"The following types implement trait..." suggets crate private items, if inside macro.

There are two crates here, library and consumer (which depends on library).

library:

#![feature(decl_macro)]

pub macro global_meow($item:expr) {
    const _: () = { 
        fn foo() {
            Meow::meow(&$item);
        }
    };
}

#[macro_export]
macro_rules! global_meow_rules {
    ($item:expr) => {
        const _: () = { 
            fn foo() {
                $crate::Meow::meow(&$item);
            }
        };
    }
}

pub trait Meow {
    fn meow(&self) {}
}

pub struct GlobalMeow;

impl Meow for GlobalMeow { }

pub(crate) struct PrivateMeow;

impl Meow for PrivateMeow { }

consumer:

static NOT_MEOW: usize = 0;

library::global_meow!(NOT_MEOW);

library::global_meow_rules!(NOT_MEOW);

fn main() {}

The current output is (when running cargo check on consumer):

# cargo check
    Checking consumer v0.1.0 (/tmp/scratcho2FpPeJDq/consumer)
error[E0277]: the trait bound `usize: Meow` is not satisfied
 --> src/main.rs:2:1
  |
2 | library::global_meow!(NOT_MEOW);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Meow` is not implemented for `usize`
  |
  = help: the following other types implement trait `Meow`:
            GlobalMeow
            library::PrivateMeow
  = note: this error originates in the macro `library::global_meow` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `usize: Meow` is not satisfied
 --> src/main.rs:4:1
  |
4 | library::global_meow_rules!(NOT_MEOW);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Meow` is not implemented for `usize`
  |
  = help: the following other types implement trait `Meow`:
            GlobalMeow
            library::PrivateMeow
  = note: this error originates in the macro `library::global_meow_rules` (in Nightly builds, run with -Z macro-backtrace for more info)

Ideally the output should look like:

# cargo check
    Checking consumer v0.1.0 (/tmp/scratcho2FpPeJDq/consumer)
error[E0277]: the trait bound `usize: Meow` is not satisfied
 --> src/main.rs:2:1
  |
2 | library::global_meow!(NOT_MEOW);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Meow` is not implemented for `usize`
  |
  = help: the following other types implement trait `Meow`:
            GlobalMeow
  = note: this error originates in the macro `library::global_meow` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `usize: Meow` is not satisfied
 --> src/main.rs:4:1
  |
4 | library::global_meow_rules!(NOT_MEOW);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Meow` is not implemented for `usize`
  |
  = help: the following other types implement trait `Meow`:
            GlobalMeow
  = note: this error originates in the macro `library::global_meow_rules` (in Nightly builds, run with -Z macro-backtrace for more info)

We should not be suggesting private types to users, since macro hygiene does mean they can't write library::PrivateMeow

This is on the latest nightly, rustc 1.64.0-nightly (06754d885 2022-07-08)

This was ran into in #99074

[compiler-errors]

@5225225, this only happens with macro items and not macro_rules! global_meow?

I included examples of macro and macro_rules!, and it seems to happen for both (see: library::global_meow_rules!)

[compiler-errors]

Oh, my bad. I didn't read closely, my eyes just went directly to the macro item. Sorry. 😅

[compiler-errors]

For the record, this is not a macro problem. See this example that does not use macros:

/// ```
/// fn needs_meow<T: playground::Meow>(t: T) {}
/// fn main() { needs_meow(1usize); }
/// ```
pub trait Meow {
    fn meow(&self) {}
}

pub struct GlobalMeow;

impl Meow for GlobalMeow { }

pub(crate) struct PrivateMeow;

impl Meow for PrivateMeow { }

(uses a doc comment to emulate two crates on playground)

The reason is that InferCtxt::find_similar_impl_candidates does not do any filtering for type privacy.