Warn about #[macro_export] mod m?

[ralfbiedert]

I haven't used macros that often, but had to make changes in a 3rd party project.

During refactoring I tried to import macros from another module. I knew I had to add macro_use, macro_export somehow, but couldn't remember details.

While rustc was somewhat helpful in warning me on my first attempt:

error: cannot find macro `test!` in this scope
 --> src/x/u/u.rs:5:1
  |
5 | test! {
  | ^^^^^^^^^^
  |
  = help: have you added the `#[macro_use]` on the module/import?

It did not warn me about doing this:

#[macro_export] 
mod m;

where in reality I should have done that:

#[macro_use] 
mod m;

This was particularly confusing since the used macro was a few modules down in the hierarchy, and I should have done #[macro_use] on each level. Since I wasn't warned, I assumed #[macro_export] mod m; was correct. Instead, I tried a few permutations adding #[macro_use] to other places, which all didn't work out.

Two things that would have worked for me:

• Warn about #[macro_export] mod m;
• Make the help text above more explicit, e.g., by suggesting that macro_use needs to be added to all parent modules (maybe with a link to the error index)

Update: using rustc 1.29.0-nightly (9fd3d7899 2018-07-07)