Find a better way of checking allocations/type-instability in unit tests

[heyx3]

Check out this code which presumably understands the Julia compiler better than I do:

https://github.com/JuliaObjects/ConstructionBase.jl/blob/master/test/runtests.jl#L427-L451

function hot_loop_allocs(f::F, args...) where {F}
    # we want to test that f(args...) does not allocate 
    # when used in hot loops
    # however a naive @allocated f(args...)
    # will not be representative of what happens in an inner loop
    # Instead it will sometimes box inputs/outputs
    # and report too many allocations
    # so we use Refs to minimize inputs and outputs
    out_ref = Ref(f(args...))
    arg_refs = map(Ref, args)
    write_output_to_ref!(f, out_ref, arg_refs...)
    out_ref = typeof(out_ref)() # erase out_ref so we can assert work was done later
    # Avoid splatting args... which also results in undesired allocs
    allocs = if length(arg_refs) == 1
        r1, = arg_refs
        @allocated write_output_to_ref!(f, out_ref, r1)
    elseif length(arg_refs) == 2
        r1,r2 = arg_refs
        @allocated write_output_to_ref!(f, out_ref, r1, r2)
    else
        error("TODO too many args")
    end
    @assert out_ref[] == f(args...)
    return allocs
end