So far it is quite common to put the `AnyUnboundSocket` in an enum
variant, e.g. the following code snippet.
```rust
enum Inner {
Unbound(AlwaysSome<Box<AnyUnboundSocket>>),
Bound(AlwaysSome<Arc<AnyBoundSocket>>),
// ...
}
```
However, this pattern is very memory inefficient because the size
difference between two enum variants is significant. The size of
`AnyUnboundSocket` is much larger than the size of
`Arc<AnyBoundSocket>`, where the latter is simply a pointer and a
reference counter.
In fact, we're about to trigger Clippy's large_enum_variant warning.
We're just below its threshold, so the warning doesn't appear.
The solution is simple: If we put `AnyBoundSocket` in `Arc`, we should
also put `AnyUnboundSocket` in `Box`. This way the sizes of different
enum variants will be similar.
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