collecting a tree iterator is horribly type unstable, with lots of run time dispatch, even when the element type is provided explicitly

[nsajko]

using JET, AbstractTrees
struct S
    c::Int
end
function AbstractTrees.children(s::S)
    c = s.c
    Iterators.map(Returns(S(c - 1)), Base.OneTo(c))
end
it = Leaves(S(3))
@report_opt collect(S, it)  # ═════ 15 possible errors found ═════
it = PreOrderDFS(S(3))
@report_opt collect(S, it)  # ═════ 9 possible errors found ═════
it = PostOrderDFS(S(3))
@report_opt collect(S, it)  # ═════ 16 possible errors found ═════

For comparison, Base.Generator collects fine without run time dispatch:

using JET
struct S
    c::Int
end
it = Iterators.map(S, 1:5)
@report_opt collect(S, it)

[KristofferC]

I think this is because it wants you to define a childstatetype:

"""
    childstatetype(::Type{T})
    childstatetype(n)

Indicates the type of the iteration state of the tree node `n` or its type `T`.  This is used by tree traversal
algorithms which must retain this state.  It therefore is necessary to define this to ensure that most tree
traversal is type stable.
"""