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

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)

Start by running the JET examples for collect(S, it) with Leaves, PreOrderDFS, and PostOrderDFS, then compare their reports with the Iterators.map example. Trace the iterator implementations behind those three entry points and determine what keeps explicit element-type collection type unstable. Done means the reported runtime dispatch is addressed for the shown cases.