$ cargo run --features "test_generator"
warning: function is never used: `test_generator`
 --> src\main.rs:8:4
  |
8 | fn test_generator() {
  |    ^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running `target\debug\cfg-yield-test.exe`
Hello, world!
$ cargo run
   Compiling cfg-yield-test v0.1.0 (C:\Users\hato2\Desktop\cfg-yield-test)
error[E0658]: yield syntax is experimental
  --> src\main.rs:14:9
   |
14 |         yield 1;
   |         ^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/43122
   = help: add `#![feature(generators)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.
error: could not compile `cfg-yield-test`.

To learn more, run the command again with --verbose.