In rustc, Oli had that as well, with many features being blocked on a complex refactor. Another example is the infamous Rust soundness bug that needs a completely new Trait solver. The project for this Next Trait solver exists, but does not necessarily have funding. (Edit: see reply, this is incorrect).
Oli says, this is not something we can rely on volunteers for that: they'll just end up burning-out, like others before them.
#RustInParis