Inconsistency in formated output with respect to functions in standard lib.

[FabienTregan]

Using log 0.4.11, the following code:

#[macro_use]
extern crate log;
extern crate stderrlog;

fn main() {
	stderrlog::new().module(module_path!()).init().unwrap();

	println!("{{{{this}}}} prints a way");
	println!("{{{{this}}}} prints {}", "in the same way");

	error!("{{{{this}}}} logs a way");
	error!("{{{{this}}}} logs {}", "in another way");
}

produces this output:

{{this}} prints a way
{{this}} prints in the same way
ERROR - {{{{this}}}} logs a way
ERROR - {{this}} logs in another way

This is due to

log/src/macros.rs

Line 36 in f06a18d

let _ = __log_format_args!($message);

checking that the message is correct when there are no arguments but ignoring the result and logging the original message.
Standard rust macros (format!, write!, println!, ...) call std::format_args! which already handles both cases:
https://github.com/rust-lang/rust/blob/ac48e62db85e6db4bbe026490381ab205f4a614d/library/alloc/src/macros.rs#L105-L110
https://github.com/rust-lang/rust/blob/4b0882cfaa33de2796eac01afeac69097fbccbd2/library/core/src/macros/mod.rs#L792-L795

If you think it is desirable to have a behavior that is analog to rust macros, I can try doing the change.

[FabienTregan]

The same code with 4.0.8 gives the expected result.

[sfackler]

We started special-casing the no-args case to minimize codegen, but it seems like that's not quite the correct behavior: https://github.com/rust-lang/log/blob/master/src/lib.rs#L1388-L1403.

That special case will need to go away anyway to support https://github.com/rust-lang/rfcs/blob/master/text/2795-format-args-implicit-identifiers.md.

[sfackler]

There's a new API on Arguments that we should be able to use to recover this optimization: rust-lang/rust#74056

[sfackler]

Fixed by #446. If Arguments.as_str stabilizes, we can use that to add the optimization back.