Lint non_structural_log

[Veetaha]

Lint explanation

Catch code that uses a tracing logging macro without using structural logging. According to the conventions that I use it is prohibited to use format arguments in tracing logging macros. This means that the message of the log event must always be a static string, and no placeholder {} syntax can be used.

The reasoning is that when we look at the logs in grafana/loki we want to see a birds-eye view of what the system does without digging into the specifics of what specific values of different variables there were when the log happened like this:

and then expand the log event to see the key-value pairs of the structural log to dig into the details of a particular log in time.

If developers don't use structural logging and interpolate values in the log message, then such a birds-eye view becomes very verbose and hard to navigate. Also, when no string interpolation is used in log events it's possible to easily build the metrics and statistics of how often a log event with message "foo bar" occurs, and graph that on a dashboard.

Example code

Bad

tracing::info!("The user `{username}` joined the chat `{}`", chat.id);

Good

tracing::info!(%username, chat = %chat.id, "The user joined the chat");

Notes

It would be easier to implement such kind of a lint if marker could see unexpanded macros, but I suppose it may be possible to craft a hacky version of this that operates on the expanded tracing macros, although it may break at any moment once tracing changes the way their macros are expanded.