Rust workaround for "the `?` operator can only be used in a closure that returns `Result` or `Option`" when trying to use `?` inside a closure inside a `.map`

question_closure_trick.rs

let const_fns: Vec<TraitItemFn> = const_fns
    .into_iter()
    .map(|item| match item {
        TraitItem::Fn(trait_item_fn) => Ok(*trait_item_fn),
        _ => return Err(Error::new(item.span(), "expected `fn`")),
    })
    .collect::<std::result::Result<_, Self::Error>>()?;