rust/src/librustc_error_codes/error_codes/E0368.md

This error indicates that a binary assignment operator like += or ^= was applied to a type that doesn't support it. For example:

let mut x = 12f32; // error: binary operation `<<` cannot be applied to
                   //        type `f32`

x <<= 2;

To fix this error, please check that this type implements this binary operation. Example:

let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait

x <<= 2; // ok!

It is also possible to overload most operators for your own type by implementing the [OP]Assign traits from std::ops.

Another problem you might be facing is this: suppose you've overloaded the + operator for some type Foo by implementing the std::ops::Add trait for Foo, but you find that using += does not work, as in this example:

use std::ops::Add;

struct Foo(u32);

impl Add for Foo {
    type Output = Foo;

    fn add(self, rhs: Foo) -> Foo {
        Foo(self.0 + rhs.0)
    }
}

fn main() {
    let mut x: Foo = Foo(5);
    x += Foo(7); // error, `+= cannot be applied to the type `Foo`
}

This is because AddAssign is not automatically implemented, so you need to manually implement it for your type.