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Concatenating two string slices &str in Rust

When concatenating two string slices, we can't use + in Rust.

rust

fn main() {
  let one = "string";
  let two = "something else";

  let three = one + two;
}

The compiler will warn us.

error[E0369]: cannot add `&str` to `&str`
 --> main.rs:5:19
  |
5 |   let three = one + two;
  |               --- ^ --- &str
  |               |   |
  |               |   `+` cannot be used to concatenate two `&str` strings
  |               &str
  |
help: `to_owned()` can be used to create an owned `String` from a string reference. String concatenation appends the string on the right to the string on the left and may require reallocation. This requires ownership of the string on the left
  |
5 |   let three = one.to_owned() + two;
  |               ^^^^^^^^^^^^^^

As the compiler help text describes, concatenation appends the string on the right to the string on the left, which requires ownership and a string slice is not an owned type.

Instead, we can take advantage of the format! macro to leave both string slices alone and create a new String.

rust

format!("{}{}", one, two);

rust

fn main() {
  let one = "string";
  let two = "something else";

  let three = format!("{}{}", one, two);
}