Diagonal Sum in Rust

Given a square grid of integers grid, with as many rows as columns, return the sum of the cells on its diagonal: the cells that go from the top-left corner to the bottom-right corner.

A grid is an array of rows, and each row is an array of integers. Rows are numbered from 0, top to bottom, and columns from 0, left to right. A cell is on the diagonal when its row number and its column number are the same: row 0 and column 0, row 1 and column 1, and so on.

Example 1:

Input: grid = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]
Output: 15
Explanation: The diagonal cells are 1, 5 and 9: 1 + 5 + 9 = 15.

Example 2:

Input: grid = [
  [7]
]
Output: 7
Explanation: A grid with one cell: that cell is the whole diagonal.

Example 3:

Input: grid = [
  [-1, 2],
  [3, -4]
]
Output: -5
Explanation: The diagonal cells are -1 and -4: -1 + (-4) = -5. The 2 and the 3 are not on the diagonal.

Constraints:

• grid has n rows and n columns, with n at least 1.
• Every value is an integer between -1000 and 1000; values can be negative.