How to push String in String in Rust?

Question

fn main() {
    let mut str = "string".to_string();

    // How do I push String to String?
    str.push_str(format!("{}", "new_string"));
}

So, how do I push the formatted string in str? I don't want to concatenate. Concatenation is kinda' the same thing as push but I want to know how I can push String instead.

@SvetlinZarev Thanks, that works. But how is &String allowed? — Axel
– Axel, Commented Jan 18, 2022 at 11:00
In case it's not obvious, push_str() accepts a &str rather than String because it has to copy the data anyway, so it doesn't gain anything by consuming the String it receives. And if you have an owned string, it's trivial to obtain a reference, as shown. — user4815162342
– user4815162342, Commented Jan 18, 2022 at 11:17

Svetlin Zarev · Accepted Answer · 2022-01-18 11:04:17Z

10

You cannot push String directly, but you can push a string slice - i.e. - &str. Therefore you have to get a slice from your String, which can be done in several ways:

By calling the string.as_str() method
By taking advantage by the automatic deref coercion. Because String implements Deref<Target=str>, the compiler will automatically convert the string reference (i.e. &String) to a string slice - &str.

Thus, it's enough to get reference to the formatted string in order to push it:

fn main() {
  let mut str = "string".to_string();
  str.push_str(&format!("{}", "new_string"));
}

answered Jan 18, 2022 at 11:04

Svetlin Zarev

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1 Comment

Jmb Over a year ago

Note that in this specific case str.push_str ("new_string") avoids a superfluous allocation and copy. And in case of more complicated format strings, write!(&mut str, "{}", 42) also avoids extra allocations and copies.

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