Data provides a settable `count` property. Its expected behavior is undocumented, but based on the implementation, it is intended to zero-extend (or truncate) the collection to the specified length.
This does not work correctly if we start with an empty Data and we try to increase the count by a small integer:
```
import Foundation
var d = Data()
d.count = 2
print(d.count) // ⟹ 0 ⁉️
d.count = 100
print(d.count) // ⟹ 100 ✓
```
It looks like this bug was introduced with the Data overhaul that shipped in Swift 5.
(This issue was uncovered by https://github.com/apple/swift/pull/28918.)
rdar://58134026
1a9f7afdec