Function count_distinct 

pub fn count_distinct<'a, V, E>(
    expr: E,
) -> SQLExpr<'a, V, <V::DialectMarker as CountPolicy>::Count, NonNull, Agg>
where
    V: SQLParam + 'a,
    V::DialectMarker: CountPolicy,
    E: Expr<'a, V>,

COUNT(DISTINCT expr) - counts distinct non-null values.

Returns a BigInt, NonNull, Aggregate expression. Works with any expression type.