rustc suggests to use path::_; with as _ trait re-exports

[jplatte]

Given the following code (playground):

//! ```
//! use playground::S;
//! S.method()
//! ```

pub struct S;
pub use m::Tr as _;

mod m {
    pub trait Tr { fn method(&self); }
    impl Tr for crate::S { fn method(&self) {} }
}

The current output for compiling the doctest is:

error[E0599]: no method named `method` found for struct `S` in the current scope
 --> src/lib.rs:3:3
  |
5 | S.method()
  |   ^^^^^^ method not found in `S`
  |
  = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
  |
3 | use crate::playground::_;
  |

I'm not sure what the output should be. Originally I hit this issue with tracing, which re-exports one trait (soon multiple) as _ in its prelude module. They are of course reachable in another path, so that one could be prefered¹, but I realized that this issue could also crop up with a situation such as what I've constructed above, where the trait is actually unnamable. In that case, either the compiler has to suggest as * import, or it has to not emit the help item at all.

¹ probably needs an annotation like #[doc(canonical)] (rust-lang/rfcs#3011)

[edward-shen]

I can verify that this problem still exists on 1.56.1 and 1.59.0, where I also ran into this via the tracing ecosystem, this time with this one-liner and tracing-subscriber:

use tracing_subscriber; // 0.3.3
tracing_subscriber::registry().init();

This results in the following rustc error:

error[E0599]: no method named `init` found for struct `Registry` in the current scope
  --> src/main.rs:2:36
   |
2  |     tracing_subscriber::registry().init();
   |                                    ^^^^ method not found in `Registry`
   |
  ::: /Users/edward-shen/.cargo/registry/src/6github.com-1ecc6299db9ec823/tracing-subscriber-0.3.3/src/util.rs:89:8
   |
89 |     fn init(self) {
   |        ---- the method is available for `Registry` here
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
   |
1  | use tracing_subscriber::prelude::_;
   |

For more information about this error, try `rustc --explain E0599`.

Not sure how common it is (considering more haven't complained about this), but the real-world implementation that leads to this bug seems like an easy path to hit.

[compiler-errors]

I've written up fix of this in several parts. Let me put up a PR.

@rustbot claim