julia> struct X   # isbits
           x::Int
       end

julia> struct Y   # immutable, but not isbits
           y::Vector{Int}
       end

julia> function caller(v::V) where V
           r = Ref(v)
           GC.@preserve r begin
               p = reinterpret(Ptr{V}, pointer_from_objref(r))
               callee(p)
           end
       end
caller (generic function with 1 method)

julia> function callee(p::Ptr)
           v = unsafe_load(p)
           foo(v)
       end
callee (generic function with 1 method)

julia> foo(x::X) = x.x
foo (generic function with 2 methods)

julia> foo(y::Y) = sum(y.y, init=0)
foo (generic function with 2 methods)

julia> const x1 = X(1)
X(1)

julia> const y1 = Y([1,2])
Y([1, 2])

julia> caller(x1)
1

julia> caller(y1)
3

julia> @time caller(x1)
  0.000001 seconds
1

julia> @time caller(y1)  # One alloc from the Ref(), one alloc from boxing the Y() instance.
  0.000002 seconds (2 allocations: 32 bytes)
3