fn test(_x: &mut String) {} fn test2(_x: &mut i32) {} fn main() { let x: usize = String::new(); //~^ ERROR E0308 let x: &str = String::new(); //~^ ERROR E0308 let y = String::new(); test(&y); //~^ ERROR E0308 test2(&y); //~^ ERROR E0308 let s = &mut String::new(); s = format!("foo"); //~^ ERROR E0308 }