rustc recommends a trait from a crate the current crate doesn't depend on

[joshtriplett]

I ran into this in a crate that depends directly on anyhow, and depends very indirectly on quick-error. When trying to use the context method, in a case that wouldn't work with anyhow because the error type didn't implement Error, rustc said:

help: the following trait is implemented but not in scope, perhaps add a `use` for it:
   |
1  | use quick_error::ResultExt;
   |

However, since the crate doesn't directly depend on quick-error, it can't add such a use.

Rough steps to reproduce:

• Have a top-level binary crate that depends on library crates c1 and c2
• Have crate c1 provide an extension trait with some requirements (such an extension trait on Result that places requirements on the error type).
• Have crate c2 depend on crate c3.
• Have crate c3 provide an extension trait that works on any type.
• Have the top-level binary crate import the extension trait from c1, and attempt to use it, but not satisfy the required trait bounds.

This should result in rustc suggesting use c3::ExtensionTrait for use in the top-level binary crate, even though the top-level binary crate doesn't depend on c3.

[joshtriplett]

I just attempted to recreate a minimal example, and apparently there's more needed than that to reproduce the issue. I'm going to see if I can reduce the original sample from wasmtime into a test case.

[joshtriplett]

OK, I have a few fairly small test cases now that reproduce the problem. They still depend on external crates, but external crates with minimal dependency trees.

Cargo.toml

[package]
name = "issue-66562"
version = "0.0.0"
authors = ["Josh Triplett <josh@joshtriplett.org>"]
edition = "2018"

[dependencies]
humantime = "1.3"

src/main.rs

use humantime as _;
fn main() {
    let run: Result<(), ()> = Ok(());
    run.context("foo");
}

Resulting errors (cargo build)

error[E0599]: no method named `context` found for type `std::result::Result<(), ()>` in the current scope
 --> src/main.rs:4:9
  |
4 |     run.context("foo");
  |         ^^^^^^^ method not found in `std::result::Result<(), ()>`
  |
  = 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 quick_error::ResultExt;
  |

Variation 1

Adding anyhow = "1.0.22" to the dependencies, and adding use anyhow::Context; to the top of the source, will result in the following:

error[E0599]: no method named `context` found for type `std::result::Result<(), ()>` in the current scope
 --> src/main.rs:5:9
  |
5 |     run.context("foo");
  |         ^^^^^^^ method not found in `std::result::Result<(), ()>`
  |
  = note: the method `context` exists but the following trait bounds were not satisfied:
          `std::result::Result<(), ()> : anyhow::Context<_, _>`
  = 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 quick_error::ResultExt;
  |

Variation 2

Adding anyhow = "1.0.22" to the dependencies, adding use anyhow as _; to the top of the source, and changing Result<(), ()> to Result<(), std::io::Error> will result in the following:

error[E0599]: no method named `context` found for type `std::result::Result<(), std::io::Error>` in the current scope
 --> src/main.rs:5:9
  |
5 |     run.context("foo");
  |         ^^^^^^^ method not found in `std::result::Result<(), std::io::Error>`
  |
  = help: items from traits can only be used if the trait is in scope
help: the following traits are implemented but not in scope, perhaps add a `use` for one of them:
  |
1 | use anyhow::Context;
  |
1 | use quick_error::ResultExt;
  |

[lolbinarycat]

triage: still broken, here's the repro in txtar format:

-- Cargo.toml --
[package]
name = "issue-66562"
version = "0.0.0"
authors = ["Josh Triplett <josh@joshtriplett.org>"]
edition = "2018"

[dependencies]
humantime = "1.3"

-- src/main.rs --
use humantime as _;
fn main() {
    let run: Result<(), ()> = Ok(());
    run.context("foo");
}