autoderef: `(&mut foo[bar]).baz()` works, but `foo[bar].baz()` fails to type check due to wrong deref

[japaric]

STR

use std::ops::{Index, IndexMut};

struct Foo;
struct Bar(Vec<i8>);

#[cfg(not(works))]
impl Index<Foo> for Bar {
    type Output = [i8];

    fn index(&self, _: &Foo) -> &[i8] {
        &*self.0
    }
}

#[cfg(works)]
impl Index<Foo> for Bar {
    type Output = Vec<i8>;

    fn index(&self, _: &Foo) -> &Vec<i8> {
        &self.0
    }
}

impl IndexMut<Foo> for Bar {
    type Output = Vec<i8>;

    fn index_mut(&mut self, _: &Foo) -> &mut Vec<i8> {
        &mut self.0
    }
}

fn test(bar: &mut Bar) {
    // these two should work
    bar[Foo].push(0);  //~ error: type `[i8]` does not implement any method in scope named `push`
    (&mut bar[Foo]).push(0);  // OK
}

fn main() {}

Version

rustc 1.0.0-nightly (4e4e8cff1 2015-01-24 22:14:14 +0000)

When <Bar as Index<Foo>>::Output == <Bar as IndexMut<Foo>>::Output == Vec<i8>, the bar[Foo].push(0) method call properly derefs bar[Foo] to Vec<i8>. But, when [i8] = <Bar as Index<Foo>>::Output != <Bar as IndexMut<Foo>>::Output = Vec<i8>, the method call wrongly derefs bar[Foo] to [i8]. In both cases explictly using (&mut bar[Foo]).push(0) does work.

cc @nikomatsakis @nick29581

[nikomatsakis]

This is not supposed to be possible. We screwed up somewhere in the definition of the Index traits -- there needs to be a constraint that the output type for IndexMut is always consistent with the Output type for Index -- in fact, I think there should just be one output type.

[nikomatsakis]

Same with Deref.

[japaric]

@nikomatsakis Should we change the definition of Index/IndexMut to be a hierarchy like Deref/DerefMut then?

// `?Sized` bounds omitted for brevity
trait Index<Index> {
    type Output;
    fn index(&self, &Index) -> &Self::Output;
}

trait IndexMut<Index>: Index<Index> {
    fn index_mut(&mut self, &Index) -> &mut <Self as Index<Index>>::Output;
}

cc @aturon