How can I concatenate two `&str` type elements of Rust Vector and add to Zero Index

Question

let mut item = vec!["2", "3", "5"];

I want to concatenate the first and second index of this vector and replace with the value of zero index.

item[0] = item[0] + item[1];

But due to the type of vector elements being &str and the result I'm getting after concatenation is String , Rust is not letting me update the value of vector.

Answer 1

One way to get to the result is through Compiler Driven Development.

Starting with:

fn main() {
   let mut item = vec!["2","3","5"];

   let first_item = &(item[0].to_owned() + item[1]);

   item[0] = item[0] + item[1];

   println!("item {:?}", item);
}

We have:

error[E0369]: binary operation `+` cannot be applied to type `&str`
 --> src/main.rs:6:22
  |
6 |    item[0] = item[0] + item[1];
  |              ------- ^ ------- &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
  |
6 |    item[0] = item[0].to_owned() + item[1];
  |              ^^^^^^^^^^^^^^^^^^

Following the compiler suggestion:

item[0] = item[0].to_owned() + item[1];

Now we get:

  |
6 |    item[0] = item[0].to_owned() + item[1];
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |              |
  |              expected `&str`, found struct `std::string::String`
  |              help: consider borrowing here: `&(item[0].to_owned() + item[1])`

Then applying again the suggestion:

  |
6 |    item[0] = &(item[0].to_owned() + item[1]);
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- temporary value is freed at the end of this statement
  |               |
  |               creates a temporary which is freed while still in use
7 | 
8 |    println!("item {:?}", item);
  |                          ---- borrow later used here

Now the problem is related to lifetime scopes, following the compiler suggestion this is the final version:

fn main() {
   let mut item = vec!["2","3","5"];

   let first_item = &(item[0].to_owned() + item[1]);

   item[0] = first_item;

   println!("item {:?}", item);
}

The compiler has driven to a solution, but in every case you have to carefully consider if this is a solution that meets the requirements of your application. In this case you must pay attention to lifetime scopes.

Another, more idiomatic solution, as already suggested, could be to use a Vec of String:

let mut item: Vec<String> = vec!["2".to_owned(), "3".to_owned(), "5".to_owned()];

item[0] = format!("{}{}", item[0], item[1]);

Answer 2

Assuming you want to do it in runtime, it is not possible. &str is non-owning type, one can't concatenate two &str 's into one for the storage they point to may be non-contigious.

You may achieve the goal using String :

let mut item = vec![String::from("2"), String::from("3"), String::from("5")];
// Vec is not empty and contains more than 1 element, it is safe to unwrap
let (first, rest) = item.split_first_mut().unwrap();
// Append the second element to the end of the first element 
first.push_str(rest[0].as_str()); 
println!("{}", &first); // 23