Docs suggest implemting try_fold on std::iter::iterator, which is not possible due to Try being unstable

[mpdn]

std::iter::Iterator docs has this "Note to implementors" on try_fold:

Most of the other (forward) methods have default implementations in terms of this one, so try to implement this explicitly if it can do something better than the default for loop implementation.

But this isn't actually possible to do in stable due to the Try trait being unsable. The type bounds cannot be specified correctly to match the iterator definition.