Smarter macros to allow writing the log message at the beginning

[JasonXJ]

Feature Request

Crates

tracing

Motivation

It is more natural to put the log message at the beginning. For example, this

info!("downloading sub-sitemap", url = sitemap.loc);

is better than

info!(url = sitemap.loc, "downloading sub-sitemap");

We can only do the latter currently. I believe this is to avoid ambiguity of whether url = sitemap.loc is a formatting argument or a structured field for the log. But I think there is a solution to this.

Proposal

We should allow putting message at the beginning. And if the message requires any formatting, it should be surrounded by [ ] (I pick these over other parentheses because it looks better to me).

Example of simple string without any formatting as the message (same example as above):

info!("downloading sub-sitemap", url = sitemap.loc);

Example of format string as the message:

info!(["downloading sub-sitemap for {domain}", domain = "example.com"], url = sitemap.loc);

To be backward compatible and compatible with the log crate, we will still support something like

info!(... "downloading sub-sitemap. domain={domain}", domain = "example.com")

This can be done by checking whether the message not wrapped in [ ] is actually a format string (i.e. contains un-escaped { }). And if it is, we would consider the following arguments to be formatting arguments just like what we are doing right now.

I feel that this is not too hard to implement, although we might need to switch to procedure macros to make it work.

Also, this is a bit off-topic, but I wonder why we have so much redundant code in macros.rs. Surely, there is a way to avoid having almost identical 250+ lines of implementation for info!(), debug!() ... Is this because of some limitation of declarative macros?