No SyntaxExtension variant can modify foreign items

[tomjakubowski]

SyntaxExtension::MultiModifier only visits true items, ImplItems, and TraitItems, not ForeignItems. This makes it impossible to write syntax extensions which operate directly on foreign items (e.g. functions declared inside an extern block).

I started work on rust-lang/rfcs#1217 out of tree but hit this issue almost right away.

[steveklabnik]

Triage: this bit still exists.

[steveklabnik]

Triage: this still exists, but syntax extensions are never being stabilized as a public api, and there's been no real comments since this issue has been opened. I'm going to close this as WONTFIX. If you really truly care about this still please let me know!

[petrochenkov]

This was actually fixed in #49350