From 607d2973da7bd5895a5e84a65ba5174527bee904 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Mon, 3 Oct 2016 14:31:30 +1100
Subject: [PATCH] Avoid overflow check in `HashMap::reserve`'s fast path.

---
 src/libstd/collections/hash/map.rs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index 5fdc6056c68f..e8ca2977241e 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -667,8 +667,9 @@ impl<K, V, S> HashMap<K, V, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn reserve(&mut self, additional: usize) {
-        let min_cap = self.len().checked_add(additional).expect("reserve overflow");
-        if self.capacity() < min_cap {
+        let remaining = self.capacity() - self.len(); // this can't overflow
+        if remaining < additional {
+            let min_cap = self.len().checked_add(additional).expect("reserve overflow");
             let raw_cap = self.resize_policy.raw_capacity(min_cap);
             self.resize(raw_cap);
         }