Allow a regex filter for RUST_LOG

[nrc]

When specifying RUST_LOG, the programmer may append /regex to the end of the spec. All results will then be filtered using that regex.

r?

[huonw]

Could you describe what is being filtered here? (i.e. that it is applied to the message that will be printed, rather than the module path.)

[huonw]

Oh, and can you specify a regex for each module, or can there only be one for the whole RUST_LOG, applying globally?

[nrc]

@huonw updated for your comments, thanks!

[nrc]

Sadly, I can't reproduce these errors locally.

[lilyball]

Am I correct in assuming the regex cannot include a comma? It might be nice to state this explicitly. Ideally there would be some syntax of escaping a comma for inclusion, such as backslash-escaping it, but it's probably ok to ignore that for now.

[lilyball]

Actually, reading the code, the regex applies to all crates, not just to a single crate, and thus can contain commas. That's very surprising. I would have expected the regex to be per-crate.

[lilyball]

To be more specific, I would expect the actual syntax to look a bit more like the following:

crate1::mod1,crate1::mod2=1,crate1::mod3/filter/,crate1::mod4/filter/=4

With this syntax, the filter is per-crate, and actually occurs before the log level. Which suggests that the following should be possible:

crate1::mod1=1,crate1::mod1/filter/=4

i.e. set the base log level for the mod to 1 but bump it up to 4 for any line that matches the filter.

Also, with this syntax, I would expect commas and equals signs within the regex to be treated as part o the regex. Similarly, I would expect to be able to backslash-escape the closing / of the regex, e.g.

crate1::mod1/foo\/bar/=1

[nrc]

@kballard this is by design. My use case is that I insert a load of debug statements in different modules, and use a tag word in each. Then I want to match the same regex for each module (just to pick out the tag).

[nrc]

@huonw r?

[huonw]

The two println!s should go to stderr.

[nrc]

I was following convention in the file. Do you prefer to change all uses to stderr?

[nrc]

@huonw ping on this question

[huonw]

Oh, it probably should; but I'm happy with just leaving as is and filing a bug.

[huonw]

r=me with the minor comments.

[lilyball]

I'm still concerned that this design is tailored very closely to one particular use-case of @nick29581, does not match the semantics that one would expect upon learning that RUST_LOG supports regexes, and provides no backwards-compatible path forward to expanding this syntax later to actually be more flexible.

[nrc]

@kballard I'm not sure what your use case is for the more flexible approach, so I'm erring in the direction of the YAGNI principle here. I think it is fine to break this in the future if there is need for the more flexible approach - there is no backwards-compatibility guarantee with logging. I also don't think the more flexible approach is necessarily what "one would expect upon learning that RUST_LOG supports regexes", that seems purely subjective.

(I should also state a broader use case which is logging multiple modules but filtering on some cross-cutting string, e.g., finding all references to a particular set of expressions across type checking and trans).

[lilyball]

@nick29581

there is no backwards-compatibility guarantee with logging

That's a fair point. Given that, I am much less concerned about committing this in its current form.

I also don't think the more flexible approach is necessarily what "one would expect upon learning that RUST_LOG supports regexes", that seems purely subjective.

RUST_LOG supports a log level per module. It seems natural to me that specifying a regex would have the same level of granularity.

YAGNI

My feeling is that while you might want to find some cross-cutting string across a bunch of modules, I might want to simply enable debug logs on several modules and filter each module to just the output I want from that module. The interesting output from one module might look different than the interesting output from a second.

[nrc]

@bors: retry