ex.rs

pub fn set_voice_note_on(&mut self, note_data: NoteData){
    // loop over all available voices
    for voice in &mut self.voices {
        // check the `VoiceState` and find one that's off
        if voice.state == VoiceState::Off {
            // send a note to an unused voice.
                voice.note_data = note_data;
                // add our voice to our vector of used voice references
                self.voices_in_use.push(voice); // <===== compiles if removed
                break
        }
    }
}