clippy::unused_peekable triggers when `peek` is called in a closure

[nikomatsakis]

Description

Example code that triggers clippy::unused_peekable (but ought not to):

fn main() {
    let mut x = vec![1, 2, 3].into_iter().peekable();
    let mut y = std::iter::from_fn(move || {
        match x.peek() { _ => () }
        None::<u32>
    });
    y.next();
}

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