Determine mutability of indirections in (ref) t.
if true, check ref t; otherwise, check just t
the type that is being checked
When the type has any mutable indirections, returns 0. When all indirections are immutable, returns 2. Otherwise, when the type has const/inout indirections, returns 1.
See Implementation
Determine mutability of indirections in (ref) t.