Evaluation order when using mutating operations with destructuring assignment

[simonbyrne]

julia> x = [1,2]
2-element Vector{Int64}:
 1
 2

julia> x[2],x[1] = x
2-element Vector{Int64}:
 1
 1

This is caused by mutating the iterator as it is being iterated: i.e. it is being lowered to

julia> x = [1,2]
2-element Vector{Int64}:
 1
 2

julia> x[2], state = iterate(x)
(1, 2)

julia> x[1], state = iterate(x, state)
(1, 3)

julia> x
2-element Vector{Int64}:
 1
 1

From @simeonschaub on Slack:

For a non-trivial lhs, we should probably always assign the rhs to temporary variables first and only then call setindex! etc.

cc: @JeffBezanson @vtjnash

[JeffBezanson]

I'm not 100% sure this is bad; iterating and assigning left-to-right is at least "obvious".