How to concatenate two strings into the third string without copying?

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I'm new to Rust and I'm currently reading The Rust Programming Language book.

I'm curious about this example:

fn main() {
    let s1 = String::from("Hello, ");
    let s2 = String::from("world!");
    let s3 = s1 + &s2; // note s1 has been moved here and can no longer be used
}

Is it possible to take ownership not only of s1, but also of s2, so s2 gets invalidated as well as s1, making s3 the only possible variable that remains usable?

ownership rust move concatenation string
1 Answers

Updated Answer

This is not possible. A String has to be a single contiguous allocation in memory. If you want to roll your own simple solution you can probably define a type like this:

struct MyString {
    parts: Vec<String>,
}

impl MyString {
    fn concat(&mut self, other: String) {
        self.parts.push(other);
    }
}

However re-implementing every useful String method for this custom type is going to be tedious and error-prone. You can find a Rust crate which implements a Rope data structure, and an-rope seems to be such a crate, but only a tiny fraction of String methods are supported.

I gave this answer when I interpreted OP's question as one about invalidating and moving variables:

Original Answer

You can invalidate any non-Copy variable by moving it somewhere else, like by passing it to the drop function:

fn main() {
    let s1 = String::from("Hello, ");
    let s2 = String::from("world!");
    let s3 = s1 + &s2; // s1 invalidated
    drop(s2); // s2 invalidated
    // now only s3 is usable
}

If this is a common pattern in your application you can just write a function which takes ownership of 2 Strings and returns the concatenated result:

fn concat(s1: String, s2: String) -> String {
    s1 + &s2
}

fn main() {
    let s1 = String::from("Hello, ");
    let s2 = String::from("world!");
    let s3 = concat(s1, s2); // s1 & s2 invalidated
    // now only s3 is usable
}
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