change: replace log with log/slog for logging

[alexandear]

This PR replaces usages of log to log/slog as discussed in #1278.

Now, the logger outputs to stderr and revive.log simultaneously if DEBUG environment variable is defined and not empty.

Example:

$ DEBUG=1 go run main.go
time=2025-03-25T13:04:29.971+02:00 level=INFO msg="Logger initialized" logFile=revive.log
time=2025-03-25T13:04:29.971+02:00 level=INFO msg="Config loaded" rules="[time-naming unexported-naming context-keys-type filename-format useless-break package-comments unused-parameter var-declaration unreachable-code unexported-return line-length-limit range increment-decrement indent-error-flow empty-lines use-any error-strings enforce-slice-style error-naming bare-return receiver-naming dot-imports context-as-argument errorf empty-block exported var-naming enforce-map-style error-return redefines-builtin-id blank-imports superfluous-else]"

For discarding output, I use slog.NewTextHandler(io.Discard, nil) because slog.DiscardHandler exist only in Go 1.24.

[ccoVeille]

LGTM 👍

Only minor feedbacks and a question

[ccoVeille]

BTW, is there a linter about slog (not necessarily in revive) about replacing slog.NewTextHandler(io.Discard, nil) ?

[alexandear]

I think no, it's not.

[ccoVeille]

Maybe we should think about it

https://github.com/search?q=%22slog.NewTextHandler%28io.Discard%2C+nil%29%22&type=code

What would be the best candidate? Revive?

[alexandear]

Perhaps it's go-critic

[ccoVeille]

BTW, is there a linter about slog (not necessarily in revive) about replacing slog.NewTextHandler(io.Discard, nil) for Go 1.24+ ?

Perhaps it's go-critic

What do you think about this Oleg ?

@cristaloleg

[cristaloleg]

Yep, this can be easily done via ruleguard rule. Feel free to submit an issue (or even a PR).

[ccoVeille]

Thanks for confirmation

It can be done here then:

https://github.com/go-critic/go-critic/blob/master/checkers%2Frules%2Frules.go

[ccoVeille]

I consider sloglint as a possible candidate also

https://github.com/go-simpler/sloglint

@tmzane do you think there could be an interest in sloglint to report people using

slog.NewTextHandler(io.Discard, nil) for Go 1.24+, while slog.DiscardHandler is available

Apparently, it's massively used

https://github.com/search?q=%22slog.NewTextHandler%28io.Discard%2C+nil%29%22&type=code

If you have interest for this, I will open an issue in your project
If you don't, I will open an issue in go-critric and will code it I think.

[tmzane]

Hi, I don't mind adding it to sloglint as in slog-focused linter, seems like a useful and easy-to-implement check. Feel free to open an issue, thanks

[ccoVeille]

Thanks for creating

• Suggest slog.DiscardHandler instead of slog.NewTextHandler(io.Discard, nil) for Go 1.24+ go-simpler/sloglint#78

[denisvmedia]

Thank you!