tracing: Replace macro-rules with procedural macros about tracing HOT 6 OPEN

I believe @Ralith is working on this. My understanding is that it is currently blocked on dtolnay/syn#661.

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I believe @Ralith is working on this. My understanding is that it is currently blocked on dtolnay/syn#661.

This is accurate. If that issue can be fixed, I am otherwise very close to a working prototype of event!().

I think the new macros could live in the existingtracing-proc-macros or tracing-macros crates. Perhaps we should use tracing-macros and move everything currently in tracing-proc-macros there (and rewrite or remove the macros currently in tracing-macros, which are very experimental).

These macros will necessarily rely on proc-macro-hack, which means that they can't be used by downstream code directly from the crate they're implemented in; an intermediate re-exporting crate is always necessary. My understanding is that tokio(-proc)-macros are intended to be user-facing, so we'll still need an internal -impl crate.

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Looks like it might not be a syn issue: dtolnay/proc-macro-hack#34

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Now isolated to a compiler issue: rust-lang/rust#62325

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Thanks for keeping us updated, @Ralith --- great to hear that progress is being made on unblocking this!

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Looks like the compiler bug has been fixed in rust-lang/rust#62393, so hopefully we can move forwards with this once the fix is on stable.

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