Add "Log messages with a custom logger" recipe

[rap2hpoutre]

This recipe shows how to create a custom logger and log various messages into it. Based on #69 and is a part of #61

Side note:

Too many errors and mistakes were made in https://github.com/brson/rust-cookbook/pull/69 (and it's now in conflit with commit like "oops", so its quality is poor). Now with the help of @dtolnay and @budziq I re-created a clean (?) PR based on this todo-list https://github.com/brson/rust-cookbook/pull/69#issuecomment-300814527

[budziq]

@rap2hpoutre Hi there was no deed to close the original PR as you could have just git push --force pulled_branch to your repository with new changes and the PR would be completely updated/rewritten :)

Anyhow. Your pull looks very good 😃 . Minor suggestions:

• the textual description should mention hyperlinked log::Log and log::set_logger and that it is writing to stdout.
• (much less important - not mandatory) I would suggest changing title to "Log messages with a custom logger impl" as "custom formatting" can be realized via https://doc.rust-lang.org/log/env_logger/struct.LogBuilder.html

[rap2hpoutre]

@budzik Thanks! Once again I would like your recommandations. What about this textual description:

Set a basic custom logger ConsoleLogger that logs various messages to stdout. Loggers implement the log::Log trait and are installed by calling the log::set_logger function. This ConsoleLogger writes to stdout.

Source: https://doc.rust-lang.org/log/log/index.html#logger-implementations

For the other suggestion, I think you are right but I think we do not need "impl", just "Log messages with a custom logger"

[budziq]

I would suggest to make the text a little more concise and in passive voice.

Custom logger ConsoleLogger is implemented with log::Log trait and installed via log::set_logger. Messages are logged to stdout.

As to your suggestion "Log messages with a custom logger" looks awesome to me.

For future reference, I would just push updated PR and ask for review instead of posting suggested text inline. It will be easier and less noisy on the thread :)

[budziq]

Almost there :)

• forgotten to update master table in intro.md
• use ex-log-custom

Please use try to use git commit --amend and then git push --force to update history without new commit.

[budziq]

ex-log-custom was ok no need to change it

[rap2hpoutre]

In fact I actually had to change ex-log-custom because it was already in use below and I used ex-log-logger everywhere else. I commited (with --amend).

[budziq]

then use ex-log-custom-logger ;) ex-log-logger is not really informative

[rap2hpoutre]

Ok I just changed from ex-log-custom-logger to ex-log-logger

[budziq]

Looks good to me 👌. Great work! I assure you that it was painful only the first time :)

@rap2hpoutre Do you feel upto creating the the minimal env_logger based examples?

[rap2hpoutre]

Great work! I assure you that it was painful only the first time :)

Thank YOU for all the time you offered me. I learned A LOT! And I am slow so thanks for your patience.

Do you feel upto creating the the minimal env_logger based examples?

Not today I think, maybe after the weekend. I think I will do this in a new PR because I can't say "yes I will do this in X hours/days", I have no idea right now. But I will continue to contribute and hope I will become a better contributor.

[dtolnay]

Nicely done!