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trait FooPow {
    fn pow(&self) -> i32;
}
 
struct Foo(i32);
 
impl<'a> FooPow for &'a Foo {
    fn pow(&self) -> i32 {
        self.0 * self.0
    }
}
 
/*impl<'a> FooPow for &'a mut Foo {
    fn pow(&self) -> i32 {
        self.0 * self.0
    }
}*/
 
fn test1(x: &Foo) {
    println!("{}", x.pow());
}
 
fn test2(x: &mut Foo) {
    println!("{}", x.pow()); // no method named `pow` found for type `&mut Foo` in the current scope
}
 
fn main() {
    test1(&Foo(42));
    test2(&mut Foo(42));
}

error: no method named `pow` found for type `&mut Foo` in the current scope
  --> prog.rs:24:22
   |
24 |     println!("{}", x.pow()); // no method named `pow` found for type `&mut Foo` in the current scope
   |                      ^^^
   |
   = help: items from traits can only be used if the trait is implemented and in scope; the following trait defines an item `pow`, perhaps you need to implement it:
   = help: candidate #1: `FooPow`

error: aborting due to previous error

Standard output is empty