trace: Determine new syntax for uninitialized fields about tracing HOT 5 CLOSED

@hawkw It's not a blocking issue for me. I think it can be added later.

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Same 😄

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Personally, I think let field would be fine, but I'd like to hear from everyone who's invested in this.

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@jonhoo, @carllerche, @davidbarsky, and @LucioFranco, just checking to see if supporting uninitialized fields in tokio-trace 0.2 is important to any of you. If so, we'll block the breaking change on this issue. Otherwise, when we agree on the new syntax, we can add it in a small-point release...

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I thought about this a bit more and I think I prefer field = _ despite the possible confusion with assignment syntax. The field = _ syntax reminds of anonymous lifetimes ('_) and placeholders in generics (Vec<_>) much more than the fact we're not assigning values to fields.

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