I try to understand how the rotation snapping feature works in Blender, since it works very strange. Especially, I want to learn how to use it in Object mode.
I can’t find any documentation how it works. And until then, I think the behaviour should be changed into something natural.
I don’t believe there is rotation snapping. I saw this post on twitter about some one wanting this for the 3d cursor. By default it won’t snap and apply rotation. I went on search and found parts of codes. I adjust ed it a bit and almost finished with it.
So far I can now let objects snap to the 3d cursor and apply the same rotation. I’ve also implemented a new bone snapping feature where you can snap to head or tail of a bone
I want to see the diff! That sounds 
It’s some classes I’ve putten together. The person asking on twitter never really reacted to post. Kinda weird if you ask something and then never respond???
I’ll send you my test file. I’m making it so that it will replace default snapping menu and then shows my version with new added features
That animation is kinda of a weird method. In edit mode you can use the 3d cursor What i dont understand is why do extrude that line to snap to. Simply use 45deg on X then use 35 on Y as rotation value in the 2 axis. Not sure its correct. But looks okay. I first tried 45 on Y as well, but looks weird. I guess thats due to ortho view/ I used my snapping thing. But you can input the degrees directly ofcourse
Is this what you need?
The bones do act a bit weird know and then. I need to work on that a bit more
The I used the 3d gizmo to have visual feedback when im adjusting the 3d cursor. That little bugger is hardly visible. But luckily we can enable gizmo properties in the menu.
PS one thing, i have not tried 2.9 yet. I just noticed i saw new inputs. I guess thats due to the some new smapping feats.
Oh, I thought you had a patch in Blender. This is Python?
I can help with anything bone-related. You might find the answers already in my addon, Chaintools.
Yes this is python.
I started with the 3d cursor snapping method and then bumped into a some code for bones. I adjsuted it a bit to me need or what i found useful. But needs a bit of work
It might not be fully what you are looking for but have a look at https://docs.blender.org/manual/en/dev/scene_layout/object/editing/transform/control/precision.html#snapping
I try to understand how the rotation snapping feature works in Blender, since it works very strange.
In my tests, it snaps the cursor to a vertex and takes the angle between the cursor position and the coordinate zero, instead of the relative delta.
This is a behaviour that makes sense. At least when the view is aligned in some natural way (the rotate snapping feature depends on the view) . But do you do this in Edit mode?
In the comments to the question at StackExchange above there is a link to an answer to a post about how to rotate a cube (sorry, but this forum thinks my post is spam and prevents a direct link) . He aligns the view and snapping works on vertices since he is in Edit mode (i.e. he has “access” to the vertices and can use them for snapping). If you are not in Edit mode, and the view is not aligned properly, the behaviour is not very understandable as shown below (images from the linked question on StackExchange):
