no warning for implicit copies into closures

[nikomatsakis]

In this program:

fn main() {
    let x = {mut f: 3};

    let y = [x];
    let z = y[0];

    let f = fn@() {
        x.f = 4;
    };
    f();
}

I get a warning for let y = [x] and a warning for let z = y[0], but no warning for the copy of x into the fn@

[nikomatsakis]

@msullivan points out this is a last use. my mistake