Unexpected error: loan of mutable local variable as mutable conflicts with prior loan

[lilyball]

I have some code that looks like

let mut map : LinearMap<~[u8], int> = LinearMap::new();

for split_vec(text, 16).each |&block| {
    match map.find_mut(&block) {
        None => { map.insert(block, 0); }
        Some(x) => { *x += 1; }
    }
}

(split_vec returns a ~[~[T]])

Unfortunately the compiler complains about

foo.rc:46:13: 46:16 error: loan of mutable local variable as mutable conflicts with prior loan
foo.rc:46             None => { map.insert(block, 0); }
                                ^~~
foo.rc:45:8: 45:11 note: prior loan as mutable granted here
foo.rc:45         match map.find_mut(&block) {
                        ^~~

I can't figure out why calling two methods that both take &mut self is a problem here.

[lilyball]

The only way I could figure out how to accomplish what I wanted was to stuff the map into a @mut box. Surely that shouldn't be necessary.

[nikomatsakis]

this is due to the lack of flow-sensitivity in the borrow checker. hopefully this will be fixed soon. dup of #5074