struct Foo{T}
       x :: T
end

foo = Foo([1,2,3])

m(foo) = foo.x

m2(foo) = (foo.x, 1)

julia> mutable struct It
       arr::Vector{Int64}
       end

julia> function it_test(it)
       push!(it.arr, length(it.arr))
       return it.arr
       end
julia> itt = It([])
It(Int64[])
julia> sizehint!(itt.arr, 100)
0-element Array{Int64,1}
julia> @time it_test(itt)
  0.000002 seconds

struct Foo{T} 
    arr::T
end

function process(f, x::Foo)
    for i=0:1, j=0:1, k=0:1
        x.arr[1] = i
        x.arr[2] = j
        x.arr[3] = k
        f(x.arr)
    end
end

foo = Foo([0, 0, 0])

process(foo) do x
    println(sum(x))
end

@time Tuple{Int}(1)
  0.000017 seconds (5 allocations: 176 bytes)

julia> @time Tuple{Int}(1)
  0.000000 seconds