Logging to stdout seems to be disabled

[azerupi]

Since some time, mdBook doesn't log anything to stdout anymore. Users don't get any feedback from mdBook, which is not good...

Related to #355

[budziq]

Well it is not really disabled we just don't log messages on Error level which are displayed by default. If we set RUST_LOG=info or debug we see some additional logs. It depends what you would like to achieve.

Cargo for instance uses dual logging system. env_logger like mdBook does for its backend/library part (that is controlled by RUST_LOG env var). And termcolor based colored output to stdout for the binary frontend app that is controlled by --verbose and --quiet cmd switches.

We can either:

• emulate cargo's split logging system
• raise the default visible loggin level to info
• use println / termcolor where we want guaranteed output to stdout.

but firstly we would need to know which logs would need to be shown by default.

[azerupi]

Cargo for instance uses dual logging system.

Interesting. I assume this is to split the output into messages for the devs and messages for the users, which is nice. We could go that route if it doesn't add a lot of complexity. Otherwise we should set the logging level to info I think, and make sure that all the messages for ìnfo, warnand errorare directed at users. We can keepdebugandtrace` for development purposes.

Having coloured output would be a nice addition too.

[budziq]

Having split logging would certainly add complexity cargo seams to prefer logging to user (with several variants of their shell.warn as opposed to seldom used warn!).

I would go with info bumping the level to info and see how do we like it. Also colored output is pretty trivial to add with https://github.com/seanmonstar/pretty-env-logger (I helped clean its docs some time ago).