Concat First and Last Char in Rust

Given a string s, return a new string made of two characters: the first character of s, followed by its last character.

Example 1:

Input: s = "hello"
Output: "ho"
Explanation: The first character is "h" and the last one is "o".

Example 2:

Input: s = "Hello World!"
Output: "H!"
Explanation: Every character counts, so the last one is the "!", not the "d". The "H" stays upper case.

Example 3:

Input: s = "ab"
Output: "ab"
Explanation: With only two characters, the first is "a" and the last is "b".

Constraints:

• s has at least 2 characters.
• s contains English letters, digits, spaces and punctuation, and doesn't start or end with a space.