Why can't compiler infer a type here ?

Compiler refuses to compile this:

use std::ops::Mul;

struct Stuff{
    a: i128
}


impl Mul<i64> for Stuff {
    type Output = Stuff;
    fn mul (self, other: i64) -> Stuff {
        Stuff{a: self.a * (other as i128)}
    }
}
impl Mul<i32> for Stuff {
    type Output = Stuff;
    fn mul (self, other: i32) -> Stuff {
        Stuff{a: self.a * (other as i128)}
    }
}


fn main() {
    let stuff = Stuff{a: 42};
    
    let y = stuff * 2;
    
    println!("{:?}", y.a);
}

Compiler says:

error[E0282]: type annotations needed
  --> src/main.rs:25:9
   |
25 |     let y = stuff * 2;
   |         ^
26 |     
27 |     println!("{:?}", y.a);
   |                      - type must be known at this point
   |
help: consider giving `y` an explicit type
   |
25 |     let y: /* Type */ = stuff * 2;
   |          ++++++++++++

For more information about this error, try `rustc --explain E0282`.

y is obviously a Stuff, why can't the compiler see that ? Note that:

• compiler manages to compile if I only have one Mul function
• compiler manages to compile if I specify y's type.