pub fn is_builder(&self) -> bool {
        use self::Type::{Builder, Struct};
        match *self {
            Builder(_, _) => true,
            Struct(ref tys) => tys.iter().all(|t| t.is_builder()),
            _ => false,
        }
    }
Returns whether this `Type` is a builder.
weld/src/ast/ast.rs