It seems that the issue here is that "last" is a reference, but the result of ".collect<Vec>()" is a temporary, so in the next invocation of the closure last would be referencing a Vec from a previous closure's stack-frame which would probably blow-up, and Rust is protecting you from the creating the corruption. Happy coding!

flatMap is supported by Node 11 onwards.

I think it may have more to do with this:

Attempting to invoke an infinitely recursive pattern of any form (including non-recursive patterns which call on infinitely recursive patterns in their pattern body, etc.) in the global scope. Extreme cases (e.g. 500 levels of non-infinite recursion) will not be tested in the test cases so a sensible recursion limit will do.

What do you mean by speed required? There are no performance tests, if anything's slow then maybe you're missing some logic.

Nice test, I will look into adding it. Thanks, I will also update the description with your comments.