Lifetime inference failure

[Florob]

In the code below Rust fails to infer an appropriate lifetime, even though y and w
clearly both live long enough to compare them.
Note that the suggested fix of giving the &str contained in y the lifetime 'a is undesirable in case y lives shorter than x and the returned value.

fn test<'a>(x: &'a Option<String>, y: Option<&str>) -> Option<&'a Option<String>> {
    let w = x.as_ref().map(|z| z.as_slice());
    if y == w {
        return Some(x);
    }
    None
}

fn main() {
    let x = Some("asdf".to_string());
    println!("{}", test(&x, Some("foo")));
}

<anon>:1:1: 7:2 note: consider using an explicit lifetime parameter as shown: fn test<'a>(x: &'a Option<String>, y: Option<&'a str>) ->
 Option<&'a Option<String>>
<anon>:1 fn test<'a>(x: &'a Option<String>, y: Option<&str>) -> Option<&'a Option<String>> {
<anon>:2     let w = x.as_ref().map(|z| z.as_slice());
<anon>:3     if y == w {
<anon>:4         return Some(x);
<anon>:5     }
<anon>:6     None
         ...
<anon>:2:13: 2:23 error: cannot infer an appropriate lifetime for autoref due to conflicting requirements
<anon>:2     let w = x.as_ref().map(|z| z.as_slice());
                     ^~~~~~~~~~
error: aborting due to previous error
playpen: application terminated with error code 101

[Ryman]

This is now fixed: http://is.gd/OKyPZK