Function set_excluded 

pub fn set_excluded<Q: HasSet>(
    columns: impl IntoIterator<Item = impl Into<Cow<'static, str>>>,
) -> impl Mod<Q>

"col" = excluded."col" for each column — the body of an upsert.

The pseudo-table is excluded in lower case and unquoted, which is how https://www.sqlite.org/lang_upsert.html spells it.