[coolreader18]

I believe you can just use `.flatten()` instead of `.flat_map(|v| v)`:

    let mut output: Vec<_> =  
        foo.stuff.into_iter()
            .flatten()
            .chain(bar.stuff)
            .collect();

I might make it slightly more clear what the intent is by doing this:

    let mut output: Vec<_> =
        Iterator::chain(foo.stuff.into_iter().flatten(), bar.stuff)
            .collect();

But still basically the same.