How is differentiation of implicit parameters implemented in Flux.jl?

Internals & Design
,
1

In the document of Flux.jl an example of differentiation of implicit parameters is given as follows:

julia> x = [2, 1];

julia> y = [2, 0];

julia> gs = gradient(params(x, y)) do
         f(x, y)
       end
Grads(...)

julia> gs[x]
2-element Array{Int64,1}:
 0
 2

julia> gs[y]
2-element Array{Int64,1}:
  0
 -2

I just wonder how is this implemented. My main question is, how can we know what global variables does a function use?