Autoderef requests a mutable slot when it shouldn't

[alexcrichton]

struct Foo<T> {
    a: T,
}

struct Bar<'a, T> {
    foo: &'a Foo<T>,
}

impl<T> Foo<T> {
    fn bar<'a>(&'a self) -> Bar<'a, T> { fail!() }
}

impl<'a, T> Deref<T> for Bar<'a, T> {
    fn deref<'a>(&'a self) -> &'a T { fail!() }
}
impl<'a, T> DerefMut<T> for Bar<'a, T> {
    fn deref_mut<'a>(&'a mut self) -> &'a mut T { fail!() }
}

fn main() {
    let a = Foo { a: Foo { a: 2 } };
    let b = a.bar();
    let c = b.bar();
}

$ rustc foo.rs
foo.rs:23:13: 23:14 error: cannot borrow immutable local variable `b` as mutable
foo.rs:23     let c = b.bar();
                      ^
error: aborting due to previous error

This should not require b to be mutable. If I replace b.bar() with b.deref().bar() it will compile ok.

cc @eddyb

[alexcrichton]

Aha! I had a feeling there was already an issue on this.

Closing as a dupe of #12825