trace: support string interpolation about tracing HOT 5 OPEN

👋 Dropping a line here in reference to a convo in the #tracing channel in Discord.

Currently we use slog in a bunch of our libs/services which has a syntax like

info!(logger, "msg: {}", 1, formatted_field="a"; "additional_field" => 4)

The fields before the ; are used for the format string, and the result of that is used as msg in the json output. The fields after are just added to the json directly.

I'm not actually a huge fan of this, mostly cause it leads to duplication of fields, and the syntax differs between then in a weird wait. Something that might be nice could be if the syntax was basically just like format!/println! but it didn't error when named arguments weren't used, and all named arguments were added as fields, but I haven't totally thought through the implications of that. Like I'm not sure how field = ?value would work if the field was used multiple times, and say we wanted it to be Display in the span/event fields, but Debug in the message

from tracing.

I suspect it should be possible to add syntax for interpolating fields into a textual message in tokio-trace's current macros. May just require some thought to make work.

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Since Rust RFC 2795 will be stabilized in 1.58, the ergonomic semantics outlined by @samschlegel above should soon be achievable. All we'll need to do is modify event! and span! to interpret format arguments as fields, and use implicit format argument captures for them so that unused fields do not become an error.

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Is there anything currently blocking this feature from happening?

from tracing.

No, it's fairly straightforwardly doable now, just nobody's had time to get to it.

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