Macros are not propagated through pub extern crate.

[Xaeroxe]

As of Rust 1.26.2 stable if I define an exported macro in crate foo then pub extern crate foo in another crate bar then consumers of the bar crate cannot use the macro in foo even if there is a #[macro_use] on the pub extern crate foo and a #[macro_use] on the extern crate bar in the consumers code.

I would expect the macros to be propagated similarly to the other items in the foo crate.

This can be worked around by having the consumer of bar depend on foo directly, but that defeats the purpose of the pub extern crate imo.

[Mark-Simulacrum]

cc @jseyfried @petrochenkov

I'm also nominating for compiler team discussion -- this feels like it might be somewhat related to recent movement on the use extern macros feature?

[Xaeroxe]

minimal-repro.zip

Here's a minimal reproduction case

[nikomatsakis]

This is tagged as a regression, but did this ever work? Maybe my memory is faulty, but I didn't think that #[macro_use] worked across crates (no matter the privacy of the extern crate in question).

[nikomatsakis]

Put another way, the behavior @Xaeroxe describes is the behavior I expect. I think that this was the purpose of #[macro_reexport] (#29638) which was never stabilized (and indeed was recently removed). We expect pub use to eventually play this role, modulo the resolution of the various issues around it.

[nikomatsakis]

triage: P-high

Marking as P-high so that we revisit next week. But I'm not convinced this is a bug (as I wrote earlier).

Thoughts @rust-lang/compiler ?

[petrochenkov]

Yes, this isn't supposed to work.
#[macro_use] extern crate ... adds macros to the prelude, but not reexports them in any way.
As @nikomatsakis said #[macro_reexport] was used for reexporting and now it's subsumed by pub use.

[nikomatsakis]

Closing as not a bug, thanks @petrochenkov.

[MightyPork]

I'm trying to use this feature and it doesn't work with pub use. Is there currently a working mechanism to reexport macros?

My use case is caused by the lack of workspace-wide dependency config in Cargo.toml, so I made a crate "deps" that was meant to reexport dependencies that need special feature flags or versions. It works fine, except for macros.

I tried pub use, pub extern crate, both with #[macro_use] and #[macro_export], and import it with #[macro_use]. Nothing got the macro into scope ("cannot find macro doc! in this scope")

[petrochenkov]

@MightyPork
Could you give some example / reproduction?
pub use generally works for macros and is actively used (e.g. for reexporting macros from libcore in libstd).

[MightyPork]

here is a minimal reproduction (tested with current stable):

debug-macro-reexport.zip

[petrochenkov]

@MightyPork
That example is supposed to use pub use bson::doc;