#![feature(format_args_capture)] support

[ghost]

It would be great if this crate supports #![feature(format_args_capture)]. It currently does not (playground):

#![feature(format_args_capture)]

fn main() {
    env_logger::init();
    let x = 0;
    log::error!("{x}");
}

The above program logs {x} without unused-variables warning because log! treats single argument specially and uses __log_format_args! to validate the argument:

log/src/macros.rs

Lines 35 to 36 in 1cd39c0

	// ensure that $message is a valid format string literal
	let _ = __log_format_args!($message);

I don't understand why log! treats single argument specially. The __private_api_log code path and __private_api_log_lit path should do the same thing (both of them construct fmt::Arguments):

log/src/lib.rs

Line 1481 in 1cd39c0

.args(format_args!("{}", message))

log/src/lib.rs

Line 1462 in 1cd39c0

.args(args)

If the (redundant?) single argument special treatment is removed, #![feature(format_args_capture)] will be supported. That is not a breaking change because log! already uses __log_format_args! to validate the argument (edit: not totally true → #407).

I'd like to help if this request is accepted.

[sfackler]

That special case existed to minimize the amount of code generated, but it's not compatible with format_args_capture as you pointed out. It should just be removed.