liblog: add the loglevel, file and linenumber to the log output

[Blei]

No description provided.

[alexcrichton]

@pcwalton has had concerns in the past about the code bloat of passing around a filename and line number.

I do agree that this is quite useful, but I would expect loggers to only take a level and a format string rather than litany of other relevant arguments. Instead, perhaps the macro could change similar to this?

macro_rules! log_with_location(
    ($lvl:expr, $fmt $($arg:tt)*) => ({
        log!($lvl, concat!("{} {}:{}", $fmt), ::log::level_to_str($lvl), file!(), line!() $($arg)*)
    })
)

[seanmonstar]

I'd actually think a Logger would rather receive a LogRecord of some such, that includes the message, the name (from directive), level, and a few other possible properties. Loggers could want to handle different messages differently (send db errors as an email).

[Blei]

I agree that code bloat could potentially be a problem, not sure to what extent 2 static arguments make much of a difference. I don't agree with the proposed solution though. What's the point of making custom loggers possible, if the majority of the formatting is already prescribed? If the two additional arguments are the main concern, then gathering them into a struct as proposed by @seanmonstar would be a nicer solution IMO.

[alexcrichton]

In that case, I like the suggestion by @seanmonstar as well, it'd also allow some nice extensibility in the future?

Would you be ok with making that change @Blei? Also, you may want to print just the filename of the paths because sometimes the path to the file can be quite long.

[alexcrichton]

(sorry it took me a bit to get back to this!)

[seanmonstar]

@alexcrichton I can make a PR with what I had in mind. One question: how would I use the fmt::Arguments provided in a new format string? I can use fmt::format(args), but I don't want to allocate a string just to feed back into a new format string.

fn log(record: &LogRecord) {
    write!(stdout, "{} {}: {}", record.file, record.line, record.args);
}

[alexcrichton]

Something along the lines of:

fn log(record: &LogRecord) -> IoResult<()> {
    try!(write!(stdout, "{} {}: ", record.file, record.line));
    fmt::write(stdout, record.args)
}

[seanmonstar]

Ouch. Would it make sense to add a Show implementation to Arguments in the same patch, to allow the example I used?

[alexcrichton]

Sure!

[alexcrichton]

Closing in favor of #13912

[hirschenberger]

@seanmonstar Was it intentional using write! instead of writeln!?

[seanmonstar]

Nope, that was a mistake. Woops!