Iterators.Flatten allocates on iteration

[hessammehr]

Apologies if this has been discussed before. I tried looking through issues and discourse to no avail (see relevant discussion here, here, and here). It appears that Iterators.flatten allocates when it's not supposed to.

using BenchmarkTools
x = [[1, 2, 3, 4], [5, 6, 7, 8]]
f = Iterators.flatten(x)
@benchmark iterate($f)
#  memory estimate:  64 bytes
#  allocs estimate:  2
val, state = iterate(f)
@benchmark iterate($f, $state)
#  memory estimate:  64 bytes
#  allocs estimate:  2

I'm not familiar enough with the output of @code_warntype to be able to dig deeper but was wondering if there has been any work/discussion to get this behaviour fixed.