IFD breaks parallel building

[DavHau]

Is your feature request related to a problem? Please describe.
Currently IFD seems to break parallel building.
I have put together a flake demonstrating the issue.

Describe the solution you'd like
When evaluation depends on several derivation outputs, build these derivations in parallel.

Describe alternatives you've considered
Wait for RFC92 and refactor existing IFD based code to make use of it.

I think this improvement would be of great value for the community, as some frameworks like haskell.nix are heavily based on IFD and would probably benefit by this change a lot.

[L-as]

I think this is a more general issue.
IFD completely blocks on a single derivation before evaluating again.
@thufschmitt Are there any plans/blockers for this? It seems easy enough to me.

[thufschmitt]

@thufschmitt Are there any plans/blockers for this? It seems easy enough to me.

Conceptually it is trivial :) In practice unfortunately I don't think it is. Unbreaking that would require the evaluator to be parallelizable, which it isn't atm. And I don't know that part enough to know exactly how much work that would be, but that doesn't look trivial at all :/

[L-as]

@aciceri