mutable iterator

mut_iter.rs

struct MutableIter<'a, A> {
    // reference to underlying buffer
    inner: &'a mut [A],
    // state to keep track of how far along we are
    index: usize,
}

impl<'a, A> Iterator for MutableIter<'a, A> {
    // associated types
    type Item = &'a mut A;
    // required methods
    fn next(&mut self) -> Option<Self::Item> {
        // optionally return the next item in the inner slice
        let i = self.index;
        self.index += 1;
        self.inner.get_mut(i).map(|item| {
            let ptr = item as *mut _;
            
            // we need to use unsafe here due to a 
            // current limitation in the compiler:
            // http://smallcultfollowing.com/babysteps/blog/2013/10/24/iterators-yielding-mutable-references/
            unsafe { &mut *ptr }
        })
    }
}

fn iter<'a, A>(over: &'a mut [A]) -> MutableIter<'a, A> {
    MutableIter {
        inner: over,
        index: 0,
    }
}

fn main() {
    let mut s = vec![1, 2, 3, 4];
    for item in iter(&mut s) {
        *item *= 2;
        println!("item: {}", item);
    }
    println!("after: {:?}", s);
}