How can I format a string like this?

static TEST: &str = "test: {}";

fn main() {
    let string = format!(TEST, "OK");
    println!("{}", string);
}

I want to construct the string "test: OK", but this doesn't work. How can I do it?

The format! macro needs to know the actual format string at compile time. This excludes using variable and statics, but also consts (which are known at compile time, but at a later compilation phase than macro expansion).

However in this specific case you can solve your problem by emulating a variable with another macro:

macro_rules! test_fmt_str {
    () => {
        "test: {}"
    }
}

fn main() {
    let string = format!(test_fmt_str!(), "OK");
    println!("{}", string);
}

(Permalink to the playground)

If your format string isn't actually know at compile and can't be used in a macro like this, then you need to use a dynamic template engine.