Should Rust give an error with this one liner to concatenate a new string to another using push_str()?

Question

I want to concatenate a String.
In my opinion the following code should give some sort of a panic or compiler error.

let mut s = String::from("abc").push_str("x");
println!("{:?}", s); // Prints ()

But this code works:

let mut s = String::from("abc");
s.push_str("x");  
println!("{:?}", s); // Prints "abcx"

Answer 1

In my opinion the following code should give some sort of a panic or compiler error.

Why should it? This code is perfectly legal.

s is now a unit value () and because you are using {:?} which is the Debug output it prints ().

This is a good time to introduce clippy to you. With cargo clippy you receive the following warning:

warning: this let-binding has unit value
 --> src/main.rs:2:5
  |
2 |     let mut s = String::from("abc").push_str("x");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: omit the `let` binding: `String::from("abc").push_str("x");`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
  = note: `#[warn(clippy::let_unit_value)]` on by default

which indicates that something is fishy in your code.