For this kind of validation, I feel like having to remember to validate the options everywhere is a bit error prone. Since all sources of this data will need this validation, it's best to implement it with a custom Deserialize implementation on Options.

In fact, the best way to do this is to just repurpose the From<OptionsWire> implementation. Something like this:

impl<'de> Deserialize<'de> for Options {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        let wire = OptionsWire::deserialize(deserializer)?;

        if wire.preview_features.is_some() && wire.preview.is_some() {
            return Err(serde::de::Error::custom(
                "Cannot specify both `preview` and `preview-features`.",
            ));
        }

        let OptionsWire {
            required_version,
            native_tls,
            ...

See also #16452 (comment).

In fact, the best way to do this is to just repurpose the From implementation...

Nice, thanks for the pointer! Centralizing into the deserializer is good.

It would make the most sense to me, then, to consolidate what you suggest into the original PR #16452 once the design there is clarified. I can do it in this PR if you all would prefer, though.