Unexpected "failed to find an implementation of trait" when combining mut self and impl for T where T is a type variable

[Dretch]

This code doesn't compile:

struct Foo { x: int }

impl io::Reader for Foo {
    fn read(&self, _bytes: &mut [u8], _len: uint) -> uint { fail!() }
    fn read_byte(&self) -> int { fail!() }
    fn eof(&self) -> bool { fail!() }
    fn seek(&self, _position: int, _style: io::SeekStyle) { fail!() }
    fn tell(&self) -> uint { fail!() }
}

impl Foo {
    fn bar(&mut self) {
        self.read_be_u32(); // read_be_u32 is defined by io::ReaderUtil, which is defined for all <T: io::Reader>
    }
}

fn main() {}

rustc fails with this error:

test-impl-for-t.rs:15:12: 15:31 error: failed to find an implementation of trait core::io::Reader for &mut Foo
test-impl-for-t.rs:15             self.read_be_u32();
                                   ^~~~~~~~~~~~~~~~~~~

I don't think that rustc should have a problem with this code.

Note that this error seems to be related to &mut pointers and immutable borrows, because if the bar function is changed to the following then rustc is happy:

let iself: &Foo = self;
iself.read_be_u32();

[emberian]

Closing as WONTFIX, due to #5087