I've been working on feature_A while another dev works on feature_B. Now, the other dev needs a small functionallity that's implemented in my feature_A, but I've not finished the whole feature itself. How should I share the code to the other dev? Is it against git-flow branching model? Can I simply merge feature_A into feature_B?
Publish a stub.
It might only exist in unit test code but get it out there fast so feature B can see what to expect from feature A. If you discover that the stub must change while you develop then inform the feature B team as quick as you can. Try hard not to change the stub but accept that no amount of hand wringing will ensure it can’t happen.
Communicate all this clearly and put the stub somewhere people can pull it. Likely on a branch that would make a good merge target for both features.