error[E0368]: binary assignment operation `+=` cannot be applied to type `Foo`
  --> $DIR/assignment-operator-unimplemented.rs:6:3
   |
LL |   a += *b;
   |   -^^^^^^
   |   |
   |   cannot use `+=` on type `Foo`
   |
   = note: an implementation of `std::ops::AddAssign` might be missing for `Foo`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0368`.