[stdlib] fix _makeMutableAndUniqueUnchecked()

Author: glessard

stdlib/public/core/Array.swift

@@ -356,7 +356,7 @@ extension Array {
   }
 
 #if INTERNAL_CHECKS_ENABLED && COW_CHECKS_ENABLED
-  @inlinable
+  @_alwaysEmitIntoClient
   @_semantics("array.make_mutable")
   @_effects(notEscaping self.**)
   internal mutating func _makeMutableAndUniqueUnchecked() {

stdlib/public/core/ArraySlice.swift

@@ -169,6 +169,16 @@ extension ArraySlice {
     }
   }
 
+#if INTERNAL_CHECKS_ENABLED && COW_CHECKS_ENABLED
+  @_alwaysEmitIntoClient
+  @_semantics("array.make_mutable")
+  internal mutating func _makeMutableAndUniqueUnchecked() {
+    if _slowPath(!_buffer.beginCOWMutationUnchecked()) {
+      _buffer = _Buffer(copying: _buffer)
+    }
+  }
+#endif
+
   /// Marks the end of a mutation.
   ///
   /// After a call to `_endMutation` the buffer must not be mutated until a call

stdlib/public/core/ContiguousArray.swift

@@ -74,6 +74,16 @@ extension ContiguousArray {
     }
   }
 
+#if INTERNAL_CHECKS_ENABLED && COW_CHECKS_ENABLED
+  @_alwaysEmitIntoClient
+  @_semantics("array.make_mutable")
+  internal mutating func _makeMutableAndUniqueUnchecked() {
+    if _slowPath(!_buffer.beginCOWMutationUnchecked()) {
+      _buffer = _buffer._consumeAndCreateNew()
+    }
+  }
+#endif
+
   /// Marks the end of an Array mutation.
   ///
   /// After a call to `_endMutation` the buffer must not be mutated until a call