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use slice_trait::*;

let a: &[i32] = [1, 2, 3].as_slice();

fn first<'a, S: Slice + ?Sized>(slice: &'a S) -> Option<&'a S::Item>
where
    S::Item: Copy,
{
    slice.as_slice().first()
}

assert_eq!(first(a), Some(&1));