From 5b8bfe047123bad63ead0370c165cd9307a07caa Mon Sep 17 00:00:00 2001
From: Stein Somers <git@steinsomers.be>
Date: Wed, 3 Apr 2019 13:01:01 +0200
Subject: [PATCH] improve worst-case performance of HashSet.is_subset

---
 src/libstd/collections/hash/set.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/libstd/collections/hash/set.rs b/src/libstd/collections/hash/set.rs
index 89d5b2ff30f9f..b9fcc2365fa7c 100644
--- a/src/libstd/collections/hash/set.rs
+++ b/src/libstd/collections/hash/set.rs
@@ -627,7 +627,11 @@ impl<T, S> HashSet<T, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_subset(&self, other: &HashSet<T, S>) -> bool {
-        self.iter().all(|v| other.contains(v))
+        if self.len() <= other.len() {
+            self.iter().all(|v| other.contains(v))
+        } else {
+            false
+        }
     }
 
     /// Returns `true` if the set is a superset of another,