Declaring modules inside decl macros 2.0

[valff]

Not sure if it is a bug or confusing error messages. Consider the following code:
Playground

#![feature(decl_macro)]

macro impl_mod($name:ident) {
  pub mod $name {
    pub struct Bar;
  }
}

impl_mod!(foo);

use foo::Bar;

fn main() {}

Output:

error[E0432]: unresolved import `foo::Bar`
  --> src/main.rs:11:5
   |
11 | use foo::Bar;
   |     ^^^^^^^^ no `Bar` in `foo`. Did you mean to use `Bar`?

warning: unused import: `foo::Bar`
  --> src/main.rs:11:5
   |
11 | use foo::Bar;
   |     ^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default

What is confusing:

1. So is it unresolved or resolved and unused?
2. "no 'Bar'. did you mean 'Bar'?" - feels like a bug.
3. Path foo is accessible, why then foo::Bar not?

rustc --version --verbose:

rustc 1.24.0-nightly (560a5da9f 2017-11-27)
binary: rustc
commit-hash: 560a5da9f1cc7f67d2fc372925aef18c96c82629
commit-date: 2017-11-27
host: x86_64-unknown-linux-gnu
release: 1.24.0-nightly
LLVM version: 4.0

[petrochenkov]

unresolved import `foo::Bar` / no `Bar` in `foo`

Name Bar defined in the macro is protected by hygiene 2.0 and cannot be named from the outside, therefore the error. (Name foo is defined outside of the macro, so it can be used from outside of the macro as well.)

Did you mean to use Bar?

This is a diagnostic bug, inaccessible names like Bar should be filtered away before giving advices.

unused import: `foo::Bar`

Diagnostic bug as well, erroneous imports should not be reported as unused.

[valff]

Name Bar defined in the macro is protected by hygiene 2.0 and cannot be named from the outside, therefore the error.

Makes sense. But it still can be exported and named by crate consumers. Is it how intended? Maybe a separate bug?

[petrochenkov]

But it still can be exported and named by crate consumers.

Do you mean foo::Bar is accessible from other crates?
Then it looks like a bug. Bar shouldn't be accessible anywhere outside of macro impl_mod.

[valff]

Do you mean foo::Bar is accessible from other crates?
Then it looks like a bug. Bar shouldn't be accessible anywhere outside of macro impl_mod.

Yes. Then here is a minimal example: https://github.com/valff/hygiene_bug

[petrochenkov]

I see.
IIRC, cross-crate hygiene (i.e. storing hygiene related data in metadata of compiled crates) isn't fully implemented yet.

[valff]

Ok, now it is clear for me. Thank you for your time!

[petrochenkov]

There are some legitimate issues reported here.

[fmease]

This no longer emits the incorrect suggestion did you mean …?:

error[E0432]: unresolved import `foo::Bar`
  --> src/main.rs:11:5
   |
11 | use foo::Bar;
   |     ^^^^^^^^ no `Bar` in `foo`

Therefore I consider this fixed unless you'd like to see a note explaining def-site hygiene?
Closing for now but feel free to raise objections and I can reopen it.

Edit: Fixed by #90202.