Revert "[stdlib][performance] skip copying old values during removeAll(keepCapacity: true)"

stdlib/public/core/HashedCollections.swift.gyb

@@ -3394,21 +3394,27 @@ internal enum _Variant${Self}Storage<${TypeParametersDecl}> : _HashStorageType {
     }
   }
 
-  internal mutating func nativeKeepCapacityRemoveAll() {
+  internal mutating func nativeRemoveAll() {
+    var nativeStorage = native
+
+    // FIXME(performance): if the storage is non-uniquely referenced, we
+    // shouldn’t be copying the elements into new storage and then immediately
+    // deleting the elements. We should detect that the storage is not uniquely
+    // referenced and allocate new empty storage of appropriate capacity.
 
     // We have already checked for the empty dictionary case, so we will always
     // mutating the dictionary storage.  Request unique storage.
+    let (reallocated, _) = ensureUniqueNativeStorage(nativeStorage.capacity)
+    if reallocated {
+      nativeStorage = native
+    }
 
-    if !isUniquelyReferenced() {
-      self = .Native(NativeStorageOwner(minimumCapacity: native.capacity))
-    } else {
-      for b in 0..<native.capacity {
-        if native.isInitializedEntry(b) {
-          native.destroyEntryAt(b)
-        }
+    for var b = 0; b != nativeStorage.capacity; ++b {
+      if nativeStorage.isInitializedEntry(b) {
+        nativeStorage.destroyEntryAt(b)
       }
     }
-    native.count = 0
+    nativeStorage.count = 0
   }
 
   internal mutating func removeAll(keepCapacity keepCapacity: Bool) {
@@ -3422,13 +3428,13 @@ internal enum _Variant${Self}Storage<${TypeParametersDecl}> : _HashStorageType {
     }
 
     if _fastPath(guaranteedNative) {
-      nativeKeepCapacityRemoveAll()
+      nativeRemoveAll()
       return
     }
 
     switch self {
     case .Native:
-      nativeKeepCapacityRemoveAll()
+      nativeRemoveAll()
     case .Cocoa(let cocoaStorage):
 #if _runtime(_ObjC)
       self = .Native(NativeStorage.Owner(minimumCapacity: cocoaStorage.count))