Function wit_type_with 

pub fn wit_type_with(
    ty: &Type,
    known: &BTreeSet<String>,
) -> Result<String, String>

Map a Rust argument/return type to its WIT spelling, where known holds the Rust identifiers of #[derive(WitType)] user types (mapped to their kebab-case record/variant name). Returns Err(msg) for unsupported types.