For a struct like this:
```
struct Small {
    x: u32,
    y: u32,
    #[serde(skip_serializing_if = "Option::is_none")]
    maybe: Option<&'static str>,
}
```
it will change the field count computation from this:
```
false as usize + 1 + 1 + if Option::is_none(&self.maybe) { 0 } else { 1 }
```
to this:
```
2usize + if Option::is_none(&self.maybe) { 0 } else { 1 }
```
For fields with many fields, the difference will be bigger, avoiding long
sequences of `1 + 1 + 1 + ...`. This is a small compile-time win.

This turns two matches into one, for each field in a struct.

This turns two matches into one, for each field in a struct.

This turns two matches into one, for each field in a struct.