Function on_conflict 

pub fn on_conflict(columns: impl IntoExprList) -> ConflictChain

ON CONFLICT (columns) — infer the unique index from a column list.

on_conflict(()) targets any conflict at all, which only DO NOTHING accepts. There is no ON CONSTRAINT form: SQLite infers the index from columns or not at all, so PostgreSQL’s on_conflict_on_constraint has no counterpart here.