error[E0271]: type mismatch resolving `<Foo as PyClassImpl>::Mutability == Mutable`
   --> tests/ui/invalid_immutable_pyclass_borrow.rs:10:33
    |
10  |     let borrow = foo.as_ref(py).borrow_mut();
    |                                 ^^^^^^^^^^ expected struct `Mutable`, found struct `Immutable`
    |
    = note: required because of the requirements on the impl of `MutablePyClass` for `Foo`
note: required by a bound in `PyCell::<T>::borrow_mut`
   --> src/pycell.rs
    |
    |         T: MutablePyClass,
    |            ^^^^^^^^^^^^^^ required by this bound in `PyCell::<T>::borrow_mut`

error[E0271]: type mismatch resolving `<ImmutableChild as PyClassImpl>::Mutability == Mutable`
   --> tests/ui/invalid_immutable_pyclass_borrow.rs:20:35
    |
20  |     let borrow = child.as_ref(py).borrow_mut();
    |                                   ^^^^^^^^^^ expected struct `Mutable`, found struct `Immutable`
    |
    = note: required because of the requirements on the impl of `MutablePyClass` for `ImmutableChild`
note: required by a bound in `PyCell::<T>::borrow_mut`
   --> src/pycell.rs
    |
    |         T: MutablePyClass,
    |            ^^^^^^^^^^^^^^ required by this bound in `PyCell::<T>::borrow_mut`