f2c054cffa3bafa4c3d685e8080fb9 Mon Sep 17 00:00:00 2001
From: Jonathan Brouwer <jonathantbrouwer@gmail.com>
Date: Tue, 8 Sep 2026 20:41:43 +0200
Subject: [PATCH 1/2] Reserve items in `Extend` implementation of `MonoItems`

---
 compiler/rustc_monomorphize/src/collector.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 4ee1abe4a1ff4..622666a12ef40 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -345,6 +345,8 @@ impl<'tcx> Extend<Spanned<MonoItem<'tcx>>> for MonoItems<'tcx> {
     where
         I: IntoIterator<Item = Spanned<MonoItem<'tcx>>>,
     {
+        let iter = iter.into_iter();
+        self.items.reserve(iter.size_hint().0);
         for item in iter {
             self.push(item)
         }

From 5bd43f35e5ec90ac97e035a70683412a5c35e9f5 Mon Sep 17 00:00:00 2001
From: Jonathan Brouwer <jonathantbrouwer@gmail.com>
Date: Tue, 8 Sep 2026 20:46:38 +0200
Subject: [PATCH 2/2] Do the same optimization in more places

---
 compiler/rustc_data_structures/src/sso/map.rs | 4 +++-
 compiler/rustc_data_structures/src/sso/set.rs | 4 +++-
 compiler/rustc_infer/src/traits/util.rs       | 2 ++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_data_structures/src/sso/map.rs b/compiler/rustc_data_structures/src/sso/map.rs
index 827c82fa46a11..018cbd96317db 100644
--- a/compiler/rustc_data_structures/src/sso/map.rs
+++ b/compiler/rustc_data_structures/src/sso/map.rs
@@ -356,7 +356,9 @@ impl<K: Eq + Hash, V> Extend<(K, V)> for SsoHashMap<K, V> {
     where
         I: IntoIterator<Item = (K, V)>,
     {
-        for (key, value) in iter.into_iter() {
+        let iter = iter.into_iter();
+        self.reserve(iter.size_hint().0);
+        for (key, value) in iter {
             self.insert(key, value);
         }
     }
diff --git a/compiler/rustc_data_structures/src/sso/set.rs b/compiler/rustc_data_structures/src/sso/set.rs
index e3fa1cbf4cc57..cf75ea4537012 100644
--- a/compiler/rustc_data_structures/src/sso/set.rs
+++ b/compiler/rustc_data_structures/src/sso/set.rs
@@ -168,7 +168,9 @@ impl<T: Eq + Hash> Extend<T> for SsoHashSet<T> {
     where
         I: IntoIterator<Item = T>,
     {
-        for val in iter.into_iter() {
+        let iter = iter.into_iter();
+        self.reserve(iter.size_hint().0);
+        for val in iter {
             self.insert(val);
         }
     }
diff --git a/compiler/rustc_infer/src/traits/util.rs b/compiler/rustc_infer/src/traits/util.rs
index cc29546adb880..0380488caf96f 100644
--- a/compiler/rustc_infer/src/traits/util.rs
+++ b/compiler/rustc_infer/src/traits/util.rs
@@ -47,6 +47,8 @@ impl<'tcx> PredicateSet<'tcx> {
 
 impl<'tcx> Extend<ty::Predicate<'tcx>> for PredicateSet<'tcx> {
     fn extend<I: IntoIterator<Item = ty::Predicate<'tcx>>>(&mut self, iter: I) {
+        let iter = iter.into_iter();
+        self.set.reserve(iter.size_hint().0);
         for pred in iter {
             self.insert(pred);
         }