[stdlib] don't copy array contents on removeAll(keepingCapacity: true)

Author: oxy

stdlib/public/core/Array.swift

@@ -1378,9 +1378,16 @@ extension Array: RangeReplaceableCollection {
     if !keepCapacity {
       _buffer = _Buffer()
     }
-    else {
+    else if _buffer.isMutableAndUniquelyReferenced() {
       self.replaceSubrange(indices, with: EmptyCollection())
     }
+    else {
+      let buffer = _ContiguousArrayBuffer<Element>(
+        _uninitializedCount: 0,
+        minimumCapacity: capacity
+      )
+      _buffer = _Buffer(_buffer: buffer, shiftedToStartIndex: startIndex)
+    }
   }
 
   //===--- algorithms -----------------------------------------------------===//

stdlib/public/core/ArraySlice.swift

@@ -1067,9 +1067,16 @@ extension ArraySlice: RangeReplaceableCollection {
     if !keepCapacity {
       _buffer = _Buffer()
     }
-    else {
+    else if _buffer.isMutableAndUniquelyReferenced() {
       self.replaceSubrange(indices, with: EmptyCollection())
     }
+    else {
+      let buffer = _ContiguousArrayBuffer<Element>(
+        _uninitializedCount: 0,
+        minimumCapacity: capacity
+      )
+      _buffer = _Buffer(_buffer: buffer, shiftedToStartIndex: startIndex)
+    }
   }
 
   //===--- algorithms -----------------------------------------------------===//

stdlib/public/core/ContiguousArray.swift

@@ -975,9 +975,12 @@ extension ContiguousArray: RangeReplaceableCollection {
     if !keepCapacity {
       _buffer = _Buffer()
     }
-    else {
+    else if _buffer.isMutableAndUniquelyReferenced() {
       self.replaceSubrange(indices, with: EmptyCollection())
     }
+    else {
+      _buffer = _Buffer(_uninitializedCount: 0, minimumCapacity: capacity)
+    }
   }
 
   //===--- algorithms -----------------------------------------------------===//