I like functional code, except that because the use of lambdas is awkward to "escape" them:

    nums.iter().map(|x| if x > 10 break(?))

This little thing is the major problem of functional chains (this is only a toy version of the issue). Because lambdas are inside the scope is hard to make them truly part of the current execution flow.

So, sometimes I change a chain of iterators to simpler loops for things like this. IF iterators where part of current scope, it will look far more natural, IMHO...