Confusing diagnostics when forgetting to import `rustc_abi` crate

[RalfJung]

Code

mod rs {
    pub use rustc_abi as abi;
}

fn foo(_x: rs::abi::Type) {}

Current output

error[E0433]: failed to resolve: could not find `abi` in `rs`
 --> src/lib.rs:5:16
  |
5 | fn foo(_x: rs::abi::Type) {}
  |                ^^^ could not find `abi` in `rs`

warning: unused import: `rustc_abi as abi`
 --> src/lib.rs:2:13
  |
2 |     pub use rustc_abi as abi;
  |             ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

Desired output

It should point at `pub use rustc_abi as abi;` and complain that `rustc_abi` does not exist.

The correct fix is to add
extern crate rustc_abi;

Rationale and extra context

Neither of the actually shown errors is helpful:

• One error says that rs::abi does not exist -- but clearly it does exist, I wrote it right there!
• The other error says that rs::abi is unused -- but it is used, there's even an error pointing at it!

If I change rustc_abi to foobar, the error changes to something much better:

error[E0432]: unresolved import `foobar`
 --> src/lib.rs:2:13
  |
2 |     pub use foobar as abi;
  |             ^^^^^^^^^^^^^ no external crate `foobar`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `playground` (lib) due to 1 previous error

I assume this is related to the fact that rustc_abi exists as a crate, it's just not been imported.

Other cases

Rust Version

Rust 1.86.0

Anything else?

No response

[theemathas]

use rustc_abi;  // this compiles
use rustc_abi::*;  // this doesn't compile

Is this intentional?

[RalfJung]

Ah, I don't know?

[danielhenrymantilla]

FWIW, the diagnostic becomes pristine the moment one uses unambiguous pathing:

mod rs {
    pub use ::rustc_abi as abi;
}

fn foo(_x: rs::abi::Type) {}

yielding:

error[E0432]: unresolved import `rustc_abi`
 --> src/lib.rs:2:13
  |
2 |     pub use ::rustc_abi as abi;
  |             ^^^^^^^^^^^^^^^^^^ no external crate `rustc_abi`

So it looks like some rustc_abi item is available in the prelude; not just the crate.

• rustc_abi! fails;

• const _: () = rustc_abi; fails;

• const _: rustc_abi = (); fails, but with a nice diagnostic!

  error[E0573]: expected type, found built-in attribute `rustc_abi`
   --> src/lib.rs:1:10
    |
  1 | const _: rustc_abi = ();
    |          ^^^^^^^^^ not a type

So we got our answer:

Note

#[rustc_abi] is in scope of the implicit/magic prelude (or the macro attribute fallback namespace), hence the bad diagnostics

[RalfJung]

Oh d'oh, so my pub use rustc_abi as abi; imported an attribute? Wow.^^