Function is_modifiable 

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

Whether a value of this type may be modified, 6.3.2.1p1.

An array is not modifiable, a const object is not, an incomplete type is not, and a struct with a const member anywhere inside it is not, which is the part that takes a walk rather than a look and the part a compiler forgets.