Borrow check error with hash maps

[catamorphism]

In trans::reachable, there is the following code:

fn traverse_public_item(cx: &ctx, item: @item) {
    let rmap: &mut HashSet<node_id> = cx.rmap;
    if cx.rmap.contains(&item.id) { return; }
    rmap.insert(item.id);
//...
}

A comment says that naming rmap and giving it an explicit type should not be necessary, but with incoming, it still is. The error if I inline rmap is:

/Users/tjc/rust2/src/librustc/middle/trans/reachable.rs:98:7: 98:14 error: illegal borrow unless pure: creating immutable alias to dereference of mutable & pointer
/Users/tjc/rust2/src/librustc/middle/trans/reachable.rs:98     if cx.rmap.contains(&item.id) { return; }
                                                                  ^~~~~~~
/Users/tjc/rust2/src/librustc/middle/trans/reachable.rs:98:7: 98:35 note: impure due to access to impure function
/Users/tjc/rust2/src/librustc/middle/trans/reachable.rs:98     if cx.rmap.contains(&item.id) { return; }

This confuses me since I thought we removed purity. Anyway, this was an XXX and I figured I would make it into a proper issue. (@nikomatsakis may know more.)

[nikomatsakis]

This is a bug in the existing borrow checker (and the error messages were not updated to reflect the absence of purity). Both problems are addressed in my branch.

[nikomatsakis]

This precise code no longer exists, but I believe the situation to be resolved (modulo #6269)