`pub` on `macro_rules!` not supported

[JulianKnodt]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=71be237d1aed61df9bbdb665bbd5a056`

I expected to be able to format with rustfmt,

but when running rustfmt there is an error despite compilation succeeding.

The implementation PR is 82296, but unsure if there's a RFC for the original idea.

[calebcartwright]

Thanks for the report. We consume the rustc internals via the auto publish crates, so we always lag behind rustc changes somewhat. This should be a straightforward update on our side but may take a little while to pull in and apply the upstream updates

[calebcartwright]

Closing as this no longer seems relevant given the upstream state (though I expect that if the feature and syntax support are brought back the visibility will be maintained now that we consume the rustc internals via rustc-dev)