[stdlib] Optimize Set.isStrictSubset<S>(of: S)

Use a temporary bitset to speed up the Sequence variant by roughly a factor of 3.

Author: lorentey

stdlib/public/core/NativeSet.swift

@@ -608,4 +608,32 @@ extension _NativeSet {
       return false
     }
   }
+
+  @_alwaysEmitIntoClient
+  internal func isStrictSubset<S: Sequence>(of possibleSuperset: S) -> Bool
+  where S.Element == Element {
+    _UnsafeBitset.withTemporaryBitset(capacity: self.bucketCount) { seen in
+      // Mark elements in self that we've seen in `possibleSuperset`.
+      var seenCount = 0
+      var isStrict = false
+      for element in possibleSuperset {
+        let (bucket, found) = find(element)
+        guard found else {
+          if !isStrict {
+            isStrict = true
+            if seenCount == self.count { return true }
+          }
+          continue
+        }
+        let inserted = seen.uncheckedInsert(bucket.offset)
+        if inserted {
+          seenCount += 1
+          if seenCount == self.count, isStrict {
+            return true
+          }
+        }
+      }
+      return false
+    }
+  }
 }

stdlib/public/core/Set.swift

@@ -740,9 +740,10 @@ extension Set: SetAlgebra {
   @inlinable
   public func isStrictSubset<S: Sequence>(of possibleStrictSuperset: S) -> Bool
   where S.Element == Element {
-    // FIXME: code duplication.
-    let other = Set(possibleStrictSuperset)
-    return isStrictSubset(of: other)
+    if let s = possibleStrictSuperset as? Set<Element> {
+      return isStrictSubset(of: s)
+    }
+    return _variant.convertedToNative.isStrictSubset(of: possibleStrictSuperset)
   }
 
   /// Returns a Boolean value that indicates whether the set is a superset of
@@ -1211,7 +1212,7 @@ extension Set {
   ///   `other`; otherwise, `false`.
   @inlinable
   public func isStrictSubset(of other: Set<Element>) -> Bool {
-    return other.isStrictSuperset(of: self)
+    return self.count < other.count && self.isSubset(of: other)
   }
 
   /// Returns a new set with the elements that are common to both this set and