c_void is *not* simply `const c_void = opaque{};`. It has unique
semantics as any pointer type may coerce to `*c_void` which is not true
for an arbitrary `*opaque{}`.
c_void is *not* simply `const c_void = opaque{};`. It has unique
semantics as any pointer type may coerce to `*c_void` which is not true
for an arbitrary `*opaque{}`.