Format_args!() with long lifetimes

language design
9

Firstly, I believe the issue is the same as with this: (i.e. not specific to format_args)

//error[E0716]: temporary value dropped while borrowed
consume(
//----- borrow later used by call
    if cond {
        &then_block(4)
//      ^^^^^^^^^^^^^ creates a temporary which is freed while still in use
    } else {
//  - temporary value is freed at the end of this statement
        &else_block(7)
    }
);

I agree that ought to compile, but I have no idea if it could be changed.

Labeled breaks and tuple evaluation do enable a workaround:

macro_rules! if_then_else {
    ($cond:expr, $then:expr, $otherwise:expr) => {
        'if_else:  {
            (
                'yes: {
                    if $cond { break 'yes }
                    break 'if_else $otherwise
                },
                $then
            ).1
        }
    }
}

With that my earlier example becomes:

pub fn apples(n: usize) -> String {
    format!("There {}!",
        if_then_else!(n==0, format_args!("are no ({n}) apples"),
        if_then_else!(n==1, format_args!("is just one ({n}) apple"),
        if_then_else!(n==2, format_args!("are two ({n}) apples"),
                            format_args!("are {n} apples")
        )))
    )
}
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