The spaces are quite easy to get around.

HINT: You should probably make your function space independent, as in working the same regardless of spacing

In the description:
"Your task is to return the first longest string consisting of k consecutive strings taken in the array."
seems to be clear:-) At least it seems have been for lots of people.
Moreover the example given removes ambiguity if there was one:

...
Two strings are the longest: "folingtrashy" and "abcdefuvwxyz".
The first that came is "folingtrashy" so 
...

I think so. There's a couple of active Rust users, so someone should be able to take a look.