Function is_complete 

pub fn is_complete(types: &Types, id: TypeId) -> bool

A complete type: one whose size is known, so an object of it can exist.

void is never complete. An array is complete when its length is known, which includes a variable length array, since the length is known when the declaration is reached even though it is not known here. A struct, a union or an enum is complete once its definition has been seen, which is a property of the declaration and not of the type expression.