rustc recommends non-existent use

[thanatos]

rustc will recommend the non-existent use statement use std::os::ext::process::CommandExt;, if the CommandExt trait is used, but not in scope.

Consider the following minimal example, a program that we would like to exec true:

use std::process::Command;

fn main() {
    let err = Command::new("true").exec();
    panic!(err)
}

This shouldn't compile, as the CommandExt trait, which .exec is part of, is not in scope. rustc says as much:

» rustc execit.rs
error[E0599]: no method named `exec` found for type `std::process::Command` in the current scope
 --> execit.rs:4:36
  |
4 |     let err = Command::new("true").exec();
  |                                    ^^^^
  |
  = help: items from traits can only be used if the trait is in scope
  = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
          candidate #1: `use std::os::ext::process::CommandExt;`

error: aborting due to previous error

Let's take the compiler's suggestion, since it is correct — a required trait is indeed not in scope:

» cat execit.rs
use std::process::Command;
use std::os::ext::process::CommandExt;

fn main() {
    let err = Command::new("true").exec();
    panic!(err)
}
» rustc execit.rs
error[E0432]: unresolved import `std::os::ext`
 --> execit.rs:2:14
  |
2 | use std::os::ext::process::CommandExt;
  |              ^^^ Could not find `ext` in `os`

warning: unused import: `std::os::ext::process::CommandExt`
 --> execit.rs:2:5
  |
2 | use std::os::ext::process::CommandExt;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

error[E0599]: no method named `exec` found for type `std::process::Command` in the current scope
 --> execit.rs:5:36
  |
5 |     let err = Command::new("true").exec();
  |                                    ^^^^
  |
  = help: items from traits can only be used if the trait is in scope
  = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
          candidate #1: `use std::os::ext::process::CommandExt;`

error: aborting due to 2 previous errors

… it both fails to resolve the import, and re-recommends that very import in the same output.

I'm not sure how rustc forms its candidates for missing use statements (which are normally spot-on suggestion, BTW) but this particular one is bugged (and somewhat wat-inducing).

The docs for Command link to std::os::unix::process::CommandExt, and indeed, that compiles:

» cat execit.rs
use std::process::Command;
use std::os::unix::process::CommandExt;

fn main() {
    let err = Command::new("true").exec();
    panic!(err)
}
» rustc execit.rs
» # success!

[kennytm]

Thanks for the report. This is a duplicate of #21934.