Compiler too restrictive for unused variables interacting with closures

I'd like to write the following code with `map`:

```
fn foobar(map: Option<&BTreeMap<u32,u32>>) {
    let temp;
    let map = match map {
        Some(a) => a,
        None => { temp = BTreeMap::new(); &temp }
    }
    // Do something.
}
```

But the following thing doesn't work:

```
let temp;
map.unwrap_or_else(|| { temp = BTreeMap::new(); &temp })
```
