Best-effort checks for `@mut` freezing are *too* conservative

[nikomatsakis]

This test fails to compile but should not:

fn take_mut_slice<'a>(v: &'a mut [int]) -> &'a int { &v[0] }

fn main() {
    let mut x: ~[@mut [int]] = ~[@mut [3], @mut [4]];
    let a = take_mut_slice(x[0]);
    let b = take_mut_slice(x[1]);
    *a + *b;
}

I get the following errors:

/home/nmatsakis/tmp/best-effort.rs:6:27: 6:30 error: cannot borrow `(*(*x)[])[]` as mutable more than once at at a time                                          
/home/nmatsakis/tmp/best-effort.rs:6     let b = take_mut_slice(x[1]);
                                                                ^~~
/home/nmatsakis/tmp/best-effort.rs:5:27: 5:30 note: second borrow of `(*(*x)[])[]` as mutable occurs here                                                        
/home/nmatsakis/tmp/best-effort.rs:5     let a = take_mut_slice(x[0]);
                                                                ^~~

[emberian]

still relevant

[metajack]

Visiting for triage. Still relevant, but nothing to add.

[thestinger]

@mut was removed from the language.