How to concatenate a char onto a string in Rust?

You can also use format!:

fn main() {
   let s = String::from("March");
   // example 1
   let s1 = format!("{}!", s);
   // example 2
   let s2 = format!("{}{}", s, '!');
   // print
   println!("{} {}", s1, s2);
}

https://doc.rust-lang.org/std/macro.format.html

Using String::push method is the easiest method:

let mut a_string = String::from("Hello World");
a_string.push('!');