Everything about iterators

In the day 8 problem, our input is a height map:

This is a 5x5 grid, and every number denotes the height of a tree. For part 1, we must find out how many trees are visible from the outside of the grid.

Two years ago, I did part of Advent of Code 2020 using the Rust language. It was a lot of fun, so let's try it again!

The problem statement