Provide roots (and leafs) method about daggy HOT 3 OPEN

This sounds very useful and would be a welcome addition!

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I started implementing it and quickly noticed that it's harder than I thought. All impls of the Walker trait require a starting node, but I about iterating over every root node, so I'm not quiet sure if I can use the Walker trait at all. I guess I have to create a new one, that doesn't need a NodeId to start with?

Or would you be fine by using the Iterator trait, but that means borrowing of the Dag.

from daggy.

@hellow554 if there doesn't seem to be a nice solution with the Walker trait I would be content with an Iterator that borrowed the Dag. It would still be very useful even if a user just wanted to collect each of the roots/leaves into a collection before then iterating over them in a way that doesn't borrow the Dag.

If you do have an approach in mind that allows for iterating over roots/leaves without borrowing the Dag but is not suited to a Walker impl, perhaps this could simply be done with an inherent impl? I don't think we need to introduce another trait if this new type is the only one likely to implement it.

Thanks for taking a stab at this btw!

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