How do I make format! return a &str from a conditional expression?

format! can't return &str because it will always allocate String. What is possible to do is to return a &str from a String, which is what you did in your code.

As the compiler hinted, the created String is immediately dropped after its creation because it went out of the current scope and one way around could be an external variable that is not bounded to the match scope. E.g.:

use std::fmt::Write;

fn main() {
    let mut buffer = String::with_capacity(20);
    buffer.push_str("It's a ");

    let x = 10;
    let category = match x {
        0...9 => "Between 0 and 9",
        number @ 10 => {
            write!(&mut buffer, "{}", number).unwrap();
            buffer.as_str()
        }
        _ if x < 0 => "Negative",
        _ => "Something else",
    };

    println!("{}", category);
}

If you want an [no_std] environment or don't want to do any dynamic allocation, you can take a look at this limited code snippet:

use core::str;

fn each_digit<F>(mut number: u32, mut f: F)
where
    F: FnMut(u8),
{
    while number > 0 {
        f((number % 10) as u8);
        number /= 10;
    }
}

fn main() {
    const BUFFER_LEN: usize = 20;
    let mut buffer = [0u8; BUFFER_LEN];

    let x = 12344329;
    let category = match x {
        0...9 => "Between 0 and 9",
        number @ 123443219 => {
            let mut idx = BUFFER_LEN;
            each_digit(number, |digit| {
                let ascii = digit + 48;
                idx -= 1;
                buffer[idx] = ascii;
            });
            str::from_utf8(&buffer[idx..BUFFER_LEN]).unwrap()
        },
        _ => "Something else",
    };

    assert_eq!("123443219", category);
}

This is 90% a duplicate of Return local String as a slice (&str), see that for multiple other solutions.

There's one extra possibility since this is all in one function: You can declare a variable for the String and only set it when you need to allocate. The compiler (obliquely) suggests this:

consider using a let binding to create a longer lived value

fn main() {
    let x = 42;
    let tmp;

    let category = match x {
        0...9 => "Between 0 and 9",
        number @ 10 => {
            tmp = format!("It's a {}!", number);
            &tmp
        }
        _ if x < 0 => "Negative",
        _ => "Something else",
    };

    println!("{}", category);
}

This is mostly the same as using a Cow, just handled by the compiler instead of a specific type.

See also:

• How can I conditionally provide a default reference without performing unnecessary computation when it isn't used?
• Are polymorphic variables allowed?