Implementing Deref and DerefMut for the same type makes it impossible to call &self methods through immutable refs of that type

[reem]

Code:

struct Ref<T> {
    inner: T
}

impl<T> Deref<T> for Ref<T> {
    fn deref(&self) -> &T {
        &self.inner
    }
}

impl<T> DerefMut<T> for Ref<T> {
    fn deref_mut(&mut self) -> &mut T {
        &mut self.inner
    }
}

trait ImmutableRefTrait { fn immutable_borrow_method(&self) { () } }
trait MutableRefTrait { fn mutable_borrow_method(&mut self) { () } }

impl ImmutableRefTrait for uint {}
impl MutableRefTrait for uint {}

fn main() {
    let a = Ref { inner: 7u };
    a.immutable_borrow_method();
}

Error:

test.rs:28:5: 28:6 error: cannot borrow immutable local variable `a` as mutable
test.rs:28     a.immutable_borrow_method();

[jdm]

Probably related to #15609.

[huonw]

I think this may be a dupe of #12825.

[eddyb]

Confirmed as unrelated to #15609, dupe of #12825, can close.

[alexcrichton]

Thanks @eddyb!