error[E0599]: no method named `exec` found for mutable reference `&mut Command` in the current scope
  --> $DIR/issue-39175.rs:14:39
   |
LL |     Command::new("echo").arg("hello").exec();
   |                                       ^^^^ method not found in `&mut Command`
   |
   = 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:
   |
LL + use std::os::unix::process::CommandExt;
   |

error: aborting due to 1 previous error

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