Function canonical 

pub fn canonical(
    ty: &Ty,
    types: &BTreeMap<Arc<str>, TyDecl>,
) -> Result<Value, Uninhabitable>

The don’t-care inhabitant — §21.3 rule 1’s “‘any value’ is the default, so it needs no expression”.

Deterministic and small: the first variant, an empty collection, zero, "". Small matters because this is what a stub returns when nobody said otherwise, and a surprising default value is worse than an obvious one.