Lifetime inference issue when destructuring in argument list

[sfackler]

I would expect that the sadface and non-sadface versions of next would be equivalent, but the sadface one is rejected and the other is not:

use std::slice;

pub struct PhfMapEntries<'a, T> {
    priv iter: slice::Items<'a, (&'static str, T)>,
}

impl<'a, T> Iterator<(&'static str, &'a T)> for PhfMapEntries<'a, T> {
    #[cfg(sadface)]
    fn next(&mut self) -> Option<(&'static str, &'a T)> {
        self.iter.by_ref().map(|&(key, ref value)| (key, value)).next()
    }

    #[cfg(not(sadface))]
    fn next(&mut self) -> Option<(&'static str, &'a T)> {
        self.iter.by_ref().map(|e| {
            let &(key, ref value) = e;
            (key, value)
        }).next()
    }

    fn size_hint(&self) -> (uint, Option<uint>) {
        self.iter.size_hint()
    }
}

fn main() {}

~ ❯ rustc --cfg sadface test.rs
test.rs:10:40: 10:49 error: borrowed value does not live long enough
test.rs:10         self.iter.by_ref().map(|&(key, ref value)| (key, value)).next()
                                                  ^~~~~~~~~
test.rs:9:57: 11:6 note: reference must be valid for the lifetime &'a  as defined on the block at 9:56...
test.rs:9     fn next(&mut self) -> Option<(&'static str, &'a T)> {
test.rs:10         self.iter.by_ref().map(|&(key, ref value)| (key, value)).next()
test.rs:11     }
test.rs:10:9: 10:72 note: ...but borrowed value is only valid for the method call at 10:8
test.rs:10         self.iter.by_ref().map(|&(key, ref value)| (key, value)).next()
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[edwardw]

A manifestation of #3696?

Works as intended now but needs a test.