feat: prefix a field with %% to use alternate Display

[michaelbeaumont]

Motivation

Fixes #1311

There's currently no way to use {:#} to format a field, which is very important for anyhow::Error in particular.

Of course this should be backported v0.1

Solution

Add a new sigil %%. I had parsing issues with # so I quickly abandoned that, though there may be a way to make it work. I'm not attached to this particular sigil at all.

Add a new wrapper similar to DisplayValue that formats with the alternate # flag. The solution I have is use format_args!, this should be OK AFAICT. I think ideally we would use the Formatter API to just switch on alternate, but it's nightly only.

[hds]

I don't see anything in this change that looks like it's changing the behavior of the event and span macros to accept a %% prefix.

Is there something that wasn't added to the change?

(Caveat: I haven't actually tested the change, but without modifying the declarative macros I don't see how the new prefix could work)

Could you please clarify this?

[michaelbeaumont]

but without modifying the declarative macros

Maybe I'm missing something but all changes in tracing/src/macros.rs? @hds

[hds]

Yeah. I'm expecting changes in there. And some tests to go with it.

Also, fair warning, we've recently found some inconsistencies in those macros and different field patterns.

It needs to be cleaned up and covered in more comprehensive tests. To keep things sane, we probably won't merge this PR until that's done.

[michaelbeaumont]

Yeah. I'm expecting changes in there. And some tests to go with it.

Yeah, they're in the PR already though.

[michaelbeaumont]

@hds the changes you requested were already part of the PR

[hds]

@michaelbeaumont Sorry about that! I missed the changes as they were automatically folded up by GitHub. I'll have a look at this one, but after we make some general fixes to the macros where we've got a bunch of inconsistencies.