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

Author: glessard

stdlib/public/core/Array.swift

@@ -1348,9 +1348,17 @@ extension Array: RangeReplaceableCollection {
     if !keepCapacity {
       _buffer = _Buffer()
     }
-    else {
+    else if _buffer.isMutableAndUniquelyReferenced() {
       self.replaceSubrange(indices, with: EmptyCollection())
     }
+    else {
+      let buffer = _ContiguousArrayBuffer<Element>(
+        _uninitializedCount: capacity,
+        minimumCapacity: 0
+      )
+      buffer.count = 0
+      _buffer = _Buffer(_buffer: buffer, shiftedToStartIndex: startIndex)
+    }
   }
 
   //===--- algorithms -----------------------------------------------------===//

stdlib/public/core/ArraySlice.swift

@@ -1067,9 +1067,17 @@ extension ArraySlice: RangeReplaceableCollection {
     if !keepCapacity {
       _buffer = _Buffer()
     }
-    else {
+    else if _buffer.isMutableAndUniquelyReferenced() {
       self.replaceSubrange(indices, with: EmptyCollection())
     }
+    else {
+      let buffer = _ContiguousArrayBuffer<Element>(
+        _uninitializedCount: capacity,
+        minimumCapacity: 0
+      )
+      buffer.count = 0
+      _buffer = _Buffer(_buffer: buffer, shiftedToStartIndex: startIndex)
+    }
   }
 
   //===--- algorithms -----------------------------------------------------===//

stdlib/public/core/ContiguousArray.swift

@@ -972,9 +972,13 @@ extension ContiguousArray: RangeReplaceableCollection {
     if !keepCapacity {
       _buffer = _Buffer()
     }
-    else {
+    else if _buffer.isMutableAndUniquelyReferenced() {
       self.replaceSubrange(indices, with: EmptyCollection())
     }
+    else {
+      _buffer = _Buffer(_uninitializedCount: capacity, minimumCapacity: 0)
+      _buffer.count = 0
+    }
   }
 
   //===--- algorithms -----------------------------------------------------===//