Rust recommends to use a non-existing module

[RalfJung]

The following program:

use std::process::Command;

fn main() {
    Command::new("true")
        .before_exec(|| {})
        .status().unwrap();
}

shows an error saying

error[E0599]: no method named `before_exec` found for type `std::process::Command` in the current scope
 --> src/main.rs:5:10
  |
5 |         .before_exec(|| {})
  |          ^^^^^^^^^^^
  |
  = 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 std::os::ext::process::CommandExt;
  |

However, following that suggestions leads to a program that errors saying

error[E0433]: failed to resolve. Could not find `ext` in `os`
 --> src/main.rs:2:14
  |
2 | use std::os::ext::process::CommandExt;
  |              ^^^ Could not find `ext` in `os`

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

error[E0599]: no method named `before_exec` found for type `std::process::Command` in the current scope
 --> src/main.rs:6:10
  |
6 |         .before_exec(|| {})
  |          ^^^^^^^^^^^
  |
  = 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 std::os::ext::process::CommandExt;
  |

[RalfJung]

Oh, this is a duplicate of #21934. Never mind.