How to get around lockstep iteration restrictions

playground.rs

#![feature(trace_macros)] trace_macros!(true);
macro_rules! doc_items {
    (@distribute $docs:tt $($item:item)*) => {
        $(doc_items!(@output $docs $item);)*
    };
    (@output [$($doc:expr),*] $item:item) => {
        $(#[doc=$doc])*
        $item
    };

    {
        $(
            $(#[doc = $doc:expr])*
            {
                $(
                    $item:item
                )*
            }
        )+
    } => {
        $(doc_items!(@distribute [$($doc),*] $($item)*);)*
    };
}

doc_items! {
    /// A
    {
        struct Foo;
        struct Bar;
    }
    /// B
    {
        struct Baz;
        struct Quux;
    }
}

fn main() {}