{"ast_node_type":"other","tokens":[{"end":66,"kind":"keyword","start":0,"text":"///"}]}
    /// }` this would be the total number of fields of the struct.
    /// The number of fields in this pattern. E.g. if the pattern is `SomeStruct { field12: true, ..
    /// This is also the same as `self.ctor.arity(self.ty)`.