Borrowck doesn't handle &mut correctly anymore

[nikomatsakis]

This USED to be an error, but doesn't seem to be anymore:

use core::either::{Either, Left, Right};

    fn f(x: &mut Either<int,float>, y: &Either<int,float>) -> int {
        match *y {
            Left(ref z) => {
                *x = Right(1.0);
                *z
            }
            _ => fail
        }
    }

    fn main() {
        let mut x: Either<int,float> = Left(3);
        io::println(f(&mut x, &x).to_str());
    } 

In particular it should not be allowed to simultaneously have an &mut and an & in scope. I recall that I changed the rules at some point around the creation of &mut pointers—that used to be considered equivalent to writing to the variable in question. I should not have changed that rule!

[nikomatsakis]

Oh, this isn't quite the bug I thought it was. This has to do with the compatible loan check. It checks that new loans are compatible with old loans but not that new loans are compatible with each other. Got a fix, testing now.