Borrowing from a vec after mutation requires a helper function to compile

[erickt]

I was working on adding a find_or_insert function to the hashmaps, and I ran into some trouble with the lifetime borrow checker. Here's my distilled code:

struct Bucket {
    value: uint,
}

struct A {
    xs: ~[Option<Bucket>],
}

impl A {
    fn find_or_insert1(&mut self, key: uint, value: uint) -> &self/uint {
        match self.xs[key] {
            Some(_) => (),
            None => { self.xs[key] = Some(Bucket { value: value }); }
        }

        match self.xs[key] {
            Some(ref bkt) => &bkt.value,
            None => die!(),
        }
    }

    fn find_or_insert_internal(&mut self, key: uint, value: uint) {
        match self.xs[key] {
            Some(_) => (),
            None => { self.xs[key] = Some(Bucket { value: value }); }
        }
    }

    fn find_or_insert2(&mut self, key: uint, value: uint) -> &self/uint {
        self.find_or_insert_internal(key, value);

        match self.xs[key] {
            Some(ref bkt) => &bkt.value,
            None => die!(),
        }
    }
}

fn main() { }

This errors with:

test.rs:13:22: 13:33 error: assigning to mutable vec content prohibited due to outstanding loan
test.rs:13             None => { self.xs[key] = Some(Bucket { value: value }); }
                                 ^~~~~~~~~~~
test.rs:16:14: 16:25 note: loan of mutable vec content granted here
test.rs:16         match self.xs[key] {
                         ^~~~~~~~~~~
error: aborting due to previous error

However, if instead I comment out find_or_insert1, it compiles fine. It would be great if there was a way to do this in one function, instead of needing a helper function work around lifetime issues.

[nikomatsakis]

I'm pretty sure this is a dup of #4903