Function structural 

pub fn structural(ty: &Ty, types: &BTreeMap<Arc<str>, TyDecl>) -> String

A type’s structure, transitively — every field of every variant, through every named type it reaches.

§4.3 asks for an operation id that is “content-derived … stable across refactors that don’t change the signature”. A hash of the type’s name satisfies neither half: it does not move when a field is added, which is the change that breaks every open tab. This is what content means.