tokio-trace: Extend macros to allow trailing commas

[kleimkuhler]

Trailing commas are optional when fields are included, and when fields
are delimited from the format string for the message.

Before these would error:

event!(tokio_trace::Level::DEBUG, foo = 3, bar = 3,);
event!(tokio_trace::Level::DEBUG, { foo = 2, bar = 78, }, "baz");

Now they do not. Tests have been extended.

Signed-off-by: Kevin Leimkuhler kevinl@buoyant.io

[hawkw]

This LGTM, and I'm happy to merge as-is.

For extra credit, I'd love it if you added examples of events with trailing commas to the RustDocs for these macros, but it's not a big deal.

[hawkw]

Thanks for adding to the tests, this is great!

[hawkw]

@kleimkuhler can we also make a similar change for the span! macro?

[kleimkuhler]

@hawkw I'm not sure I like how this currently is. An unlimited number of trailing commas are valid anywhere - not just the last field.

event!(
   tokio_trace::Level::DEBUG, 
   foo = 1,,,,,,,,
   baz = 2,
   ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
);

is valid. Let me clean this up to allow only 1 trailing comma on the final field.

[kleimkuhler]

After some investigation, this is an issue that is fixed by the Kleene operator being stabilized as a macro repetition operator. Unfortunately, this RFC was merge and is only stable on 1.32 with edition = "2018".

As mentioned here, the best way to validate a trailing comma is introducing an identical rule that has a trailing comma. I will follow this up with those changes. This essentially doubles our rules for each macro, so after it is ready to go we can evaluate if that is something we want to introduce. It makes the macros look cluttered.

[hawkw]

@kleimkuhler thanks, this is great --- I've merged now that the CI build has passed.